Quote Martin Walker:

---

Quote Paul Woodlock:

---

Pity you didn't live closer to Peterborough, I would have let you borrow a few packs Only potential problem with that, is you might not wanna give them back

I look forward to your test results and findings. Finding someone with both ETF, Mini-Traps, and willing to get some rockwool is cool.


Paul

---

That's a strange coincidence - I was born in Peterborough (Eastfield Road if that means anything to you - my family moved further South when I was five years old).

Somehow I doubt that I'll get enough time to drive to Bude and buy those slabs before Christmas though - too many other commitments and deadlines I'm afraid


Martin

---

Cool! - Yes I know Eastfield Road Very well. I lived only a few yards from there for 10 years, before moving to my present location.

How's Cornwall? The last time I was there was in 1973. My parents and I used to Holiday there, at a caravan site near Mullion Cove.

My clearest memories are that of the tin mines, where extremely deep vertial mine shafts were, at that time, NOT surrounded by any safety fences. That would never happen today.


btw - Travis Perkins do deliver within a couple of days. Might be worth it, offsetting the cost of driving and timetaken for your good self


Paul

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

Kevin,

> at low frequencies the walls act like a kind of waveguide ... the rise up to 100Hz is simply caused by the materials absorption range. Obviously none of these materials do a briliant job way down in the 20, 30, 40 Hz range <

Give this man a fine Cuban cigar. Corners focus sound just like cupping your hand over your ear, or a parabolic dish. And as you rightly observed, the low end rolloff is not the downside of a peak, but simply the natural falloff of porous absorber materials.

The final piece of the puzzle is even simpler: I assume that most acoustic labs are a similar size. So it stands to reason that frequencies emphasized in one lab will be similar to those of other labs. The range from 80 to 125 Hz is very wide! As I said earlier, that wide span hides the very detail needed to more closely analyze what's really going on when you measure only at widely spaced frequency ranges. If you have five labs whose dimensions are all within 10-20 percent of each other, one's corner peaks might be at 95 Hz and another's at 105 Hz. But without using something sophisticated like ETF - or noise analysis that can resolve to much finer than 1/3 octave - there's no way to know where the peak really is.

--Ethan

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

Jeff,

> We measured the narrowband, high frequency absorption of a roughly 8'x8' sample of material - the blue curve <

This is very interesting, and clearly shows that even at higher frequencies comb filtering in the room is skewing the readings. I'm sure the material isn't really absorbing that selectively!

Can I assume that graph is with the test microphone at a single position? Also, is this standard noise that was analyzed to a very fine resolution, or the output from a more sophisticated analyzer?

-Ethan

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

Quote Ethan Winer:

---

As I said earlier.... ...But without using something sophisticated like ETF - or noise analysis that can resolve to much finer than 1/3 octave - there's no way to know where the peak really is.

--Ethan

---

Hello all,
Ethan:

How does this address the posts of Jeff & the Foz regarding the nature of 1/3 octave pink noise measurements?
(namely, that all the energy is measured, peaks included)

Ido (yet another studiotips inhabitant)

Quote:

---

Ido (yet another studiotips inhabitant)

---

Blimey, they're taking over

Welcome Ido

Quote:

---

How does this address the posts of Jeff & the Foz regarding the nature of 1/3 octave pink noise measurements?
(namely, that all the energy is measured, peaks included)

---

Not answering for Ethan (he's a big boy and can do that for himself ) but if I read correctly the post to which you refer, he doesn't appear to dispute that. When he says that in order to determine the actual frequency of the peak we're talking about, one needs to use a measurement and data presentation regime caable of showing greater than averaged, 1/3 octave banded resolution, I don't have a problem with that. In this case, I feel that Ethan is right that in exploring this "100Hz" peak phenomenon a 1/3 octave plot is of limited use.

On the other hand, Jeff has already shown that the question can be one of data presentation rather than necesarily a problem with the accuracy of the measurements. Just because the plot that's presented "in public" conforms to the standard (rough) method of presentation that doesn't mean that the lab didn't log much more detailed results. It's convenient and usually "accurate enough" broadly to describe absorber performance in averaged 1/3 octave bands; it's only really a big problem when investigating more detailed acoustic bahaviour. This isn't a question of whether or not all the energy is measured but one of the presentation of the results in averaged 1/3 octave bands causing the detail to be blurred so that the specific frequency and bandwidth of a given peak/dip isn't visible.

