Folks,

I hope nobody minds, but I figured it's best to start a new topic for this rather than tack it onto the end of the existing cue balls thread.

Last night I spent the evening with my genius friend and lifetime mentor Bill Eppler. Not only is Bill an expert on all matters audio, he can also explain advanced concepts in a way even I can understand. Where else could one get a college-level education for the cost of three beers and a dinner?

Bill and I discussed some very interesting stuff, and by the end of the night I had learned a lot and walked away with even more to think about and share / ask / discuss here.

Cue balls:

In the Cue Balls thread I believe we all ended up agreeing that at higher frequencies sound does in fact travel like a cue ball, but at lower frequencies - below the Schroeder frequency for the room - the "ray tracing" model breaks down and sound waves instead propagate as pressure patterns in the room. This brings up two new questions:

1. How can some low frequency room modes involve four or six surfaces when nothing is actually bouncing off those surfaces at an angle like a ray?

2. If non-axial modes do travel so they touch four or six surfaces, as shown in all of the text books, why is the first non-axial mode's frequency higher than the first axial mode which has a shorter path length?

Diffraction:

In my Acoustics FAQ is this drawing of a 2 by 4 foot absorbing panel that's 4 inches thick:



In the accompanying text I explain that with a panel this thick, having four inches of edge surface all the way around increases the 8 square feet of front surface area by 50 percent. Therefore, this is what accounts for absorption coefficients greater than 1.0, because the edge surface is not included in the calculations that convert Sabins to an absorption coefficient. While this panel is considered by the conversion formula to have 8 square feet of surface, it's really 12 square feet when you include the edges that are also exposed to the room during testing.

It has been suggested that this explanation is wrong, and the true cause of coefficients greater than 1.0 is diffraction of sound waves at the edges of the front surface. When a sound wave travels along a surface and reaches the end, the surface impedance changes suddenly and the wave then wraps around that edge. Sort of like water that's travelling in a pipe and is contained by the pipe. When the water reaches the end of the pipe it is no longer constrained to the size of the inside diameter, and so is free to expand and spread out. And since there had been outward pressure against the pipe walls, when the impedance changes suddenly at the end of the pipe the water does in fact start to spread outward.

But this does not change the fact that the higher absorption is still caused by having more surface area! Whether a wave that travels along the surface wraps around the corner and is then absorbed by the edge, or it's just that the edge is present in the room in the first place, either way it's still the edge that's absorbing. Moreover, I'm not convinced that diffraction is a big contributor at the frequencies the panel is absorbing. Frequencies that are in the range the panel can absorb will enter the panel, rather than skate along the surface. Perhaps at extreme angles of incidence, and with very dense material, a midrange frequency will skate rather than sink in. But it seems to me in that case much of it will skate again anyway when it wraps around the edge.

Impulses:

In the Cue Balls thread I was asked how a sound wave could travel like a cue ball, and I said:

Quote:

---

Suppose you make a very short click sound in a wave editor program, let's say with fast rise and fall times and 1 millisecond duration. Now play that click through a loudspeaker in a room. The speaker cone lurches forward for a moment sending a sound wave "bouncing around the room" until it runs out of energy.

---

Now, I was considering only the high frequency content of that wave. However, what some folks may not realize is that a single impulse - no matter how brief - has significant energy at frequencies lower than the pulse length might imply. As proof, consider a 1 millisecond impulse that repeats every 10 milliseconds. Clearly you will have energy at 100 Hz. And if it repeats once per second you can see there is energy at 1 Hz. So by extension even if it never repeats there is still energy down to as low a frequency as you care to measure.

--Ethan

--------------------
The acoustic treatment experts

Ethan:
It has been suggested that this explanation is wrong, and the true cause of coefficients greater than 1.0 is diffraction of sound waves at the edges of the front surface. When a sound wave travels along a surface and reaches the end, the surface impedance changes suddenly and the wave then wraps around that edge. Sort of like water that's travelling in a pipe and is contained by the pipe. When the water reaches the end of the pipe it is no longer constrained to the size of the inside diameter, and so is free to expand and spread out. And since there had been outward pressure against the pipe walls, when the impedance changes suddenly at the end of the pipe the water does in fact start to spread outward.

The fire brigade won't like this. They want the water coming out of the hose to be directed, wouldn't they?
So do sound engineers designing horns and line arrays.

Ethan:
Now, I was considering only the high frequency content of that wave. However, what some folks may not realize is that a single impulse - no matter how brief - has significant energy at frequencies lower than the pulse length might imply. As proof, consider a 1 millisecond impulse that repeats every 10 milliseconds. Clearly you will have energy at 100 Hz. And if it repeats once per second you can see there is energy at 1 Hz. So by extension even if it never repeats there is still energy down to as low a frequency as you care to measure.

Read about Dirac pulses and then come back on this, please.

Highest regards,

Bert

I hope those beers were Belgian?

Congratulations Ethan:

You have now added a Canadian sports term to acoustics!

Quote:

---

Perhaps at extreme angles of incidence, and with very dense material, a midrange frequency will skate rather than sink in

---

Andre

Ethan,

Before Hugh thinks that me is starting another war on you (you know that we had some very interesting discussions here and there without going at each others throats ):

When you have an impulse in your time window of ETF or whatever, and you window it, cut it of, at 1 msec, that is something completely different than a dirac pulse, and that is what your buddy is suggesting, IMO.

When you window your impulse at 1 msec and you apply a FFT, you will see that the thing is really limited in freq bandwidth. Try it, and you'll see.

Regards (no sarcasme, really),

Bert

A dirac pulse is a theoretical concept that has all freqs in it.
But as infinite high freq's have infinite small time information, you'll get an infinite high impulse because you try to store all this information in a infinite small amount of time.
So it has a very high spike in the start of the impuls. The lower freqs need time to develope, as states the uncertainty theory of Werner Heisenberg, so a real dirac pulse also has infinite long time resolution. You are not allowed to limit it to just 1 msec.

Bert

Ive always dreamed about turning this into a forum about quantum mechanics

Then we should definitely discuss Bell's Theorem!

Cheers,

Jim

--------------------
...rubbish at words 'n' stuff.

Bert,

> When you have an impulse in your time window of ETF or whatever, and you window it, cut it of, at 1 msec <

That's not what I'm talking about.

> a dirac pulse <

Nor is that. I'd never heard of that, but this first link Google returned says it's a myth because its infinitely short and thus impossible to realize. So while this type of pulse may have some academic interest, it's clearly irrelevant to what I described.

--Ethan

--------------------
The acoustic treatment experts

> a dirac pulse <

In the real world would that be pink noise?

Bert,

> The fire brigade won't like this. They want the water coming out of the hose to be directed, wouldn't they? <

Sure, and this amplifies my point even better about how small the effect of diffraction really is. And of course fire hoses (like garden hoses) use nozzles to direct the flow. But my basic premise remains - what really increases absorption is the presence of all that edge surface, regardless of anything else going on.

Right?

--Ethan

--------------------
The acoustic treatment experts

Quote Ethan Winer:

---

Bert,

> The fire brigade won't like this. They want the water coming out of the hose to be directed, wouldn't they? <

Sure, and this amplifies my point even better about how small the effect of diffraction really is. And of course fire hoses (like garden hoses) use nozzles to direct the flow. But my basic premise remains - what really increases absorption is the presence of all that edge surface, regardless of anything else going on.

Right?

--Ethan

---

hmmm?

Ethan, have a look at the picture in my studio diary of the huge makeshift absorber I was testing in my room. The absorber itself made a great difference for the better as I've already said.

However take a look at the MDF tops I put on the rockwool. I tested with and without those MDF tops, and they actually made NO Difference worth even worrying about. I was pleased by this of course, as I wanted some more worktop space in the studio. But I was quite prepared to leave that top edge ( of quite apreciable area ) absorbent if it HAD have made a difference.

And if the diffraction effect was that small how comes many pro studios soffit mount their monitors to cure the diffraction around the speaker cabinet. Even though soffit mounting excites mroe mdoes than free standing speakers, and thus requires a lot more rear wall absorption.

My Genelecs Soffit mounted sound so much better than freestanding, and it ain't just because I've got a better room now. The previous room was pretty good in fact acoustics wise. Just not as good as the new one, and had NO soudnproofng

I'm not directly disagreeing with you, as I don't know enough to do that, I'm just throwing some questions into the discussion.


Paul

--------------------
Pauls Studio Build Diary at http://forum.studiotips.com/viewforum.php?f=1

Paul,

> I tested with and without those MDF tops, and they actually made NO Difference worth even worrying about. <

Sure, at low frequencies. But I bet blocking all that surface would make a very large difference at higher frequencies like those measured when testing absorber panels. And to better test that directly you'd want to measure reverb time rather than frequency response.

> And if the diffraction effect was that small how comes many pro studios soffit mount their monitors to cure the diffraction around the speaker cabinet. <

My understanding is the main benefit of soffit mounting is to avoid SBIR, where sound leaving the rear of the speaker causes comb filtering off the front wall (the wall behind the speakers). And I'm not saying that diffraction does not occur or is insignificant! I'm saying that regardless of whether diffraction causes wrapping or not, it's still the presence of the additional edge that increases the absorption. At least that seems like the most plausible explanation.

How else would the act of diffraction alone increase a panel's absorption?

--Ethan

--------------------
The acoustic treatment experts

Quote Ethan Winer:

---

> a dirac pulse <

Nor is that. I'd never heard of that, but this first link Google returned says it's a myth because its infinitely short and thus impossible to realize. So while this type of pulse may have some academic interest, it's clearly irrelevant to what I described.

---

Well, just for linguistic clarity, it doesn't say it's a myth, but rather a mathematical fiction (which is very different than a myth, especially where said mathematical fiction serves as a predictive tool). Impossible to realize in the context of seismic activity (per said link), but in other descriptions of the use of Dirac pulses (e.g., medical use), they say that it can be realised only approximately, and that the Dirac function serves as a predictive tool.

Perhaps you guys are indeed talking about different things but, after reading a few of the links in the google search Ethan linked to, I feel I'm none the wiser as to how this may or may not be relevant in this context (perhaps because of lack of some fundamental understanding).

Any good links on Dirac pulses that might give some more entry level info on what they are and how they would be used in this context, Bert? I guess I'm seeing conflicting information as to whether a Dirac impulse is extremely short or infinitely long.

--------------------
Scott
--Where are we going and why am I in this handbasket?

Hey Scott,

When you try to store all waves to the infine small freqs you wil have waves that are so short that they will store all energy in an infinite amount of time. So you get an infiniteamount of energy in an infinite slice of time.
That sounds like it must go wrong, doesn't it?

But that's a dirac pulse. In reality we try to get close to this ideal, like in ETF of Mlssa or whatever.

But of course the low energy is smeared over time (longer waves) and doesn't appear as this spike of the highs.

When you FFT such a spike it analyzes all waves in it (amplitudes ad eventually phase) and gets you to the freq domain.

But you understand: a spike like this, because it is always describes as an infinite puls, seems a very short pulse.

But to get to he low freqa, you have to examine the proper amount of time, the tine a wave needs to develope.

So a dirac is infintite in time as well as in freq.

When you take a 1 msec pulse as ethan says, he might consider it as a gunshot (also much highs in the impulse) but it has only time resolution to 34 cm = 1000 Hz.

Ethan sas I don't understand his impulses that yield low freqs at 1 msec, so: go for it, my man.

Goddammit, this took an hour, I'm just back from a gig, drunk and stoned

When you try to store all waves to the infine small freqs you wil have waves that are so short that they will store all energy in an infinite amount of time. So you get an infiniteamount of energy in an infinite slice of time.
That sounds like it must go wrong, doesn't it?

Infinite SMALL amount of time, of course

Bert,

> go for it, my man <

I already did.

If the 1 millisecond pulse repeats at a slower rate than 1 KHz - or if it never repeats! - there is still low frequency energy. Or maybe FFT algorithms can see into the future and can tell if a repeat of the pulse is going to happen soon?

--Ethan

--------------------
The acoustic treatment experts

Scott,

> it doesn't say it's a myth, but rather a mathematical fiction <

You're right, my bad for lumping the two together. The main point is that infinitely narrow pulses are a theoretical exercise, where what I'm talking about is reality that can be easily measured.

--Ethan

--------------------
The acoustic treatment experts

If the 1 millisecond pulse repeats at a slower rate than 1 KHz - or if it never repeats! - there is still low frequency energy.

I really don't get it, Ethan. Could you please explain?
A 1 millisecond pulse is a pulse lasting 1 millisecond, and thus has a bandwidth of 1 millisecond which in the frequency domain is 34 cm = 1000 Hz?

Bert,

> I really don't get it <

Okay, let's say you use an audio editor program to make a pulse that's 1 millisecond long with fast rise and fall times. Now insert 9 milliseconds of silence, then paste the pulse at the end of the silence. Now you have a 1 ms pulse, 9 ms of silence, and the pulse again. Repeat this sequence a couple of hundred times so you have essentially created a 100 Hz square wave with a 10 percent duty cycle.

Now analyze the file. What frequencies do you have?

Then do it again but with 10 seconds between each pulse, and analyze again. What frequencies do you have now?

--Ethan

--------------------
The acoustic treatment experts

Now you have a pulse train with a time length of much more that 1 msec.

Bert,

> Now you have a pulse train with a time length of much more that 1 msec. <

Okay, so analyze just one pulse's worth of the train. This is why I joked about an FFT algorithm being able to see into the future. The LF content is there, even before other pulses arrive later. Even though it may not be intuitively obvious, I'm telling you there is content below 1 KHz!

Do you have a spectrum analyzer program to try a test yourself?

--Ethan

--------------------
The acoustic treatment experts

I'm aware of the validity of your remarks, but your 1 msec pulse statement doesn't make sense. You are using a delta function (pistol shot) in a pulse train like MLS does, and your 1 kHz remark is confusing
You do nothing with a 1 kHz filter or someting, you're just exiting a speaker or whatever with a delta function (dirac pulse), an approximation of a perfect pulse.
IMO this makes your statement about the pulse bouncing through the room, in the cue ball thingie, less valid.
You in fact use a wideband signal

Bert

Bert,

> I'm aware of the validity of your remarks ... You in fact use a wideband signal <

Yes, a narrow pulse is wide band!

Here's another way to look at it: Flip the polarity so instead of thinking of the pulse as narrow up, it's narrow down. Now you should be able to see that there's lots of energy for a long time, yes?

If you still don't get it let me know and I'll ask Bill for a math-based explanation. I'm not a math guy, so Bill knows to explain things to me without any math. But I'm pretty sure there's a formula that will prove the point.

Also, nobody has any comments on edge surface and diffraction? Or an explanation for why non-axial modes that hit all four surfaces start at a higher frequency even though it would seem the path length is longer?

--Ethan

--------------------
The acoustic treatment experts

Quote Ethan Winer:

---

.....Or an explanation for why non-axial modes that hit all four surfaces start at a higher frequency even though it would seem the path length is longer?

--Ethan

---

I'm not saying this is correct, it is only a theory of mine, and part of why I don't think the cue ball notion is correct.....

If you look at the rectangle described by a tangential mode off 4 surfaces in a rectagular room, I can certainly see why this can be considered a 'path'

But I look at it as more of Boundaries containing a space, rather then paths.

Now, any rectangle 'inside another rectangle' ( at an angle ) HAS to have smaller area than the outer ( room ) rectangle.

I kind of visualise a 'container' for resonance to occour in. Even with an axial mode of only two surfaces, the 'container' they make for trhe resonance is bigger than the 'container' for the tangential moe, hence the axial mode has a lower freqeuncy.

That's the way I currently see it, and although I appreciate it could be way off the mark, it does conform to the tangential freq being lower than the axial.


Paul

--------------------
Pauls Studio Build Diary at http://forum.studiotips.com/viewforum.php?f=1

Bert wrote:

Quote:

---

Ive always dreamed about turning this into a forum about quantum mechanics

---

http://www.glafreniere.com/sa_plane.htm
(at least this page relates to sound, however, there are many more pages.)

Paul,

> I kind of visualise a 'container' for resonance to occour in. <

One problem with the non-axial animations that Jeff linked is they don't show how a wave oscillates between the surfaces. That is shown for the axials, and an equivalent image for non-axials would help clear this up.

--Ethan

--------------------
The acoustic treatment experts

Quote Ethan Winer:

---

Paul,

> I kind of visualise a 'container' for resonance to occour in. <

One problem with the non-axial animations that Jeff linked is they don't show how a wave oscillates between the surfaces. That is shown for the axials, and an equivalent image for non-axials would help clear this up.

--Ethan

---

It certainly would. This is the very bit Im having trouble getting my head around.

Perhaps I need a 4 dimentional head


Paul

--------------------
Pauls Studio Build Diary at http://forum.studiotips.com/viewforum.php?f=1

Quote:

---

One problem with the non-axial animations that Jeff linked is they don't show how a wave oscillates between the surfaces. That is shown for the axials, and an equivalent image for non-axials would help clear this up.

---

Hey Ethan, I just came across this, but I've never seen this concept before. Maybe this is SIMILAR to what you are refering to.
web page

fitZ

Quote bert stoltenborg:

---

A 1 millisecond pulse is a pulse lasting 1 millisecond, and thus has a bandwidth of 1 millisecond which in the frequency domain is 34 cm = 1000 Hz?

---

huh? I've not really been following the detail of this thread, but I don't understand this statement, bert.

Bandwidth isn't measured in time. And the frequency domain has nothing to do with wavelengths or the speed of sound.

Perhaps I should have been paying more attention to your debate with Ethan, but this seems seriously confused and confusing!

hugh

--------------------
Technical Editor, Sound On Sound

Just read the thread Hugh.

Rick,

> I just came across this, but I've never seen this concept before. <

I'd take all of that with a huge grain of salt. This is at the top of the page:

"Nobody really knows yet exactly, in a scientific sense, how and why PrimaSounds works."

And it goes downhill from there, through this gem when it continues on the next page:

"It can create a personal and direct experience of the Infinite - God. The chords can provide a musical bridge to your fundamental vibration, your deepest, center tone."

--Ethan

--------------------
The acoustic treatment experts

Ethan,

I'm limiting my input to your initial post. The rest requires time I don't have.

Quote Ethan Winer:

---

1. How can some low frequency room modes involve four or six surfaces when nothing is actually bouncing off those surfaces at an angle like a ray?

---

Because it's a resonance and not a ray.

Quote:

---

2. If non-axial modes do travel so they touch four or six surfaces, as shown in all of the text books, why is the first non-axial mode's frequency higher than the first axial mode which has a shorter path length?

---

Explained on page 73 of Newell's Recording Studio Design.

Quote:

---

Therefore, this is what accounts for absorption coefficients greater than 1.0, because the edge surface is not included in the calculations that convert Sabins to an absorption coefficient.

---

My only comment on this is the only comment I ever have on this: The Sabine equation (used to calculate absorption in the reverb room method) does not calculate percentages. It calculates absorption.

************

On impulses: I have posted elsewhere on the uncertainty principle as it applies to impulse response measurement. This is nothing new. Dates back to the 1930s. See my post on this thread (and note that it's page 3 of 3) for further discussion.

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

--------------------
All the best,
Savant

Jeff,

> Because it's a resonance and not a ray ... Explained on page 73 of Newell's Recording Studio Design. <

Thanks. I looked up that reference, but your two statements seem contradictory in that light. Newell's book says that each "leg" of travel accounts for half a cycle, but doesn't that then imply a ray?

> The Sabine equation (used to calculate absorption in the reverb room method) does not calculate percentages. It calculates absorption. <

That doesn't really explain anything. Either the edge surface adds to the total absorption or it doesn't. If it doesn't, then what causes absorption coefficients greater than 1.0?

> I have posted elsewhere on the uncertainty principle as it applies to impulse response measurement. <

Bert and I have been arguing whether a pulse can contain energy at frequencies lower than the pulse length would imply. You're talking about the accuracy of measurements made with a short gate time. These are not the same thing!

Thanks.

--Ethan

--------------------
The acoustic treatment experts

Quote:

---

Either the edge surface adds to the total absorption or it doesn't. If it doesn't, then what causes absorption coefficients greater than 1.0?

---

Diffration of the sound. Read ASTM C423. I undersatnd from past posts that you have a copy of that standard.

Andre

Bert and I have been arguing whether a pulse can contain energy at frequencies lower than the pulse length would imply. You're talking about the accuracy of measurements made with a short gate time. These are not the same thing!

Ethan,

Jeff is of course making sense (as always).
His post states what I discussed with you: a pulse contains low freq info, but only if your time window is large enough to give the info the time to develope.
When you limit a pulse to 1 msec, as you implied to do, you per definition don't have low freq information.
So Jeff is NOT talking about something different!

Bert

Andre,

> Diffration of the sound. Read ASTM C423. <

Please read the above posts again. I understand that diffraction bends waves around an edge. But how does that by itself increase the apparent absorption of a thick panel? If it's not the presence of the extra edge surface, then what?

--Ethan

--------------------
The acoustic treatment experts

Bert,

> a pulse contains low freq info, but only if your time window is large enough <

Who said anything about time windows? I sure didn't. A pulse that's 1 millisecond long has energy lower than 1 KHz. This is all I ever stated.

Are you now agreeing that a 1 ms pulse has energy at frequencies lower than 1 KHz?

--Ethan

--------------------
The acoustic treatment experts

Quote Ethan Winer:

---

Andre,

> Diffration of the sound. Read ASTM C423. <

Please read the above posts again. I understand that diffraction bends waves around an edge. But how does that by itself increase the apparent absorption of a thick panel? If it's not the presence of the extra edge surface, then what?

---

Ethan:

The spec (ASTM C423) specifies that the edges of test material are to be covered. The spec includes several drawings of suggested methods for covering the edge materialo.

Please read the spec.

Andre,

> Diffration of the sound. Read ASTM C423. <

I just looked that up, and saw no explanation. In fact, the document author acknowledged not even understanding the principle. I quote:

Quote:

---

diffraction effects make the measured results greater than the ideal to a degree not yet completely understood

---

Well, gosh, I sure understand it. As explained above in my first post in this thread.

--Ethan

--------------------
The acoustic treatment experts

Andre,

> The spec (ASTM C423) specifies that the edges of test material are to be covered. <

That doesn't explain why diffraction around the corners increases absorption!

Again, please tell me specifically what is wrong with the analysis and drawing shown in my initial post. If having 50 percent more surface exposed is not the root cause of absorption coefficients greater than 1.0, then what is the cause?