I think everyone agrees (no doubt they'll say if they don't ) that the averaged 1/3 octave banding presentation can show skewed/smeared results but averaging results as shown in Jeff's plot (higher up the thread) does still include all the energy absorption and give a fair idea of the overall performance of an absorber. The worst thing they do is disguise really peaky behaviour, make things look smoother/more spread than they really are which makes them most useful only as a guide to performance and for rough comparisons between similarly tested products. They do however have to be understood by those reading them. (In extreme cases it's statistically possible for an plot to show a huge dip/notch in absorption with a very tight bandwidth centred right on one of the 1/3 octave bands and yet, if the absorption either side of the notch is high, the plot could show high averaged absorption for that centre frequency. e.g., in Jeff's plot there's a big dip at 4450Hz yet the 1/3 octave plot shows a rising absorption trend. The same would be true for very small bandwidth spikes in the absorption data.)It would be fair to say that if anything, they present broadband absorbers in a somewhat flattering way. This isn't a fault - the info is accurate within the constraints of the display method - it's just that the plot doesn't always have enough resolution for the job in hand.

Quote Ethan Winer:

---

Kevin,

Kevin says > at low frequencies the walls act like a kind of waveguide ... the rise up to 100Hz is simply caused by the materials absorption range. Obviously none of these materials do a briliant job way down in the 20, 30, 40 Hz range <



Ethan replies ..Give this man a fine Cuban cigar. Corners focus sound just like cupping your hand over your ear, or a parabolic dish. .....

--Ethan

---

Oh Dear!

Some understanding of what causes room modes is necessary. Not like we haven't been through this before.
***And no doubt it'll be gone through again But not everyone has spent time finding out about this and it can be hard work if one's not familiar with it - everyone has to start somewhere 0VU ***

Focussing sound by "cupping your hand over your ear" or using curved surfaces is due to reflections of TRAVELLING WAVES

Room Modes are caused by STANDING WAVES.

And just so it's clear..

Travelling waves are where the crests and troughs of the wave MOVE. As in the the ripples caused by dropping a rock into a pond.

Standing Waves are waves where the crests and troughs do NOT move.

In a Travelling Wave ENERGY is transferred from one location to another, while the medium of the wave doesn't move.

In a Standing Wave, energy is stored ( as long as the standing wave is continued to be excited. ) in various amounts in fixed locations in the room.


I linked before ( in this thread ) to this simple animation before which shows the pressure ( energy ) is at max at the boundaries.

In this animated graph made by David French at RO, the y-axis's on both sides of the graph are the walls. The x-axis can be seen as the 'floor' for all intents and purposes and denotes the distance between the walls.


You could also think of it as two parrellel brickwalls built outside.

As you can see the maximum pressure is at the boundaries.




So let's build another two opposing brick walls to produce a rectangular enclosure. Creating.....CORNERS!!!


Now at these corners we will have the maximum pressure from TWO standing waves.

Add a ceiling to the structure, and then we create another standing wave between floor and ceiling.

So in a tri-corner we have the max pressure from THREE standing waves.

And, once again, this is why treatment is best placed in tri-corners and bi-corners to absorb standing waves.


I would respectfully postulate that some background in BASIC Wave Behaviour is quite neccesary for those having an interest in acoustics, and is really beginners stuff for this subject
***Like I said - everyone starts somewhere Paul, thanks for another useful post, and I really like that diagram/animation. I agree that this is basic stuff but you have to remember that there are lots of people reading this who either *are* beginners at it or just have a passing curiosity about acoustics. This is a fairly new forum for SOS and a lot of the apparently basic questions have yet to be asked/answered. I know you've put in a lot of time learning about this but not everyone has - which is where the patient people with the answers come in If Hugh had a bottle of wine for every time he'd explained sample theory he'd either have a very large cellar or be a chronic aloholic
0VU ).



Paul

Edited by 0VU (11/12/04 05:34 PM)

Ido,

> How does this address the posts of Jeff & the Foz regarding the nature of 1/3 octave pink noise measurements? <

As was already explained, the only way to know where the peaks and nulls really occur is to analyze at a finer resolution than 1/3 octave. If you have a bunch of labs all a similar size, and in those labs you test a bunch of porous absorbers that all fall off at lower frequencies, and you test only at coarse intervals, all of the absorbers will appear to have a peak at the same frequency.

Jeff's earlier post is interesting and relevant here, and it supports what I said about the need for a truly reverberant field. Obviously whatever porous absorber Jeff tested does not really have such a screwball curve of absorption versus frequency. And that graph started at 700 Hz and went up from there! When you do tests like that at even lower frequencies, comb filtering and other "selective and directional" effects cloud things even further. By the time you get to 100 Hz, even a lab's large reverb room is riddled with these problems. This is why labs have to test many times in quick succession to get a reasonable result. Even then, if you run the same series of test five times in a row you'll get five different answers.

As I said before, for the exact same reason nobody measures low frequency response in a room using 1/3 octaves, measuring low frequency absorption at 1/3 octaves is equally misleading.

--Ethan

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

Paul,

> Some understanding of what causes room modes is necessary. <

What do modes have to do with the focusing effect of a corner, tri-corner, or concave surface? You can build a small corner section of a wall outdoors (or a big satellite dish) and get a focusing effect, even though clearly there are no modes involved.