--Ethan

--------------------
The acoustic treatment experts

Quote:

---

If having 50 percent more surface exposed is not the root cause of absorption coefficients greater than 1.0, then what is the cause?

---

1. The is NOT any more surface area exposed. This is explicitly in the standard.

2. The cause diffraction effects.

Andre,

> The is NOT any more surface area exposed <

Which part of the FOUR labels marked "Edge" do you not understand?

--Ethan

--------------------
The acoustic treatment experts

Folks,

Here's proof that a narrow pulse has content at frequencies below the period length might imply:

I created a single pulse with a duration of 1 millisecond in Sound Forge. I then applied a high-Q boost at 120 Hz with an EQ plug-in. Click HERE to download the Wave file to hear it, and below is a screen capture clearly showing the 120 Hz component that was brought out by the EQ.

Any questions?

--Ethan



--------------------
The acoustic treatment experts

Ethan:
"A pulse that's 1 millisecond long has energy lower than 1 KHz. This is all I ever stated.

Are you now agreeing that a 1 ms pulse has energy at frequencies lower than 1 KHz?"

No dude. You are deliberately not understanding people. That IS a discussion technique, but not a very fine one. Now go on questioning and redefining 500 years of physics. Tip:
You can start by questioning the benefits of using round wheels on cars by emphasizing the great brake capabilities of square ones.

Bert,

> No dude <

What part of my screen cap and Wave file do you not understand? Or put another way, if there was no energy below 1 KHz, then how could EQ have brought out what is obviously a 120 Hz component?

--Ethan

--------------------
The acoustic treatment experts

Ethan,

Quote Ethan Winer:

---

Thanks. I looked up that reference, but your two statements seem contradictory in that light. Newell's book says that each "leg" of travel accounts for half a cycle, but doesn't that then imply a ray?

---

No.

Quote:

---

> The Sabine equation (used to calculate absorption in the reverb room method) does not calculate percentages. It calculates absorption. <

That doesn't really explain anything. Either the edge surface adds to the total absorption or it doesn't. If it doesn't, then what causes absorption coefficients greater than 1.0?

---

The requirement to calculate an absorption coefficient causes absorption coefficients greater than 1.0.

{SNIPPED the impulse stuff. No time for such ing.}

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

--------------------
All the best,
Savant

Quote Ethan Winer:

---

Folks,

Here's proof that a narrow pulse has content at frequencies below the period length might imply:

I created a single pulse with a duration of 1 millisecond in Sound Forge. I then applied a high-Q boost at 120 Hz with an EQ plug-in. Click HERE to download the Wave file to hear it, and below is a screen capture clearly showing the 120 Hz component that was brought out by the EQ.

Any questions?

--Ethan

---

Greetings Ethan

What you see on your screenshot is the ringing artifacts of the EQ algorithm you used.

The pulse is merely exciting EQ filter circuitry into resonance. It's the same whether it's '1's and '0's or whether it's resisitors,capacitors, inductors and op-amps.

It's a *bit* like an 'impact' in some ways to the EQ. Like when you hit a drum with a stick. it doesn't matter how short the impact of the stick is, the drum will ring at it's resonant frequencies.

If you could get a speaker cone with slew rate and damping specs to reproduce a perfect square edge 1ms pulse in a room, I wouldn't doubt that all the modes would be excited into resonance.

Paul

--------------------
Pauls Studio Build Diary at http://forum.studiotips.com/viewforum.php?f=1

Quote:

---

The requirement to calculate an absorption coefficient causes absorption coefficients greater than 1.0.

---

Huh? Could you please explain this. I understand Ethans original question, and every additional question since. Why is everyone beating around the bush. If I expose the edge to a room, and cover the face, it will absorb at the same coefficent as the face, providing the angle of incidence is the same, no? Then what in sams hell is so hard to understand his question? I too would like to know.
fitZ

It just dawned on me. If the ASTM standard does not allow edges to be used in absorption tests to rate the coefficient, that is understandable. What is NOT understandable here, is why no one understands Ethans question. In fact, if Paul really didn't understand it, he wouldn't expose the edges in his new mobile absorbers. Hmmmmm.
fitZ

Edited by Rick Fitzpatrick (27/04/05 04:37 AM)

Guys, it's getting a little warm in here again. Deep, calming breaths everyone...

hugh

--------------------
Technical Editor, Sound On Sound

Rick,

Quote Rick Fitzpatrick:

---

Quote:

---

The requirement to calculate an absorption coefficient causes absorption coefficients greater than 1.0.

---

Huh? Could you please explain this.

---

Certainly. The ASTM C423 standard measures absorption. It does not measure absorption coefficients. The absorption is expressed in sabines. How absorption coefficients and such things are calculated is somewhat arbitrary in the grand scheme of things. So I'm not perceived as being a pest about this, let's take a look at one of my company's products:

The specimen of 2" Sonomatt absorbed 58.59 sabines at 1000 Hz when tested in accordance with ASTM C423, "A" mounting, at RAL. The specimen area was 64 ft² - i.e., that's how much of the floor it covered in the test chamber.

Now, what can we deduce from this?

For that test, two (2) 48"X96" panels were used. So, we could say the absorption of Sonomatt is 58.59/2 = 29.30 sabines per panel.

The floor area covered is 64 ft². Therefore, we could say the absorption is 58.59/64 = 0.92 Sab/ft². And we know this to be the definition of the absorption coefficient per the standard. So we can report it as such.

Of course, we also know that the exposed edges of the panels add approximately 3.3 ft² to the area of the panels. Thus, we could say the absorption is 58.59/67.3 = 0.87 Sab/ft². Note that we would probably need some sort of qualifier here to distinguish this number from the 0.92 number above.

One step further, we could figure out the actual surface area of the panel - it is sculpted foam, after all - and calculate yet another arbitrary absorption value.

Taking this ad absurdum, we could use chaos theory to determine the actual exposed surface area of all the pores in the foam. We would find that this area is, in fact, approaching infinity. Thus, our absorption coefficient is getting close to 0.

************

I hope this shows that the absorption measured - in sabines - is the only absolute value. To answer Ethan's question, it is not how we calculate the absorption coefficients, but rather it is because we calculate absorption coefficients that leads to values greater than 1.0. Yes, edge effects are included in that. So are diffraction effects. So is the fact that, when using the Sabine equation, it's completely reasonable to expect numbers greater than 1.0 even if you take all the edge and diffraction stuff into account. Think about it:

We could take a large foam or fuzz panel, 24" thick in the center, pyramidal in shape so the edges taper to nothing at the boundaries. There are no "edges" to calculate. Yet we'd very likely see absorption coefficients greater than 1.0.

There are three important things to remember:

1. A full test report disclosing mounting, the area used to calculate absorption coefficients, etc. is crucial to understanding the behavior of a particular acoustical product.

2. As long as we're all following the prescribed methods for testing and mounting, reasonable comparisons between materials can be made. E.g., you can easily compare the absorption coefficients of Sonomatt (above) to those of fiberglass panels, Tectum wall panels, RPG foam panels, etc. because we all follow the same test guidelines and procedures.

3. Folks, I cannot stress this enough: ABSORPTION COEFFICIENTS CALCULATED FROM C423 TESTS ARE NOT PERCENTAGES. The sooner we stop thinking about them as such, the sooner we can move on from this.

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

--------------------
All the best,
Savant

Quote Rick Fitzpatrick:

---

It just dawned on me. If the ASTM standard does not allow edges to be used in absorption tests to rate the coefficient, that is understandable. What is NOT understandable here, is why no one understands Ethans question. In fact, if Paul really didn't understand it, he wouldn't expose the edges in his new mobile absorbers. Hmmmmm.
fitZ

---

Greetings fitz


Out of interest, when I tested the 'makeshift' version of the PortaTraps, I found that placing an 18mm Sheet of MDF on top of them ( edge ) made absolutely no difference to the ETF results. ( With the test mic at the listening position )

So indeed the largest edge is indeed covered. I've left the side edges exposed simply as a 'just in case'

What I *do* understand though, is that a 1ms pulse doesn't contain LF infomation.


Paul

--------------------
Pauls Studio Build Diary at http://forum.studiotips.com/viewforum.php?f=1

Paul,

> The pulse is merely exciting EQ filter circuitry into resonance <

Yes, exactly. But if the pulse did not have any energy at 120 Hz then it would not have triggered the EQ to resonate at that frequency. This is my entire point.

I made a similar point earlier, somewhere else, about flutter echo and wall-wall spacing. If you clap your hands in a hallway or stairwell with a wall-wall spacing of 5'8" (equal to a 1/2 wavelength frequency of 100 Hz) you will not hear 100 Hz nearly as much as the higher frequencies that are more dominant in a hand clap. So in that case you'll hear mainly a "boing" somewhere around 300 to 500 Hz or whatever.

--Ethan

--------------------
The acoustic treatment experts

Rick,

> I understand Ethans original question, and every additional question since. Why is everyone beating around the bush. <

Thanks for stating the obvious. I was starting to wonder if I'd lost my ability to communicate!

--Ethan

--------------------
The acoustic treatment experts

Paul,

> I found that placing an 18mm Sheet of MDF on top of them <

As I already explained earlier, you measured the wrong thing.

You measured the change in low frequency response where what changes much more is reverb time.

> What I *do* understand though, is that a 1ms pulse doesn't contain LF infomation. <

See my post above where I explain about frequency content needed to excite ringing. Sorry Paul, but you're wrong on this one. A 1 ms long pulse does indeed have content all the way down to DC.

--Ethan

--------------------
The acoustic treatment experts

Jeff,

As always, thanks for clarifying. I'll add just a few points.

> The floor area covered is 64 ft². Therefore, we could say the absorption is 58.59/64 = 0.92 Sab/ft² <

Yes.

> the exposed edges of the panels add approximately 3.3 ft² to the area of the panels. Thus, we could say the absorption is 58.59/67.3 = 0.87 Sab/ft². <

Yes again, and that is explained clearly in my first post of this thread.

> We could take a large foam or fuzz panel, 24" thick in the center, pyramidal in shape so the edges taper to nothing at the boundaries. <

Yes, and in my Acoustics FAQ I make the point that absorption for some products, like tube traps, is best presented using Sabins (only) for this exact reason.

I agree with everything else you wrote too. But none of that explains how diffraction alone can make a panel appear to absorb more than it really does. Again, if the increase is not due to the presence of an edge, then what is the explanation?

--Ethan

--------------------
The acoustic treatment experts

Ethan,

Quote Ethan Winer:

---

I agree with everything else you wrote too. But none of that explains how diffraction alone can make a panel appear to absorb more than it really does. Again, if the increase is not due to the presence of an edge, then what is the explanation?

---

I really am sorry, but you lost me. I really don't know what you're asking about anymore. What panel is it that appears to absorb more than what is measured? What increase are you talking about?

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

--------------------
All the best,
Savant

Jeff,

> What increase are you talking about? <

Look at my first post in this thread. I show a panel and explain that the edges add to the total surface exposed to the room while testing, but the edges are not included in the formula that calculates absorption coefficients from Sabins. With me so far? Good!

Now, let's take two hypothetical panels that absorb 100 percent on their front surface, and also 100 percent on their edges. This is not really so hypothetical, because rigid fiberglass and acoustic foam both absorb pretty much 100 percent at the upper midrange frequencies.

It is my contention that 1) a thicker panel will have more Sabins than a thinner panel, and 2) the absorption coefficients will be higher for the thicker panel.

Even though both panels absorb 100 percent, the thicker one will measure better when expressed as Sabins and will also measure better when expressed as absorption coefficients. This is for one reason and one reason only: The additional surface area of the edge.

If you agree with this, then we're done and I've made my point. If you do not agree with this, please explain as completely and explicitly as you can where I've gone wrong.

Thanks.

--Ethan

--------------------
The acoustic treatment experts

Quote Ethan Winer:

---

Even though both panels absorb 100 percent, the thicker one will measure better when expressed as Sabins and will also measure better when expressed as absorption coefficients. This is for one reason and one reason only: The additional surface area of the edge.

If you agree with this, then we're done and I've made my point. If you do not agree with this, please explain as completely and explicitly as you can where I've gone wrong.

---

Thwe standard for determining absorption specificies that the edges be covered.

For teh benefit of otheres, the standard gives examples of using masking tape, sheet metal, wood strips etc, to cover the sides.

There is no additioanl surface area of the edge of the test material.

Andre

Andre,

> Thwe standard for determining absorption specificies that the edges be covered. <

Several mounting options are specified in ASTM C-423 and E-795. But the common "A" mounting method, which is all we've been discussing here, does not specify sealing the edges unless those edges are not exposed when the product is used as intended. Clearly this is not the case for products sold by RealTraps, or for the generic panel shown in my first post here.

Maybe your misunderstanding of the standard is why you've been confused about this all along? However, if you believe I've missed something important, please provide a paragraph number or whatever.

--Ethan

--------------------
The acoustic treatment experts

Boy am I confused!

This thread seems to be about (at least) 2 different issues, but, in order to break it down to manageable bits of information for gaining further understanding, here is how I'm following the "impulses" issue of this thread:

Quote Ethan Winer:

---

Impulses:

In the Cue Balls thread I was asked how a sound wave could travel like a cue ball, and I said:

Ethan Winer Quote:

---

Suppose you make a very short click sound in a wave editor program, let's say with fast rise and fall times and 1 millisecond duration. Now play that click through a loudspeaker in a room. The speaker cone lurches forward for a moment sending a sound wave "bouncing around the room" until it runs out of energy.

---

Now, I was considering only the high frequency content of that wave. However, what some folks may not realize is that a single impulse - no matter how brief - has significant energy at frequencies lower than the pulse length might imply. As proof, consider a 1 millisecond impulse that repeats every 10 milliseconds. Clearly you will have energy at 100 Hz. And if it repeats once per second you can see there is energy at 1 Hz. So by extension even if it never repeats there is still energy down to as low a frequency as you care to measure.

---

Bert suggested that Ethan might be talking about a Dirac pulse, which, as I understand it, would contain ALL of the frequencies in the spectrum.

Quote bert stoltenborg:

---

A dirac pulse is a theoretical concept that has all freqs in it.
But as infinite high freq's have infinite small time information, you'll get an infinite high impulse because you try to store all this information in a infinite small amount of time.
So it has a very high spike in the start of the impuls. The lower freqs need time to develope, as states the uncertainty theory of Werner Heisenberg, so a real dirac pulse also has infinite long time resolution. You are not allowed to limit it to just 1 msec.

---

But Ethan says this is not what he is talking about:

Quote Ethan Winer:

---

Bert,

> When you have an impulse in your time window of ETF or whatever, and you window it, cut it of, at 1 msec <

That's not what I'm talking about.

> a dirac pulse <

Nor is that. I'd never heard of that, but this first link Google returned says it's a myth because its infinitely short and thus impossible to realize. So while this type of pulse may have some academic interest, it's clearly irrelevant to what I described.

---

Quote Ethan Winer:

---

Bert,

> go for it, my man <

I already did.

If the 1 millisecond pulse repeats at a slower rate than 1 KHz - or if it never repeats! - there is still low frequency energy. Or maybe FFT algorithms can see into the future and can tell if a repeat of the pulse is going to happen soon?

---

OK . . so Ethan isn't talking about a Dirac impulse that contains all frequencies, but rather in impulse that contains a single frequency (correct?), with the theory that it "has lower frequency energy", as he says is proved by the test for which he supplies a screen shot.

[Ethan: what is the meaning of "has" here - contains, possesses, or something else? Sorry, I'm a language person -- these distinctions can make a huge difference in understanding.]

So Paul explains:

Quote Paul Woodlock:

---

What you see on your screenshot is the ringing artifacts of the EQ algorithm you used.

The pulse is merely exciting EQ filter circuitry into resonance. It's the same whether it's '1's and '0's or whether it's resisitors,capacitors, inductors and op-amps.

It's a *bit* like an 'impact' in some ways to the EQ. Like when you hit a drum with a stick. it doesn't matter how short the impact of the stick is, the drum will ring at it's resonant frequencies.

If you could get a speaker cone with slew rate and damping specs to reproduce a perfect square edge 1ms pulse in a room, I wouldn't doubt that all the modes would be excited into resonance.

---

Quote Ethan Winer:

---

Paul,

> The pulse is merely exciting EQ filter circuitry into resonance <

Yes, exactly. But if the pulse did not have any energy at 120 Hz then it would not have triggered the EQ to resonate at that frequency. This is my entire point.

I made a similar point earlier, somewhere else, about flutter echo and wall-wall spacing. If you clap your hands in a hallway or stairwell with a wall-wall spacing of 5'8" (equal to a 1/2 wavelength frequency of 100 Hz) you will not hear 100 Hz nearly as much as the higher frequencies that are more dominant in a hand clap. So in that case you'll hear mainly a "boing" somewhere around 300 to 500 Hz or whatever.

---

[Ethan: please note again the use of "have" in your statement above, with regard to my earlier question as to what "has" means in the context of your statement.]


So . . . keeping in mind that (1) I'm a drummer, and (2) I've got a really foggy recollection of the minimal physics I learned in high school (not being a math guy, I tended to have quite a few other priorities at that time, e.g., music, arts, cars and chicks, not necessarily in that order ):

I followed to a certain extent the idea of a pulse that repeats every 10 ms creating energy at 100 Hz-- though it seemed to me that perhaps it wasn't the cleanest explanation, given that it seemed to me there must be other factors involved (for starters, just imagining what it would sound like sending such a pulse to my speakers, and the possible variations in the pulse itself, taking into account what happens when you send an electrical pulse through a speaker, and also the variations in types and sizes of drivers that you might send such an impulse through, and how they would react).

So, fast forwarding a bit since Ethan says he's not talking about Dirac pulses . . . on to Paul's explanation that the pulse is simply exciting resonance. Makes some sense to me, and I especially liked the analogy of a drumstick hitting a drum head (me being a drummer and all), causing the drum to resonate at its fundamental frequency, and also exciting the various overtones, etc. that would be inherent in the drum, given its diameter, depth, tension of the head, and the degree to which the head is evenly tensioned in all directions (which would be related to individual lug tensions and how true the bearing edges are).

So . . . following on with the drum analogy (unless there is any major flaw with that analogy that anybody could reasonably point out):

Following Ethan's theory and comparing the stick to Ethan's 1 ms impulse . . . does the stick itself contain the energy of all these frequencies, or is it my arm that propels it, or is the result of a combination of things? I guess, given my experience that the weight of the stick and the size and shape of the bead on the stick will have some influence on the timbre of the drum (even more so with cymbals) resulting from said impact, this should be taken into account in all of this. (I suppose it would be most comparable to the wave shape of the impulse?).

Again, in my foggy recollection of all things physics that I might have studied in school, I'm guessing terms like stored/potential energy, velocity (which I'm assuming would translate in this case to amplitude?), and perhaps mass might come into play in any explanation of all of this?

Can anyone break this down to basic physics and explain (in a way that I, and possibly even other drummers, can understand ) why Ethan's and/or Paul's theory/explanation does or does not work?

Ethan, if your impulse has (contains? possesses? other?) energy at lower frequencies, how specifically does it differ from a Dirac pulse (or a slice of a Dirac pulse)? I'm somehow thinking your impulse either contains the low frequency or it doesn't, and, if it doesn't, then would I be wrong in assuming that it is rather a function of an equation of things (per my question above)? If the latter is the case, is this relevant to your assertions here? Further to this, I am also unclear as to how this in the end relates to the cue ball/ray tracing thing, so any explanations along those lines would be helpful as well.

This discussion is actually also peripherally interesting to me in that I'm thinking some of this information may perhaps relate to something Walter Sear said, in a Tape Op interview, about why digital recording (as currently implemented) sucks, related to the way he says upper harmonics affect lower frequencies. I'm still trying to get my head around it (or was it bollox?), so maybe this discussion will give me some further background or basis for getting a handle on that. But that's perhaps another post, or maybe a whole other thread.

--------------------
Scott
--Where are we going and why am I in this handbasket?

Greetings

Thanks Scott


The other thing I forgot to mention, when I created a 1ms pulse and EQed it at low frequecnies ( Same as Ethan's test ), I noticed this.....

Although you can quite clearly see the resonance of the EQ algorithm starting at where the pulse ends, I notice that the pulse itself wasn't altered in anyway. Now if it had LF content in it, it would have altered shape after being EQed.

I also tried EQing at a frequency that the pulse did contain, and indeed the pulse shape, as predicted, did change.

To me that proves there isn't any LF Content in a 1ms pulse.



NOW..... before I write off the notion that a sqaure single pulse of any duration contains all freqs to DC.... ( I can see it contains DC btw. The top edge is certainly DC )

In theory, it might be shown mathematically that a single pulse can be broken down by fourier ( or other ) calcs to show all freqs.... BUT........

In reality a single 1ms pulse ( which is in fact two transients off opposite direction ) cannot contain those LFs.....becuase.....

The future cannot be predicted.


Take a sine wave at,say, 100Hz.. Now, starting from zero and going positive, trace out the sinewave for 3/4's of the wave cycle and stop. Now, it looks like 3/4's of a wavecycle of a sinewave, and indeed by measuring half the wavecycle we can conclude it's 100Hz. But is it really 100Hz? What if at the end of this waveform it transients back to zero and starts that 3/4 of a 100Hz sinewave again. Now we have a wavecycle of 133.3333333333Hz

Altough it was made up of bits of a 100Hz sinewave the lowest frequency it can contain is 133.3333333333333333333Hz

Ok, that was a long winded way of explaining what I want to say, but think you get the drift


Paul

p.s another, and perhaps silly, but slightly amusing analogy is.....

Although DC could be said to contain No frequencies, one could show mathematically that DC contained all frequencies TWICE, one with reverse polarity and identical amplitude to the other one

--------------------
Pauls Studio Build Diary at http://forum.studiotips.com/viewforum.php?f=1

Scott,

A delta function/dirac pulse contains all energy, all sine waves in a signal. All sines are of equal energy. As an infinite high freq has infinite small time, it will pack it's energy in an infinite small amount of time, thus creating an infinite spike in the time domain. The lower freqs are smeared over time. When you wnat to see these lower freqs you need to consider a certain amount of time to give the sines the time to develope.

Ethan speaks about a 1 msec pulse. In my view that is a pulse containing sines with wavelength shorter than 1 msec, or am I insane?
Such a signal CANNOT contain frquencies under 100o Hz (1 msec = 34 cm = 1000 Hz).