> Room Modes are caused by STANDING WAVES <

How so? Certainly nodes (not modes) are caused by standing waves, but room modes are "caused" by whoever designed the room and established its dimensions. The word mode is short for mode of vibration and as such it merely describes a propensity to vibrate at a certain frequency if excited. This is unrelated to standing waves, which as you rightly pointed out are places in the room where no wave motion is occurring.

--Ethan

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

Quote Ethan Winer:

---

Paul,

> Some understanding of what causes room modes is necessary. <

What do modes have to do with the focusing effect of a corner, tri-corner, or concave surface? You can build a small corner section of a wall outdoors (or a big satellite dish) and get a focusing effect, even though clearly there are no modes involved.

---

Ethan

I'm well aware of the focussing effect of travelling waves. With both sound and light.

It's quite obvious to everyone what this thread is about. i.e Initially the comparative performance of various devices in dealing with standing waves in people studios. Yes?

We ARE NOT talking about the focussing of travelling waves by concave suraces, or even the focussing by corners. We are TALKING ABOUT STANDING WAVES.


As I've previously explained, Standing waves cause a build up of ENERGY in corners, in manner that is nothing to do with focussing travelling waves.

Quote:

---

> Room Modes are caused by STANDING WAVES <

Ethan replies....

Quote:

---

How so? Certainly nodes (not modes) are caused by standing waves, but room modes are "caused" by whoever designed the room and established its dimensions. The word mode is short for mode of vibration and as such it merely describes a propensity to vibrate at a certain frequency if excited.

---

---

Ok, so "Room Modes are caused by STANDING WAVES" isn't the most eloquent way of putting it.

Room Modes ARE standing waves. 'Modes' is the collective term for all the standing waves in a room. A 'mode' is simply one standing wave at one freqeuncy. Tehre are different types of modes dending on HOW MANY SURFACES are involved.

Quote:

---

This is unrelated to standing waves, which as you rightly pointed out are places in the room where no wave motion is occurring.

--Ethan

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I'm flabbergasted! Stunned Even! That last sentance of yours shows a lack of understanding of the whole situation.

"unrelated to standing waves" !!!!!??? ??!!!! you're joking with us? aren't you?

"..which as you rightly pointed out are places in the room where no wave motion is occurring." I didn't say this. Discarding the travelling waves from the speakers and reflections in a practical example of a room.....

with standing waves ALL places in the room have NO wave motion. I actually explained the difference between standing waves and travelling waves. There is no wave motion. ANYWHERE! That's why they are called Standing waves!!!!!!

***Edited to remove inappropriate comments. HR***


Paul

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

Edited by Hugh Robjohns (12/12/04 01:06 PM)

Quote:

---

Ethan Winer:
What do modes have to do with the focusing effect of a corner, tri-corner, or concave surface? You can build a small corner section of a wall outdoors (or a big satellite dish) and get a focusing effect, even though clearly there are no modes involved.

---

Sorry, I'm confused. You're describing focusing by reflection to produce co-incidence at a focal point, as in a concave surface, cupped hand, "small corner section of a wall", etc.. As I understand it, the increased modal energy in corners is due to reinforcement by co-incidence of the antinodes of multiple standing waves (axial, tangential and oblique) existent at modal frequencies rather than reflection of waves in motion producing a co-incidence at a focal point. Though I would've also thought that in practical corner absorbers, at frequencies affected by the absorber, some boundary wall reflections are absorbed by the corner absorber covering an area of wall adjacent to the corner itself. (Not that this is relevant to the standing wave thing - just a bit of extra absorption.)

Quote:

---

Paul Woodlock:> Room Modes are caused by STANDING WAVES <

Ethan Winer:
How so? Certainly nodes (not modes) are caused by standing waves, but room modes are "caused" by whoever designed the room and established its dimensions. The word mode is short for mode of vibration and as such it merely describes a propensity to vibrate at a certain frequency if excited.

---

Good point. Room modes are a function of it's design whilst nodes are a product of the standing waves which are a characteristic energy dispersion associated with resonant modes of vibration in excitation (in this case the room's modal frequencies).

Quote:

---

The word mode is short for mode of vibration and as such it merely describes a propensity to vibrate at a certain frequency if excited. This is unrelated to standing waves, which as you rightly pointed out are places in the room where no wave motion is occurring.

---

Interesting concept. The modes of vibration in a room actually are quite strongly related to standing waves! Modes of vibration effectively are the resonant frequencies of a system. Those frequencies at which a system (in this case a room full of air) will resonate most readily when excited; they require less energy to excite them into vibration, and importantly, will sustain vibration more efficently and hence longer for a given energy input, than non-modal frequencies. Standing waves exist when a room is excited at modal frequencies; they produce nodes and antinodes in different parts of the room. They're not "places in a room where no wave motion is occurring"; nodes and antinodes exist in fixed positions but standing waves exist throughout a room, not just in places. Standing waves are pressure gradients which vary only according to the position at which they're sampled, rather than varying with time at a given position, i.e they're perceived as changing as you move around but remain the same when you're static.