Ethan describes pulses longer than 1 msec, his 1 msec statement doesn't make sense. He describes a braodband signal, containing lower freqs.

So he tries to emphasize something sustaining his cueball theory with short wavelengths, but than uses longer wavelengths to get to standing waves.

He is mixing up two signals to get his theory valid.

An impulse HAS all freqs in it, it's a pulse to excite a system like a speaker able to generate multiple freqencies. But when you filter this speaker in the time or frequency domain at 1 msec or 1000 Hz, it will have no output below these freq's.

I'm sorry I cannot describe this more compact.

Bert

Ethan's file should have a scale on the time axis. When it has a 1 msec length you can only FFT 1 msec of time, and I bet my foreskin that you won't have any freq content in MLSSA or whatever.
See Jeff's and my remarks on Heisenberg's uncertainty principle.

Ethan's file has a much longer duration than 1 msec, so it's logical that he can see lower freqs.
His 1 msec claim doesn't make sense, is only meant to sustain his short pulse-cueball theory.

Thanks, bert . . . but, unfortunately, I don't feel like I'm any the wiser.

Maybe we are misunderstanding each other?

First, we've already established that the length of the file (and the tail of the wave) is the ringing artifacts of the EQ filter algorithm Ethan used . . . the pulse excited the resonance.

There is a very short spike (which I would assume is the pulse we are talking about) at the beginning of the file . . . but it is too compressed in Ethan's picture to see what is in it. What does the wave form of the impulse itself? Or is it a 1 ms snippet of of pink noise?

Sorry if I'm being a moron here, BTW, but some of this really is new territory for me. I've always had some intuitive sense for acoustics, but I have not dealt with the science of this subject as deeply as I would like to have done.

--------------------
Scott
--Where are we going and why am I in this handbasket?

Hehehe,

Moron.... I know who you are.... :-)

But this spike should be windowed at 1 msec.

When you have a spike with lots of time afterwards, you get low freq information.

What you should do is take the spike, put a digital window over it with a time length of 1 msec, and cut the rest of the signal away. THEN you have a 1 msec pulse.
And when you FFT that pulse, you have resolution above 1 kHz, nothing under 1 kHz.

It's not pink noise, it's a wave of fe 100.000 Hz, one of 99.9999Hz, 99.998 Hz etc, etc, bundled together.
The short wave has the same energy than the longer waves, but the energy is concentrated in a shorter time! That's why it has this big spike!
When you limit the time to a window from the beginning of the impulse to 1 msec, you max freq resolution in the lows will be 1 msec = 1000 Hz.


Bert

Don't know how to make attachments here, so I do this at home:

http://forum.studiotips.com/viewtopic.php?t=1622

Guys,

Wow, this has gotten way more complicated than necessary!

Scott:

> Ethan says this is not what he is talking about <

Right, I am not talking about restricting the length of a time windowed measurement. I am talking about a single electrical pulse with a duration of 1 millisecond.

> so Ethan isn't talking about a Dirac impulse that contains all frequencies, but rather in impulse that contains a single frequency (correct?) <

No, I'm saying that a single impulse with a fixed duration contains energy at all frequencies. Note "contains" rather than "has" if that makes it clearer.

> how specifically does it differ from a Dirac pulse <

I never heard of a Dirac pulse before Bert mentioned it. But from what I read, the main difference is that a Dirac pulse has an infinitely short duration and thus is a theoretical curiosity. Versus something you can actually create and test in practice, as I have done.

> I am also unclear as to how this in the end relates to the cue ball/ray tracing thing <

It doesn't relate to that at all. It was just an aside I brought up because I knew at least one of the guys here didn't understand the concept of a short pulse containing low frequencies.

> Walter Sear said, in a Tape Op interview, about why digital recording (as currently implemented) sucks <

That statement alone shows some pretty extreme ignorance, no? Not that I want to split this off into yet another tangent!

> upper harmonics affect lower frequencies <

No they don't, other than via nonlinearity in the form of IM distortion or aliasing. The notion of "subharmonics" is a common misconception, but there's no truth to it.

Paul:

> I notice that the pulse itself wasn't altered in anyway <

Either your audio editor program is defective, or you didn't zoom in enough. I just edited the waveform image I posted earlier to show the wave zoomed in enough to see how the top became angled after applying EQ boost.

> In theory, it might be shown mathematically that a single pulse can be broken down by fourier ( or other ) calcs to show all freqs. <

Then why are you still arguing? I am serious! Further, what part of my previous explanation here do you not understand:

Quote:

---

if the pulse did not have any energy at 120 Hz then it would not have triggered the EQ to resonate at that frequency.

---

Bert:

> Ethan speaks about a 1 msec pulse. In my view that is a pulse containing sines with wavelength shorter than 1 msec, or am I insane? <

Do you really want me to answer that?

> But when you filter this speaker in the time or frequency domain at 1 msec or 1000 Hz, it will have no output below these freq's. <

Who said anything about loudspeakers or time filtering? I made the statement that a narrow pulse contains lower frequencies than the pulse length implies. That's all I ever said. And showing the effect of adding low frequency EQ to a short impulse proves the point beyond all doubt. Unless you don't understand that EQ works only if the frequencies affected are present in the original source!

--Ethan

--------------------
The acoustic treatment experts

Quote Ethan Winer:

---

.....

Paul:

> I notice that the pulse itself wasn't altered in anyway <

Either your audio editor program is defective, or you didn't zoom in enough. I just edited the waveform image I posted earlier to show the wave zoomed in enough to see how the top became angled after applying EQ boost.

---

Firstly Ethan I'm not arguing for the sake of it.

I did see that after making my last post. The reason I didn't see it before was because I created the pulse at 100% ( 0dbFS )

I did the test again with a 32bit float audio file, so I could see any effects of EQing that were over 0dBFS.

Yes, you're right, the pule *does* get altered. but I would contend that this is becaseu the ringing starts at the upward transient, and is simply added to the pulse in the result.

Quote:

---

> In theory, it might be shown mathematically that a single pulse can be broken down by fourier ( or other ) calcs to show all freqs. <

Then why are you still arguing? I am serious! Further, what part of my previous explanation here do you not understand:

if the pulse did not have any energy at 120 Hz then it would not have triggered the EQ to resonate at that frequency.

---

I'm still 'discussing', becuase I'm not sure that statement is correct. A pulse is two transients ( on up and one down ).

One could argue that a perfect transient doesn't contain any frequencies, becuase it is of infinitely small time duration.

Quote:

---

Bert:

> Ethan speaks about a 1 msec pulse. In my view that is a pulse containing sines with wavelength shorter than 1 msec, or am I insane? <

Do you really want me to answer that?

> But when you filter this speaker in the time or frequency domain at 1 msec or 1000 Hz, it will have no output below these freq's. <

Who said anything about loudspeakers or time filtering? I made the statement that a narrow pulse contains lower frequencies than the pulse length implies. That's all I ever said. And showing the effect of adding low frequency EQ to a short impulse proves the point beyond all doubt. Unless you don't understand that EQ works only if the frequencies affected are present in the original source!

--Ethan

---

regardless of Bert's time window explanations for a moment, why is it not possible an IMPACT of energy can cause ringing in an EQ at any frequency?

What I'm saying Ethan, is that while a 1ms pulse might contain the 'building blocks' of lower freqeuncies, it cannot actually contain those frequencies, as those lower freqeuncies need, in the real world, a certain amount of time to realise.

Please explain how you can get 100Hz, which smallest element is 10ms long in a 1ms pulse??

[5 minutes later ]

Here ya go Ethan..... This proves that your initial test with EQ was wrong in it's conclusion....


Try this test....

1] Create a Pulse 8 seconds long with a period of silence either side of it. Use a 32bit float file so you can see any 'overs'

2] Apply a LF boost with high Q as you did in your original test.

3] Note the result.


The result is....


You can quite clearly see the ringing of the EQ TWICE. Firstly at the initial upward transient, and secondly at the downward transient ( end of long pulse.)

And this is what I'm saying...

You don't need any frequency to excite a resonant entity at it's resonant frequencies. The analogy of the drumstick hitting the drum is sound IMHO. The drumstick deosn't contain any frequencies, it only contains kinetic energy, which is transferred to the drum on impact.

The same happens with EQing a pulse. In a pulse there is TWO impacts. When it starts and when it finishes.


I repeat I'm not arguing with you for the sake of it, or indeed becuase it's 'you'. I would contend this with anyone who suggested the same. And I find it all very interesting.


Paul

--------------------
Pauls Studio Build Diary at http://forum.studiotips.com/viewforum.php?f=1

Ethan,

Quote Ethan Winer:

---

Now, let's take two hypothetical panels that absorb 100 percent on their front surface, and also 100 percent on their edges. This is not really so hypothetical, because rigid fiberglass and acoustic foam both absorb pretty much 100 percent at the upper midrange frequencies.

---

100% of what? ABSORPTION COEFFICIENTS CALCULATED FROM C423 TESTS ARE NOT PERCENTAGES.

Quote:

---

It is my contention that 1) a thicker panel will have more Sabins than a thinner panel, and 2) the absorption coefficients will be higher for the thicker panel.

---

This statement is not a "contention." It's stating the obvious. But it's always good to be clear about your assumptions!

Quote:

---

Even though both panels absorb 100 percent, the thicker one will measure better when expressed as Sabins and will also measure better when expressed as absorption coefficients. This is for one reason and one reason only: The additional surface area of the edge.

---

Again, 100% of what? ABSORPTION COEFFICIENTS CALCULATED FROM C423 TESTS ARE NOT PERCENTAGES.

And edge effects are not the only reason the thicker panel is better. It's a thicker panel. Thin and thick could both be measured with the edges taped and the absorption would still be better for the thicker panel.

Quote:

---

If you agree with this, then we're done and I've made my point. If you do not agree with this, please explain as completely and explicitly as you can where I've gone wrong.

---

I don't know what you're looking for me to agree with. I don't know what point you are trying to make. I have made each and every one of my explanations as complete and explicit as I can. Where more clarification was requested, I gave it.

If your "point" is that edge effects can influence absorption measurements, then I would request we move on from stating the obvious. Edge effects have been known about for decades. (I could give you dozens of references, but I know how you feel about those...)

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

--------------------
All the best,
Savant

Paul,

> I'm not arguing for the sake of it. <

Of course, I know you're not, but much of what you are arguing is simply wrong. I am certain that Jeff Szymanski understands that a 1 ms pulse contains energy at 120 Hz, so maybe he'll pipe up and confirm it so we can avoid wasting yet more time on this. In the mean time I'll gladly carry on.

> the ringing starts at the upward transient <

You bet - the transient itself contains low frequencies. Just like dropping a book onto a table. Or the big bang, which had tons of LF content.

Stick a battery across your loudpseaker terminals and watch the woofer cone lurch forward. It doesn't matter if you leave the battery in place, or remove it after 1 millisecond. Either way, the LF energy that was present went through the crossover to the woofer's voice coil.

> why is it not possible an IMPACT of energy can cause ringing in an EQ at any frequency? <

It does, because the IMPACT itself contains low frequencies. So maybe we agree after all? Maybe now you can see that regardless of the pulse length, there is in fact low frequency energy?

> Please explain how you can get 100Hz, which smallest element is 10ms long in a 1ms pulse?? <

Okay, now I see where you're going wrong. Try to envision a small portion of a 120 Hz sine wave. Say, the first millisecond of the wave as it's just starting to curve upward. See what I'm getting at? The curve is what defines the frequency, not the absolute length! You don't need a full cycle for a frequency to exist, as evidenced by pink noise which often contains partial cycles at various frequencies.

Likewise, you can have a partial arc only a few inches long, but that belongs to a circle 60 feet in diameter.

> You don't need any frequency to excite a resonant entity at it's resonant frequencies. <

Yes, you do! Take a tambourine track and boost the bejeezus out of it at 30 Hz. Do you hear a change? I rest my case.

--Ethan

--------------------
The acoustic treatment experts

Jeff,

> ABSORPTION COEFFICIENTS CALCULATED FROM C423 TESTS ARE NOT PERCENTAGES. <

Rather than go 'round and 'round on semantics, let's try a different approach:

Do you agree or disagree with my basic premise that a panel having an edge surface of four inches will measure approximately 50 percent higher absorption than a (theoretical) panel that's infinitely thin, and that the increase is due mainly to the presence of the edge?

--Ethan

--------------------
The acoustic treatment experts

Quote Ethan Winer:

---

Paul,

.....

> Please explain how you can get 100Hz, which smallest element is 10ms long in a 1ms pulse?? <

Okay, now I see where you're going wrong. Try to envision a small portion of a 120 Hz sine wave. Say, the first millisecond of the wave as it's just starting to curve upward. See what I'm getting at? The curve is what defines the frequency, not the absolute length! You don't need a full cycle for a frequency to exist, as evidenced by pink noise which often contains partial cycles at various frequencies.

Likewise, you can have a partial arc only a few inches long, but that belongs to a circle 60 feet in diameter.
....--Ethan

---

I just concentrate on this bit, as Iv'e got to go out in a few minutes, so lack of time.

Yes, Ethan, that's exactly whta I was trying to explain in my exampkle in an earlier post.

If you remember I had a scenario where there was 3/4 of a full wavecycle.

So, OK, let's imagine this 'small portion' of a 120Hz wave.

Yes, I see exactly what you are getting at.

but the point I was making is 'we can't see into the future'.

What if that very small portion of that 120Hz sinewave is repeated at 1ms. ( so it looks a like saw blade )? Then it's Lowest freq component is 1Khz.

That's what I was saying earlier.....

A 1ms pulse may contain the 'building blocks' ( I.e the short segment ) of a 120Hz wave, but unless the wave completes a full cycle it cannot be termed 120Hz for definite.

And similarly with your circle and arc analogy.... The few inch long arc segment of a 60ft dia. circle might suddenly chagne direction completely if more than a few inches were observed.

Both the full circle or frequency IMHO are only being IMPLIED, and not actually present.

This may seem a pedantic difference, but IMO it's an important one.


Paul

--------------------
Pauls Studio Build Diary at http://forum.studiotips.com/viewforum.php?f=1

Paul,

> the point I was making is 'we can't see into the future'. <

There's no need to see into the future to determine the frequency components present here and now.

> What if that very small portion of that 120Hz sinewave is repeated at 1ms. ( so it looks a like saw blade )? Then it's Lowest freq component is 1Khz. <

No - the tops of each segment are angled just like the top of the (now-edited) impulse wave image I posted above.

> unless the wave completes a full cycle it cannot be termed 120Hz for definite. <

This simply is not true. As I explained earlier, pink noise routinely contains partial cycles that never complete. But we both know that you can still use pink noise to measure a room's response at those frequencies.

> Both the full circle or frequency IMHO are only being IMPLIED, and not actually present. <

Again, this is simply not the case, and again I wish Jeff would chime in because I know he understands this.

--Ethan

--------------------
The acoustic treatment experts

Please can I ask how you chaps are building your pulses and exactly which EQ and which settings you are using? This is very interesting and I want to understand more.

I have just been trying my own experiments with this applying low freq EQ to high freq pulses and get nothing.

My immediate reaction on seeing the spike effect in Ethan's pictures was - bug in the EQ algorithm or a feature of digital EQ.

I don't understand why the spike should be so short. If the lower frequency component is present in the pulse, when it is EQed, why doesn't the effect of the EQ last the duration of the pulse? Why does it only appear as a spike at the start?

(Aside #1: does the initial phase of the pulse have any effect?)

(Aside #2: What happens when you treat a low freq pulse with a high freq EQ?)

Mark

Mark,

> how you chaps are building your pulses <

I started a new empty file in Sound Forge, then used the "Generate Wave" function to create a short (half second) square wave at 1 KHz. I then zoomed all the way in to "sample" level, and set the volume on either side of a single half-cycle near the start to zero. (I also had to disable "snap to zero crossing" to avoid muting the portion I wanted to keep.)

> exactly which EQ <

I have the UltraFunk plug-in suite, so I used that parametric EQ. I set the center frequency to 120 Hz, the boost to full which is 18 dB, and the Q to max which is 24.

> bug in the EQ algorithm or a feature of digital EQ. <

Nope, you can do this with all analog gear and see the results on an oscilloscope, and I've done that many times in the past. But it's a lot more difficult than using Sound Forge and plug-ins!

> why doesn't the effect of the EQ last the duration of the pulse? <

The EQ causes ringing, and that ringing extends the short impulse.

> Why does it only appear as a spike at the start? <

Because the ringing and sustain haven't happened yet.

> Aside #1: does the initial phase of the pulse have any effect? <

No.

> What happens when you treat a low freq pulse with a high freq EQ? <

The high frequency content of a pulse depends on the rise and fall times. If these times are fast then high frequencies are present.

--Ethan

--------------------
The acoustic treatment experts

Quote Ethan Winer:

---

The EQ causes ringing, and that ringing extends the short impulse.


--Ethan

---

Ahhh, so that decaying sine wave isn't the original pulse, it is the ringing from the EQ? The original pulse itself only lasted for the duration of the blown-up picture?

I'm slow at this

Mark

Mark,

> Ahhh, so that decaying sine wave isn't the original pulse, it is the ringing from the EQ? <

Yes.

> The original pulse itself only lasted for the duration of the blown-up picture? <

Yes again.

I made a video recently that explains resonance and ringing as relates to room acoustics. You'll find it 3rd in the list here:

www.realtraps.com/videos.htm

In fact, it was while watching that video again that I realized EQ'ing an impulse is a good way to prove it contains low frequencies.

Another video in that series explains all about comb filtering, and you may find that interesting too.

--Ethan

--------------------
The acoustic treatment experts

Quote MarkEdmonds:

---

Please can I ask how you chaps are building your pulses and exactly which EQ and which settings you are using? This is very interesting and I want to understand more.

I have just been trying my own experiments with this applying low freq EQ to high freq pulses and get nothing.

My immediate reaction on seeing the spike effect in Ethan's pictures was - bug in the EQ algorithm or a feature of digital EQ.

I don't understand why the spike should be so short. If the lower frequency component is present in the pulse, when it is EQed, why doesn't the effect of the EQ last the duration of the pulse? Why does it only appear as a spike at the start?

(Aside #1: does the initial phase of the pulse have any effect?)

(Aside #2: What happens when you treat a low freq pulse with a high freq EQ?)

Mark

---

Greetings Mark

As we use the same DAW program, I'll explain how I did it in Cubase SX3


I recorded an empty file of, say 10 seconds long at 32bit float.

I then zooomed in and manually drew the pulse with the pencil tool. Set the ruler to 'Seconds' rather than bars and beats, so you can accurately get the pulse to the right length.

Drawing the pulse at 100% ( 0dBFS ) is easy becuase you just run the pencil above the 100% line and voila it's done. Then alter the gain by about -5dB to leave some visible headroom above the pulse.

Save.


Then use the range tool to highlight before and a few seconds after the pulse, and use Offien Processing with the EQ plug of your choice. I used UAD-1 Cambridge EQ and PEQ.


btw - interestingly the UAD-1 precision EQ actually had 'preringing' i.e the ring rose from nothing before the pulse started.




Also, PEQ preringing aside, Ethan is correct in that the ringing is nothing to do with the digital nature of the EQ. It does indeed happen with analog EQs/filters to.

Example: Take an analog synth VCF.. Turn the resonance right up on some synths and the ringing will last forever. i.e it goes into self oscillation and as long as power ( energy ) is applied it will last forever Anyone into 'synths' shoudl be familiar with this.

Even amplifiers, when you pump a square wave through them, will ring on the up and down transient portion of the wave. AFAIK and IIRC, the damping factor wil determine how much. Bert is probably the best person to clarify that one.

Speaker cones will also over shoot and ring from a transient ( 9v battery across the speaker terminals )



Paul

--------------------
Pauls Studio Build Diary at http://forum.studiotips.com/viewforum.php?f=1

Ethan,

Quote Ethan Winer:

---

Rather than go 'round and 'round on semantics, let's try a different approach:

---

It's not semantics. ABSORPTION COEFFICIENTS CALCULATED FROM C423 TESTS ARE NOT PERCENTAGES. Or, let me put it this way:

ABSORPTION COEFFICIENTS CALCULATED FROM C423 TESTS ARE NOT PERCENTAGES.

How many times should I repeat myself? Here it is again: ABSORPTION COEFFICIENTS CALCULATED FROM C423 TESTS ARE NOT PERCENTAGES. Read it carefully. Then read it again. Rephrase it any way you like: WHEN ABSORPTION COEFFICIENTS ARE CALCULATED USING THE METHOD PRESCRIBED IN ASTM C423, THE RESULTING NUMBERS ARE NOT PERCENTAGES. It's not semantics.

Quote:

---

Do you agree or disagree with my basic premise that a panel having an edge surface of four inches will measure approximately 50 percent higher absorption than a (theoretical) panel that's infinitely thin, and that the increase is due mainly to the presence of the edge?

---

If a panel is infinitely thin, then the panel that is not infinitely thin - e.g., the 4" panel you propose - is infinitely more absorptive. But, again, where the heck are you going with this? I don't understand why you cannot accept the fact that edge effects happen and let it go. Why do you have to attempt to ascribe some sort of made-up universal truth to it? This is ridiculous.

And as for the impulse argument, I stopped following it what seems like eons ago. I'm not about to get back into it. One BS waste of time is enough.

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

--------------------
All the best,
Savant

Quote Ethan Winer:

---

Jeff,
Do you agree or disagree with my basic premise that a panel having an edge surface of four inches will measure approximately 50 percent higher absorption than a (theoretical) panel that's infinitely thin, and that the increase is due mainly to the presence of the edge?

---

OK . . . not saying that there is no truth to your theory about the effect of edges (I certainly would be in no position to do so at this point), but . . . taking this back to drummer-level logic, I'm not sure I can agree with the edge effect as being the primary cause for the increase (or at least I cannot currently understand why this would be the case). Perhaps I am missing something?

Here's how I'm thinking about this:

So get myself a bulldozer, right? Actually, better yet, a really beefy truck with a snow plow on the front end. Then I build three dirt berms (piles of dirt, for those who may not be familiar with this word).

One is maybe four feet thick and, for the sake of practical argument, perhaps four times the width of the truck, and of course the berms are four feet thick at the ends (which I would equate to the edges).