Greetings 0VU

Thanks for replying ( and describing it all more eloquently than I )

If only one thing is learnt from this thread, it's that at least you know what we're dealing with here.


0VU says:

Quote:

---

Room modes are a function of it's design whilst nodes are a product of the standing waves which are a characteristic energy dispersion associated with resonant modes of vibration in excitation (in this case the room's modal frequencies).

---

Yup! and to clarify.....

Nodes are the point of NO DISPLACEMENT of a standing wave. 'NODE' = NO D(E)isplacement. It's where the name ( relating to standing waves ) comes from.

Anti-Nodes are the points of maximum displacement.

And furthermore, in a real room where there are also travelling waves, the wavelength of an LF travelling wave is too long to be focussed by a corner.


Paul

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

Ethan,

You started with a supposed revelation to the acoustic world, to quote:

Quote:

---

“……The reason all labs measure a bump at 100 Hz is because that's the frequency they're measuring! As they say, if your only solution is a hammer, every problem looks like a nail. And if you test at only a few frequencies, you will find data at only those frequencies. Yes, it's as simple as that! This yet again shows the failing of standard lab tests when trying to measure absorption at very low frequencies….”

---

Now you say:

Quote:

---

“..As was already explained, the only way to know where the peaks and nulls really occur is to analyze at a finer resolution than 1/3 octave. If you have a bunch of labs all a similar size, and in those labs you test a bunch of porous absorbers that all fall off at lower frequencies, and you test only at coarse intervals, all of the absorbers will appear to have a peak at the same frequency…”

---

So, do you take back the first quote or not?
If I follow you, You initially thought 1/3 octaves measure specific frequencies (not all), thereby rendering all worldwide acoustic RT absorption data useless.
If you still stick to this, I would suggest you’re posting this revelation to the alt.sci.physics group (where the worldwide acoustic experts dwell, not to mention Eric Desart).

It does seem to me that you have realized you're misconception, and that you have presently diverted the subject to “resolution”.
So this means that you agree that any energetic peak will manifest itself in 1/3 octaves, and that there is only the issue of resolution, or, knowing specific peak frequencies?
If this is the case, this is a theoreticaly valid argument, as OVU stated, only that to me, it doesn’t have much practical impact.

Regarding Jeff’s graph, we don’t have enough details to start comparing it with lab characteristics, not yet anyways.

Ido

Paul,

***Edited to remove inappropriate coments: HR***

What may be the most amazing thing in this entire thread is that every single RealTraps detractor has never once used a RealTraps product to treat a room, yet they denounce them anyway. Contrast that to the 40-odd unsolicited praise quotes from satisfied RealTraps customers on our site - many of them respected engineers and producers with a lot more industry credibility ***Edited HR***. Every quote includes a full name and address (and web site if applicable) rather than anonymous initials. We have gotten only wildly favorable reviews from magazines, and in every single case the reviewer also bought the review traps with his/her own money. Including the two glowing reviews that will be out in a few weeks - those reviewers also liked the traps so much they bought them - this comes to eight great reviews and not one negative comment (aside from UK pricing).

We have sold thousands upon thousands of MiniTraps, and even though we offer an unconditional money-back guarantee without even charging a re-stock fee, I can count the number of returns on one hand. And all but one of those were from "audiophiles," as opposed to musicians and recording engineers. (The last return was from an audiophile who told me he can hear the difference between speaker cables but couldn't detect a change after adding four MondoTraps.)

So on the one hand we have a huge number of satisfied customers praising MiniTraps, and a handful of dissenters who never even saw a MiniTrap let alone used one. I find this amazing. And it certainly shows that claims of "no malice" is a lie.

***Edited to remove inappropriate comments: HR***

In the mean time, I have to leave now to play another Nutcracker concert this afternoon, and I'll address the remaining questions later or tomorrow as time permits.

--Ethan

***INAPPROPRIATE COMMENTS AND PERSONAL INSULTS WILL NOT BE TOLERATED BY ANYONE ON THIS FORUM. STICK TO THE TECHNICAL MATTERS PLEASE. Hugh Robjohns***

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

Edited by Hugh Robjohns (13/12/04 09:46 AM)

Quote kevinmcdonough:

---

From my limited understanding of acoustics i remember reading something that said the reason that bass trapping works so well in the corners is that, at low frequencies the walls act like a kind of waveguide and send the energy along the walls towards the corners. If my memory is correct i can kind of see how this would work as the bass frequencies have such long wavelengths and come out far more omi-directionally than the high frequencies.

---

Strange thing! I had noticed something like this on an L-shaped room by placing my head near the short wall. Your description of the guide-wall describes what I was hearing better than anything!

Quote Ethan Winer:

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It appears nothing has changed at all, and your main technical arguments still rely on name calling. Name calling so offensive and inappropriate it has to be edited repeatedly by the mods.