The second is 12 feet thick, same thickness at the ends, same width, etc.

Now . . . if I back the truck way the hell up (I'll get to the third berm in a minute), and get it running full speed (don't try this at home!), and run it straight through each of these dirt berms, which one slows down the truck more (or stops it altogether)?

The one that's 12 feet thick, right? Is it because of the thickness of the edges, or is it because of the amount of mass that has to be moved to get to the other side of each berm?

So let's try it with the third berm, which is 12 feet thick (and, for the sake of keeping some consistency with the analogy, we'll say each of the berms are taller than the truck), and we have frame the berm on the side ends and the top end with concrete slabs.

I'm thinking that concrete slabs aren't going to make a whole lot of difference in how effective either berm is in slowing down or stopping the truck . . . and if it does make a difference, it wouldn't make the amount of difference that the difference between four feet of dirt and 12 feet of dirt.

Would I be correct in that assumption? And am I oversimplifying this to the point that there is something in particular in using this analogy that I am overlooking that makes it an ineffective analogy?

Again, sorry if I'm not keeping up with the class, but I figure if I have to watch you guys taking up this much space discussing these minutiae, I might as well learn something. I fully (and proudly) admit to being a drummer, and perhaps I'm showing my own limitations to my detriment, but I figure if you guys can explain it to me, then everybody else should be able to come to some common understanding about it as well -- especially since this debate has been raging on at various levels of civility for a couple of years.

And, while it may sound like it, I'm seriously not trying to be sarcastic in asking these questions, or being "intentionally thick" to make any particular point -- I really am this thick (OK, now I'm being a little facetious), and I'm serious in trying to gain some understanding here.

I don't have any particular interest or investment in who "wins" or "loses" the argument, I just would like to understand it, and perhaps for all of us to come to some common understanding of it.

--------------------
Scott
--Where are we going and why am I in this handbasket?

Quote:

---

WHEN ABSORPTION COEFFICIENTS ARE CALCULATED USING THE METHOD PRESCRIBED IN ASTM C423, THE RESULTING NUMBERS ARE NOT PERCENTAGES. It's not semantics.

---

Hmmmmmm. Then why does this state that it does.


Definition:

Practically, the sound absorption is the absorption “footprint” of the specimen in metric sabins. The sound absorption coefficient is the percentage of incident sound energy that, rather than being reflected, is converted to other forms of energy such as heat. Using a reverberation chamber, the sound absorption is computed as the difference of the Sabine absorption with and without the object under test present in the reverberation chamber."

This was from a site that the url was so long, you would kill me. But if you want, I could post it. I tried to change it, but to no avail
fitZ

Edited by Rick Fitzpatrick (29/04/05 07:31 AM)

Paul:
Even amplifiers, when you pump a square wave through them, will ring on the up and down transient portion of the wave. AFAIK and IIRC, the damping factor wil determine how much. Bert is probably the best person to clarify that one.

Guys,

This is getting out of hand. I cannot keep up with all these posts, but nobody seems to understand what I mean.

When you draw a spike in the time window of a wave editor, this has nothing to do with a pulse that has a certain timelength.

You see a spike, but also a lot of flat time information.
This flat time information contains the low freq information.

A pulse with a length of 1 msec in a wave editor should be highlighted over exact 1 msec. The marked piece of 1 msec, when FFT-ed with the analyze tool, will not contain freq under 1000 Hz. That's all. You can EQ what you want at 100 Hz, you see nothing or the analyzing tool sucks.

BUT YOU HAVE TO REALLY LIMIT THE PULSE TO 1 MSEC, AND NOT USE A SHORT SPIKE WITH LOTS OF FLAT TIME INFO!!!!!!

DAMMIT!!!!

Thanks for your efforts to explain and demonstrate, Bert. I think I'm slowly beginning to understand some of this . . . I still need to go and download Ethan's pulse and play with it a bit as well. Didn't have time yesterday.

Quote Rick Fitzpatrick:

---

This was from a site that the url was so long, you would kill me. But if you want, I could post it. I tried to change it, but to no avail
fitZ

---

Rick, if you copy the link, you can use the Instant UBB Code function just below the post window to post it. Click on "URL". A window pops up where you can paste the URL. Click "OK", and you'll get a second window. Type in some text like "Click Here" or whatever, and then click "OK" again. This will create a short link for your long URL.

--------------------
Scott
--Where are we going and why am I in this handbasket?

Rick,

Quote Rick Fitzpatrick:

---

The sound absorption coefficient is the percentage of incident sound energy that, rather than being reflected, is converted to other forms of energy such as heat.

---

By definition, that is certainly the case. And absorption measured using the impedence tube method is calculated as such. A simple level difference calculation that results in a ratio that gives an absorption coefficient. Multiply by 100 and you've got yourself a percentage. For this method,

a = [I(i) - I(r)] / I(i)

Where:
a = absorption coefficient
I(i) = Incident sound intensity
I(r) = Reflected sound intensity

It should be obvious that this value cannot exceed 1.0 since the reflected sound intensity will always be lower than the incident sound intensity.

If I(r) < I(i), then
I(i) - I(r) < I(i)
Which leads to
[I(i) - I(r)] / I(i) < 1.0

These are not the same absorption coefficients as the ones you calculate using the reverb room method.

Quote:

---

Using a reverberation chamber, the sound absorption is computed as the difference of the Sabine absorption with and without the object under test present in the reverberation chamber."

---

Also true. And this doesn't say the absorption coefficients from reverb room tests are percentages. The absorption before and after a test sample is in place is measured using differences in decay. For this method,

a = (A2 - A1) / S

Where:
a = absorption coefficient (In this case, a is often called the Sabine absorption coefficient to distinguish it from the normal, theoretical absorption coefficient explained above.)
A2 ~ d2 = the decay rate after the specimen has been added in dB/s
A1 ~ d1 = the decay rate of the empty chamber in dB/s
S = Surface area of the specimen

So, we could rewrite:

a ~ (d2 - d1) / S

(Note that I am using the "~" symbol to denote "is proportional to". I have omitted the constants involved in the equations to keep things simple.)

Since there is no reference to decay in the denominator of the above, one can hardly consider this the calculation of a ratio. Since it's not a ratio, the resulting absorption coefficient cannot be treated as a percentage.

Final note:

a =/ a

I.e., the absorption coefficient calculated using the difference ratio of sound intensity is not equal to the absorption coefficient using decay difference divided by surface area.

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

--------------------
All the best,
Savant

Quote:

---

These are not the same absorption coefficients as the ones you calculate using the reverb room method.

---

Thanks Jeff. I didn't see a caveat. And I wasn't aware there is more than ONE type of absorption coefficient.
As non-scientific person, I took the definition at face value.
Good to see you were on your toes though.
(just kidding) Sometimes, as a bystander, I have to question these things by professinals as trust is only as good as proof, yet proof is meaningless if you can't read a standard. As the song says, I won't get fooled again.
The trouble is, as a consumer, you can't actually QUESTION the product manufacturing "standard", unless you BUY the damn thing.

For all intents and purposes, that is EXACTLY why I do this.

For YEARS, I, and I'm certain other "consumers" too, when reading product specifications, such as "x part built, MEASURED or calculated according to ASTMx(or other) standard", assumed this was a government imposed standard of "quality" or something. It was only when trying to find out exactly what "absorption coefficient" exactly meant, that I finally started questioning things. What irritates me is, there is actually NO proof that a manufacturer does indeed follow a "standard", as a consumer is NOT allowed to know what the damn standard SAYS. In other words, as a consumer, I simply have to TRUST your word. Even
if I question it, YOU can't back it up because YOU arn't legally allowed to tell me what it says EITHER. And don't tell me the test results are your proof. Been there, done that.
In fact, I had to email an Acoustical products company, to contest what one of their Lab reports stated on one of their products. Not only did they misconstrue various data in relating specimen size, they did it on THREE pages. Wha the f... How can a reputable lab do that? Hence my mistrust in even so called "lab results" reports. I still have yet to hear back from them. Anyway, maybe this explains my questioning things like this.
fitZ

Quote:

---

Rick, if you copy the link, you can use the Instant UBB Code function just below the post window to post it. Click on "URL". A window pops up where you can paste the URL. Click "OK", and you'll get a second window. Type in some text like "Click Here" or whatever, and then click "OK" again. This will create a short link for your long URL.

---

Holy moly, lern somthin new evra daigh.. Thanks Scott, I was wondering how people did that. Actually, I tried that, but when submitted, it didn't have the actual link, so it confused me. DOH! Now I know..what a moron.
fitZ

Jeff,

> ABSORPTION COEFFICIENTS CALCULATED FROM C423 TESTS ARE NOT PERCENTAGES. Or, let me put it this way: <

Who said anything about percentages? I did refer to a hypothetical material that absorbs "100 percent" earlier, but that was just to simplify the example. You're the only one who keeps talking about percentages.

> But, again, where the heck are you going with this? I don't understand why you cannot accept the fact that edge effects happen and let it go. <

I do understand that edge effects "happen," though I was beginning to think maybe I was the only one! Where am I going with this? Okay, since you asked: Eric Desart has claimed several times that I'm wrong for stating that edge surface is the primary cause of absorption coefficients larger than 1.0. So I'm hoping to learn what specifically in my explanation is wrong. So far, all I've seen is a lot of beating around the bush, and intentional avoidance of the issues and direct questions I've asked. So here's another direct question, which I hope you'll think about and then answer honestly:

Do you agree with the basic premise of my statement, that edge surface is the primary reason absorption coefficients can be greater than 1? And if not, why not?

> And as for the impulse argument ... One BS waste of time is enough. <

Thanks very much for your assistance. That's exactly the type of intentional avoidance I was referring to.

--Ethan

--------------------
The acoustic treatment experts

Scott,

> So get myself a bulldozer, right? <

With all due respect, that example is so far afield of what's being discussed I can't see how it applies.

Scott, this is not complicated! You have a hunk of material with a certain amount of surface area. You increase the surface by 50 percent via 4-inch thick edges but don't take that increase into account when computing the absorption coefficient. Voila, all of a sudden the absorption seems higher than possible.

Is this really that difficult a concept?

--Ethan

--------------------
The acoustic treatment experts

Quote Rick Fitzpatrick:

---

Quote:

---

Rick, if you copy the link, you can use the Instant UBB Code function just below the post window to post it. Click on "URL". A window pops up where you can paste the URL. Click "OK", and you'll get a second window. Type in some text like "Click Here" or whatever, and then click "OK" again. This will create a short link for your long URL.

---

Holy moly, lern somthin new evra daigh.. Thanks Scott, I was wondering how people did that. Actually, I tried that, but when submitted, it didn't have the actual link, so it confused me. DOH! Now I know..what a moron.
fitZ

---

Fitz,

Another method is:
Go to > http://tinyurl.com
There you can make URLs extremely short.
The advantage is that this short URL remains valid (does nor expire), no matter where you use it.
This does help making short URLs, no matter how long the original. And you can still use it outside the forum.

Of course Scott's suggestion is as well valid.

I just suggest an alternative for hopeless cases.

--------------------
The important thing is not to stop questioning. Curiosity has its own reason for existing ............... Albert Einstein

Bert,

> nobody seems to understand what I mean. <

Now you know how I feel.

Seriously, I know exactly what you mean, but it's not relevant to anything I've been talking about!