Since calm rational technical replies from me do not appear to influence you, I'll take a different approach. I've said often that you can tell who's full of crap and who knows what they're talking about by seeing who has to resort to name calling. Here's what we have so far in this thread from you . . . etc., etc., etc.

---

I would just like to point out that, as all of the mods here have pointed out to everyone here (both publicly and privately), as soon as things start into personal insults, accusations, etc., all productive debate stops. It only takes one from either camp to start slinging arrows, and the arrows will immediately start flying both ways. Seems certain parties on both sides of the fence here want to accuse the other of lying with an evil agenda. Whether this is true or not on either side, I suspect neither side believes this of themselves, and the accusations don't get us anywhere closer to an in-depth understanding of the issues at hand.

Please . . . try to give each other a little bit of room in the debates here, and let's try to minimise the bloodshed? It seems there is a bit of longstanding bad blood between Ethan and the StudioTips guys . . . whatever the case, it seems pretty clear that the old methods of debate are not going to clear this matter up among you, so I would request of all of you to take a different, more creative approach.

Now . . . if you all don't debate fairly and stop with the back and forth sniping, I swear I'll turn this car around and NONE of you will get to visit Santa today! And I'll call him up and make sure he knows you're not to receive any presents on Christmas (or Hanukkah) morning!



And you guys were just starting to do so well, too. Just as soon as the debate starts getting good, it starts to go all wonky again.

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

Can we have some more pretty, moving graphs please?

They're very soothing.

Quote Sean Monkey:

---

Can we have some more pretty, moving graphs please?

They're very soothing.

---

Hehe . . . seconded. Some very interesting (and helpful) graphics have been included in this thread . . . quite appreciated. Further, the animated modal pressure variation graph above indeed reminds me of a Christmas tree with flashing lights, and puts me in a bit of the holiday spirit.

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

Maybe skip the manners lessons, and cut and paste acosutics lessons and try to stick to the topic.

Paul:

Please don't get off topic, and please don't get Ethan started arguing your private theories versus his... yall doing that just changes the subject on a thread that IMO has some import.. and it turns into another thread the like of which the internet self-help acoustics forums already have far too many. Next thing you know this thread will turn into a discussion of 1/4 wave nulls from all boundaries, 1 Hz pink noise, or some new "creative" acoustic priciple will get made up out of thin air to explain some part of the basic physics of acosutics that don't need explaining.

Please don't take this request as an insult.. I sincerely appreciate the fact you and Ethan want to grow in your understanding of these priciples, but maybe yall could do that in private or at least start a different thread - maybe over at an acoustic science forum - then yall could count on getting herded back to the known art by an audience schooled in the core science of modelling waves.

My $0.02



Ethan:

You seem to be trying to discredit the StudioTips "4 Devices" RAL data by changing the subject... again. Contrary to your.. uh... claim. The data label for the 100 Hz band does NOT cause the peak in this region... it merely provides a name for it.

Also, acoustic labs use RTA [real time analyser] to measure absorbtion and it works... you yourself pay for these tests, and sometimes you publish elements of the results. If you feel such tests are not valid, why do you buy em?

Suggesting that ETF and waterfall graphs are needed for this sort of testing is WRONG. These test were not designed to identify particular frequencies... such would be meaningless for the purposes of comparing the 4 low frequency devices as absorbers. And integrating the data per frequency would make the level of accuracey currently extant in the results IMPOSSIBLE.

But just for fun.. pretend that perfectly accurate intrinsic aborption for the devices had been determined by the tests... pretend for a moment that the SuperChunk peaks out in absorption at exacly 88.7 Hz, and the MinTrap at 97.3 Hz.




Now the question..

















Wait for it...
























So what?








What has that got do with anything.

As explained in some detail earlier in the thread [for lay purposes anway] all frequencies are measured in the test... and integrated. The bumps in the graph are real and talking about what software and graphs might in your opinion best be used to redesign the methods of the world's acosutic testing labs so they might identify the precise frequency of highest absorption in the MiniTrap is not only off topic, it makes no sense.

After all.. who cares? Is someone trying to build a room with this resoance? Are folks currenly buying the devices in order to treat a discrete frequecy - if so which one?

FWIW... if you want to know the precise peak of a MiniTrap in a particular room I would think a swept sine wave would do the job just fine. OTOH knowing the MiniTrap's intinsic peak in a reverb chamber may well not translate to a user's room - therefore I can't see how gathering such the information serves any purpose.

What would you propose a user do with this information?

PS: What is the volume of the IBM lab where you have measured MiniTraps?

--------------------
http://readyacoustics.com
http://forum.studiotips.com

Quote Foz:

---

Maybe skip the manners lessons, and cut and paste acosutics lessons and try to stick to the topic.

---

Lighten up for gods sake man!

I have been lurking around this debate for days and have declined to enter the fray. It's educational (I'm sure the only reason the mods have kept it alive) and I know more than I ever wanted or needed to know about acoustic theory/testing/labs etc.

I have had studios purpose built for me with the assistance of professional acoustic consultants. My only interest is do I end up with an environment I am happy to make music in. I don't think as a studio owner/engineer/musician I really need to know most of this. I turn to experts when I need them, just as a sesnible person uses a lawyer to buy a house.

I think the real trap (pun intended) is the assumption, by most proponents of the various arguments, that if we humble musos could only understand all this stuff and "see the light" we'd all be better at what we do.

Er, frankly, it's boring. Writing songs is what I do.

However, setting myself up as the "Simon Cowell patsy" here, I would far sooner buy Ethan's products rather than Paul's because, (a) as Max stated in post #1, they work, and (b) however right he is, Paul's undoubted evangelism combines with a distinct lack of subtlety/skill in the debating arts, which leaves me feeling like some ignorant schoolboy being told to go and stand in the corner. And that is a real turn-off.

Oh yes, big thanks to Jeff for his generous contributions from Auralex too.

--------------------
Dynamite with a laser beam...

Guys,

That was a really disappointing little spat. It was unhelpful and unconstructive, and merely serves to make you both look rather silly and pathetic. The science is interesting and worthy of continued discussion, but not at this kind of cost.

The other moderators and I are doing our best to keep on top of this thread, but we all have better things to do with our time, and we need to sleep occasionally.

If we can't trust people not to hurl personal abuse and insults around then there are only three options. Either we lock the thread again and kill it completely, or we instigate a moderation queue so that everyone's posts have to be read and approved before they appear... which would probably kill the thread again anyway, or we start banning people! I don't think any of want to have to go down any of these routes, but it's certainly heading that way.

Last warning. Play nice. It's Christmas. Goodwill to all men and all that!

Hugh

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

Sorry if I'm stepping on toes again... all I ever wanted to do was discuss the science behind the RAL data.

Merry Christmas

--------------------
http://readyacoustics.com
http://forum.studiotips.com

I was thinking mainly of the personal comments made by Paul and the follow up by Ethan, actually Foz.

But if everyone could manage their frustrations a little better and stick to clear, concise technical opinions, we'd all benefit a lot more from what is, after all, an interesting and thought-provoking thread.

hugh

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

I couldn't agree more Hugh.

Have a glass-a-nog on me!

--------------------
http://readyacoustics.com
http://forum.studiotips.com

Before you guys get too carried away with my graph, it was a single-bounce absorption test, not a chamber test. It used FFT, and did not measure decay directly. I only posted it as a visual example of the variations in absorption that are possible over a range of frequencies.

That's all it is: It's not "proof" of anything except what is possibly present in some measurements. If you were to filter the pink noise decay in the chamber method to a narrow enough band, you would likely see similar behavior.

And one note to Ethan: ETF is not "more sophisticated" than the lab methods for measuring decay. I thought I commented on that already, but perhaps not. Dual-FFT, TDS, MLS and other such techniques employ mathematical "short-cuts" to derive their results. Directly measuring the decay, such as is done in the C423 method, can be (and is) much more precise. And you can filter to whatever bandwidth you'd like. All you have to remember is (as Eric reminded me recently) that you will affect the time domain when you make changes to the frequency domain. (This actually applies to both the precision methods employed by labs, as well as by the less "sophisticated" - though that is a poor choice of words, being so subjective - methods like ETF.)

Best regards,

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

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

Jeff,

> it was a single-bounce absorption test, not a chamber test <

Thanks for clarifying.

> ETF is not "more sophisticated" than the lab methods for measuring decay. I thought I commented on that already <

Yes, I saw that comment, but it didn't seem to address what I had brought up. Yes, I understand that pink noise can be analyzed to very narrow bandwidths. But do labs actually have the tools to do that? ETF can analyze to about 3/4 Hz resolution. Can any/most labs do that with the tools they have?

--Ethan

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The acoustic treatment experts

Scott,

> The data label for the 100 Hz band does NOT cause the peak in this region ... What has that got do with anything. <

It has everything to do with the subject at hand. If the question is "Why do corner mounted absorbers have a peak at 100 Hz?" the first thing to do is find out if the peak is really at 100 Hz or somewhere else. Then, once you have the ability to measure to a fine enough resolution you can see if the peak is at exactly the same frequency in all rooms or if it changes frequency based on the size of the room. Such an investigation is impossible with 1/3 octave resolution! Which is why I brought up ETF in the first place.

Now can you see the relevance? If not, let me know and I'll try to think of another way to explain it.

--Ethan

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The acoustic treatment experts

Quote:

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Quote:

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The data label for the 100 Hz band does NOT cause the peak in this region ... What has that got do with anything.