Going back to my very first mention of a 1 millisecond pulse, all I said was you could send such a wave to a loudspeaker. I said nothing about being sure to trim away all leading and trailing silence. In fact, even if you did trim away all the silence, that silence still exists in the air and so still contributes low frequencies. Yet somehow that innocent statement triggered accusations of large hygienic inverted water bottles.

--Ethan

--------------------
The acoustic treatment experts

[ ****** ] you, Ethan.

Quote Ethan Winer:

---

Scott,

> So get myself a bulldozer, right? <


With all due respect, that example is so far afield of what's being discussed I can't see how it applies.

---

OK . . . but it would be a lot more helpful to me if you simply explain to me in some concrete terms why it applies, or where lies the fault or weakness in my analogy.

As I thought I made clear (though stated with a little lighthearted humour), I'm asking these questions without prejudice. I'm just trying to learn and understand here.

Quote:

---

Scott, this is not complicated! You have a hunk of material with a certain amount of surface area. You increase the surface by 50 percent via 4-inch thick edges but don't take that increase into account when computing the absorption coeffficient. Voila, all of a sudden the absorption seems higher than possible.

---

For starters, if you read my post, I did not dismiss the idea that the edges may have an effect for acoustic absorption (though I admittedly dismissed it to at least a degree in the dirt analogy, and I can understand that there would be other factors at play here when it comes to acoustic absorption compared to solid matter going through dirt). Rather, I questioned the statement that the difference was more (or primarily related to edge surface than to the thickness of the absorber. I felt I wanted to be sure I understood this more completely before I moved on to looking for further answers as to the edge issue.

Sorry . . . one of the primary ways we learn is by questioning things and dissecting them. If it is unacceptable for me to respectfully question things here in good faith, then what kind of place is this for learning?

I gave an analogy so that you and the others would understand how I was thinking of this situation. You are more than welcome -- and indeed I am actively encouraging you (or anyone else) -- to point out in a concrete manner where I am wrong so that I can learn. A good teacher will find where the student is and will find a way to bring the student closer to the teacher's understanding, rather than just telling him his question is stupid or irrelevant. Sorry, not a valid debating technique either, if you won't respectfully back up why it is that the question is not relevant, and unfortunately I see too much of this coming from certain people on both sides (and have for far too long now). I laid out where I was in my understanding (with the full understanding that doing so publicly exposes my limitations) in the hopes that someone would help bring me closer to what you guys understand.

This is not about ego for me, and it is not about discrediting anybody. Believe me . . . there is no margin whatsoever for me in discrediting anybody, or in being right over anyone here, as every one of you has more background in this than I. I'm honestly trying to understand.

Quote:

---

Is this really that difficult a concept?

---

I could have done without the condescension. It is these kinds of statements (along with some others like "what you are talking about is simply wrong") that I keep trying to tell you and the others are SIMPLY NOT PRODUCTIVE, and only serve to alienate the people you are discussing or arguing with. I'm continuing to see more of this from various parties here, and this whole thread continues to get further agitated, with defenses and egos being thrown up, and it becomes less and less educational for anybody.

This issue has gone on broken record stylee for several years now, and still continues to rage on with little real progress, and this kind of aproach is why. It's time to change the damn record.

For all the accusations flying both ways, and for all the claims from people on both sides of this thing that their true intent is to help people, and that they want to make sure that the real truth is known and understood, avoid misunderstandings, etc., there are a few people in this thing who actually ARE trying to achive those goals, but there are still some of you who can't seem to drop the defenses and egos long enough to remember what they said they were arguing all of this for and to actually try to help to the people they claim to want to reach.

Now I'll ask you . . . IS THIS SUCH A HARD CONCEPT TO UNDERSTAND?!?!?!

Sorry . . . I have a lot of patience with people (in many cases well beyond what I should, and even more often more than most people would allow for), but every once in a while I have to let it out and call a spade a spade. You guys don't owe me to catch me up on my physics, but this is, after all, a forum for sharing knowledge. And the bottom line is that this crap is getting really irksome. It isn't the fact that you guys are arguing these points, but rather the way some of you have chosen to argue them that is the problem. I'll say again that I am not at all alone in this assessment. And unfortunately sometimes it seems that some of you are more interested in pointing fingers and assigning sole blame than in finding a solution, or even in helping clarify your basis in argument. As I said, I'm not interested in taking sides in all of this. I only look for a solution for reasonably peaceful coexistence among you. While I don't wish to tar everyone with exactly the same brush, this is simply NOT one sided, no matter how much some of you wish to believe it is. And, again, I'm not alone in my assessment of this. I've gone to bat for you guys (after everyone else had pulled all their hair out and plucked their eyes out over this) and have been willing to try to work with both sides to see if we can find some solution to all of this. And if you are going to ask me to give you some benefit of the doubt and to take you at face value as being sincere, I would appreciate the same respect in return.

Again, I'm not just saying this stuff to one person. I'd like to think people should be able to be honest enough with themselves at least to figure out for themselves what applies and what doesn't.

Can we please get back to a civilised discussion of the issues and facts, without beating each other up or feeling threatened by legitimate (or at least honest intentioned) questions?

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Scott
--Where are we going and why am I in this handbasket?

Quote Ethan Winer:

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It has been suggested .....

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Quote Ethan Winer:

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Okay, since you asked: Eric Desart has claimed several times that I'm wrong for stating that edge surface is the primary cause ....
--Ethan

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I see that ... it ... finally got a name ....
  
Maybe it should be good then that you refer to the message this comes from (after numerous other times over 2 years and more of explaining, asking and begging, starting from very gentle):
  
http://forum.studiotips.com/viewtopic.php?p=15973#15973
The part of the topic starting after the dark red bold line.
  
It should also be good that you state the related quotes from the standard itself:
  
ASTM C423:2000

• 5.2 The sound absorption coefficient of a surface is a property of the material composing the surface.
  It is ideally defined as the fraction of the randomly incident sound power absorbed by the surface, but in this test method it is operationally defined in 4.2.
  The relationship between the theoretically defined and the operationally measured coefficients is under continuing study.
  Note Eric: point 4.2 is description lab method
   
• 5.3 Diffraction effects* usually cause the apparent area of a specimen to be greater than its geometrical area, thereby increasing the coefficients measured according to this test method.
  When the test specimen is highly absorptive, these values may exceed unity.
    
  Note Eric: The * refers to related studies in the standard itself:
    
  * Chrisler, V.,
  "Dependence of Sound Absorption Upon the Area and Distribution of the Absorbent Material,"
  Journal of Research, National Bureau of Standards, Vol 13, 1934, p. 169:
    
  * Northwood, T. D., Grisaru, M. T., and Medcof, M.A.,
  "Absorption of Sound by a Strip of Absorptive Material in a Diffuse Sound Field,"
  Journal of the Acoustical Society of America, Vol 31, 1959, p. 595:
    
  * Northwood, T. D.,
  "Absorption of Diffuse Sound by a Strip or Rectangular Patch of Absorptive Material,"
  Journal of the Acoustical Society of America, Vol 35, 1963, p. 1173.
    
• 11.10 Since diffraction effects make the measured results greater than the ideal to a degree not yet completely understood, no adjustments shall be made in the coefficients for this cause.
  

  
More recent:
  
Proceedings of Noise Con 90
David A. Nelson, P.E., INCE Bd. Cert.,
  
"Diffraction Effect" in Sound Absorption Tests: Why is the sound absorption coefficient greater than 1.00?

Lab meas. of sound absorption is based on the effect of a patch of material on a diffuse sound field in a reverb chamber.
The math used in the analysis presumes that sound travels with equal probability in all directions.
This is more or less true throughout the room, except over the sample.
For a highly absorptive sample sound travels INTO the specimen, but very little is reflected back.
The discontinuity in the wave field at the edge of the specimen create a diffraction effect that warps the sound field to make the specimen appear as much as a quarter-wavelength larger in each direction.
This increases the sound absorption coefficient to such a degree that it often exceeds the theoretical limit of 1.00. Does this mean the data is invalid? No. The sound absorption coeff. reported are for specimens of the given size in a diffuse sound field: application of these numbers to continuous surfaces may substantially overstate actual performance.

  
A definition
  
http://zone.ni.com/devzone/nidzgloss.nsf/webmain/78A89E994ADC83CB862568C60 05CDB58
  
I can enter lots more references, but since you choose to explain anywhere rather than reading about it, this will do for now.
  
The fun thing is that you know the content of the standard, but wisely do not refer to it.
You know that I describe diffraction different as your poetic license in this context.
At least try to represent what others told if you refer to them. Not stories about skating sound.
You doubt diffraction without knowing it (but it's a nice word isn't it?).

I specially bought edition 4 now of the Master Handbook of Acoustics of Everest (I had ed. 2 already).
It explains modes rather well (not math but Everest is meant for other target group). It nowhere refers to rays but VERY clearly to wave acoustics.
It even RELATIVATES its own picture, here witheld as proof.
Books are not meant to find usable pictures out of its context but to read.
And Hugh, your related confirmations aren't very professional.
I don't feel like buying newer editions just to see that the content is close to abused, while simultaniously used as proof to substanciate points.

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The important thing is not to stop questioning. Curiosity has its own reason for existing ............... Albert Einstein

Quote Ethan Winer:

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Bert,

> nobody seems to understand what I mean. <

Now you know how I feel.

Seriously, I know exactly what you mean, but it's not relevant to anything I've been talking about!

Going back to my very first mention of a 1 millisecond pulse, all I said was you could send such a wave to a loudspeaker. . . .

Yet somehow that innocent statement triggered accusations of large hygienic inverted water bottles.

--Ethan

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Quote bert stoltenborg:

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[ ****** ] you, Ethan.

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Scott
--Where are we going and why am I in this handbasket?

Scott,

> I'm asking these questions without prejudice. <

Of course, I know that!

> though I admittedly dismissed it to at least a degree in the dirt analogy <

Right, and that analogy seemed so far off it didn't make sense to me as a reason for dismissing my point. I meant no condescension, honest.

> I questioned the statement that the difference was more (or primarily related to edge surface than to the thickness of the absorber. <

Okay, I see the confusion and I'll gladly explain. This is exactly why I referred earlier to absorbing material that absorbs 100 percent, so it was clear that 100 percent of the sound that strikes either the front surface or edges is absorbed. Note that the idea of material absorbing 100 percent is not so far fetched, and is close to reality for some materials at higher frequencies.

Now, if we assume that the material absorbs everything that strikes it, the only factor that affects total absorption - as measured in a lab's reverb room - is how much material there is. So if you compare the absorption of a 2x4 foot panel that's one inch thick with another that's four inches thick, the thicker one has 37.5 percent more total surface area exposed in the lab, yet the same 8 square feet of front surface is used when converting the (now larger) Sabins to an absorption coefficient.

If this still is not clear, and my explanation in the first post in this thread is still not clear, please let me know and I'll keep trying to explain it.

> this whole thread continues to get further agitated <

Agreed, and mea cupla for my participation in that. It's getting very frustrating to see people who absolutely do understand what I'm saying intentionally pretend they don't understand, just so they can use the rolleyes smiley a few more times. I am not including you in this! All I meant was that your dirt analogy was very far afield of how sound waves pass through material that absorbs.

> Can we please get back to a civilised discussion of the issues and facts <

Yes sir!

--Ethan

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The acoustic treatment experts