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It has everything to do with the subject at hand. If the question is "Why do corner mounted absorbers have a peak at 100 Hz?" the first thing to do is find out if the peak is really at 100 Hz or somewhere else. Then, once you have the ability to measure to a fine enough resolution you can see if the peak is at exactly the same frequency in all rooms or if it changes frequency based on the size of the room. Such an investigation is impossible with 1/3 octave resolution! Which is why I brought up ETF in the first place.

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I think it may be best to clarify what ethan means by this (and another point, below) to perhaps save further arguments. What he is saying is that the peaks on the graph may not all be exactly 100Hz, but that one may be 95Hz and 105Hz or whatever but they are all 'rounded off' to 100Hz beause that is the nearest point that is measued on the graph. (In the same way that the numbers 19.7, 19.9, 20.4 and 20.1 would all be shown as 20 if they were 'rounded off' to the nearest whole number.)

I would argue however, at least in my opinion, that this point is a little pedantic. There clearly is a marked peak in all the tests at "around 100Hz" if thats a better way to put it, and perhaps it'd be more benificial for everyone could all discuss why and not argue about the exact frequency, as i know that at least for myself i am genuinly curious and interested to find the reason for this.


Also while we're on the subject of clarifying things, Ethan has also mentioned a few times the point that the graphs have been made to start at 1 instead of zero, which he claims was to make his product apear more peaked than before. To expand on this (and feel free to correct me if i have mis-understood) he is saying that by starting the graph at 1, but still having the graph take up the same physical space, you are stretching it vertically and therefore exagerating the peaks in it so that, at a casual glance the plot seams more peaked than it otherwise would be.

Again however, for my money at least, this is again making a big point out of a small issue. For example, if the graphs were ranging from 1 to 100 say, and all the information plotted fell in between 90 and 95, then you would have a relativly flat line. By then running this graph from 80 to 100 (in effect, zooming in the vertical scale) the same line could then become very peaky looking. If the case was this extreme i could understand your point but by simply chopping the zero to 1 section of a graph that only goes to a little above 7 wouldn't make all that much overall difference.

And anyway, ALL the measurement lines on the graph were subject to the same 'vertical zooming' so they ALL lok a little more peaked, by the same amount. Yours wasn't singled out, which although you didn't actually say your manner would suggest.


Please understand here Ethan that i am not trying to attack you or suggest your wrong. On other forums where i have seen you post you have always been quick to offer advice and be generaly helpfull when people have asked, as have the StudioTips guys on their forum which i have reciently joined and learned a lot from.

Neither have i used any Realtaps products so have no direct route for comparison and am not puting them down in any way.

Rather i just feel that if we could all move away from arguing about what i woud suggest are minor or unnessesary points and have a more constructive discussion (which everyone seems to want to) we could all try and learn something.


Indeed, looking at the graph Ethan has on his website he shows a Mini-trap and a Mondo-Trap both in a corner and against a wall. Even on his own graph it seems to suggest that the mini trap has a 'peaky' shape at around 100Hz compaired to the same measurement flat against the wall (although with a smaller difference between the peak and theflatter part that the StudioTips graph), and the mondo trap also seems to show an increase in bass when corner mounted, but this time it has two peaks a smaller one at 80 Hz and a larger one at 125Hz when compared with its flat agains the wall version. Unless the lab at which this was tested in has the same or very similar dmensions to the lab the studiotips one was done in, then i'd suggest that this phenominum was very likley to be a result of the corner mounting and would happen regardless to a certian extent of the room it was used in.


This, combined with the StudioTips graph, would suggest to me at least that the peak is more to do with the corner mounting than the actual device in question.

kev

Quote kevinmcdonough:

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....Rather i just feel that if we could all move away from arguing about what i woud suggest are minor or unnessesary points and have a more constructive discussion (which everyone seems to want to) we could all try and learn something.


Indeed, looking at the graph Ethan has on his website he shows a Mini-trap and a Mondo-Trap both in a corner and against a wall. Even on his own graph it seems to suggest that the mini trap has a 'peaky' shape at around 100Hz compaired to the same measurement flat against the wall (although with a smaller difference between the peak and theflatter part that the StudioTips graph), and the mondo trap also seems to show an increase in bass when corner mounted, but this time it has two peaks a smaller one at 80 Hz and a larger one at 125Hz when compared with its flat agains the wall version. Unless the lab at which this was tested in has the same or very similar dmensions to the lab the studiotips one was done in, then i'd suggest that this phenominum was very likley to be a result of the corner mounting and would happen regardless to a certian extent of the room it was used in.


This, combined with the StudioTips graph, would suggest to me at least that the peak is more to do with the corner mounting than the actual device in question.

kev

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Greetings Kevin

I agree the exact frequency is a bit pointless in this debate. Why and how this peak is happening is more to the point.

Although it would be nice to see some test data from corner absorption devices with larger or smaller front face widths

You see I have this conspiracy theory about the ca. 100 Hz peak.

Using the speed of sound in air as 340m/s, a 1/4 wavelength of 100Hz is 850mm or 33.4".

Looking on the Diagram below you'll the front face width of a MegaLENRD is.... 33.9" hmmmm?



So maybe the front face of the absorber has something to do with this peak? Maybe it's related to the change in medium at the front face between air and air+absorption. And this causes a 'resonance' in the absorption performance, the freqeuncy of which is determined by the dimension(s) of the front face.

I suppose the first thing to do to try and suuport that wild theory is to test a corner absorber which has a 16.7" wide front face and see if it has a peak at around 200Hz.

I dunno, just some crazy thoughts


Paul

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

Kevin,

> I would argue however, at least in my opinion, that this point is a little pedantic <

Not at all. If the goal is to understand why a peak occurs at all, the first step is to identify the frequency and see if it changes in different situations. Otherwise, with 1/3 octave averaging all you have is guesses.

> which he claims was to make his product apear more peaked than before <

Not so much more "peaked" but to force the absorption at higher frequencies farther down the scale and therefore to appear inferior in comparison. But I agree with you that it's time to move forward and stop arguing about motive and get back to the science of acoustics.

> if the graphs were ranging from 1 to 100 say, and all the information plotted fell in between 90 and 95 <

I would agree if all the data were in the top ten percent or whatever, but that's not the case here where the lowest data point is under 2!

> This, combined with the StudioTips graph, would suggest to me at least that the peak is more to do with the corner mounting than the actual device in question. <

Yes, as I have explained repeatedly.

--Ethan

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The acoustic treatment experts

Thank you for your replies Ethan.


Also to Paul, very interesting indeed, I was actually thinking along the same lines.

Looking at Ethans graphs, like I said above the mini trap seems to peak at 100Hz, but the Mondo trap at 80Hz and 125Hz.

As the mondo trap is 9" longer than a mini trap i was also thinking along the lines that surface area, or face width like you suggest, could have something to do with it.

My girlfriend came in late from a party last night and kept me up with argumens and complaints of illness untill nearly 7 this morning so i'm far too tired for Maths just now, but its certianly something i'm gonna look more into.

kev

Quote:

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There clearly is a marked peak in all the tests at "around 100Hz" if thats a better way to put it, and perhaps it'd be more benificial for everyone could all discuss why and not argue about the exact frequency, as i know that at least for myself i am genuinly curious and interested to find the reason for this.

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Kevin:

I would suggest that it would be better to say:

There is a peak in the 100 Hz band for all 4 devices.

Stated this way you make it clear that we are talking about a range of frequencies... all of which are measured - not a particular frequency.

As to device/room dimension being the defining cause of the peak in the 100 Hz band – IMO this is doubtful. Because dimension is a defining cause of many types of acoustic phenomena it is attractive/logical IMO to surmise that that the 100 Hz peak phenomena is also driven by dimension [pipe resonances are so why not corner absorption peaks] – and I myself had initially done so - but this supposition suffers when scrutinized.

Though dimension could be reasoned to have some effect - I have come to believe that this attribute is not defining. Look at Eric’s "playing with baffles" document and you see that even when laid flat on the ground absorbent materials located at a room's boundaries encounter this peak.

The commonality of the existence of this peak across samples of materials, and across rooms of differing dimension, and across different specific mountings, suggests that the defining factor transcends mere dimension of the device [or the room].

There is something intrinsic happening. As Eric once put it in a StudioTips thread dim in my memory, its as though there is a “crossover frequency” one encounters in absorption curves when devices are installed at the intersection of room boundaries. A point on the band where an additional physical phenomena begins to occur when absorbers are located along room boundaries. Testing is required – but it seems clear to me that measuring the room volume and device face area aint gonna be enough to explain it.

Ethan:

You are wrong to think that ETF will measure absorption properties in finer granularity than RTA tests done in a real lab. Just because you can plot the amplitude of discreet frequencies as measured from discreet locations using the product does not mean you can measure a device’s intrinsic absorptive properties to the same resolution. It’s a statistics thing… and you are lost in the idea that because you can measure a sine wave in your room [or any room], and plot the results with a $100 piece of software … that this means you and your laptop are a mobile acoustics lab that can measure anything – at any location - to single Hz resolution – and then report valid and reproducible results.

NO!

As explained at length previously, you are measuring the device and the room, and how they interact. And also as previously explained, taking ETF into a real lab [a known room] does nothing to reduce the statistical hurdles that must be cleared in order to gather reliable data on material absorption properties.

ETF integrates bands of input frequencies just like any other collection / integration scheme, and plots em it suffers all the limitations of math and physics that the RTA’s used in real labs encounter... and more, because the integration process reduces accuracy [example: if you apply reverse integration to ehance resolution, you lose accuracey). Whether you are applying this process to data collected by ETA or any other measurement system is immaterial. Reverse integration, etcetera is going to REDUCE accuracy.

Applying artful integration / manipulation to the input data to make a waterfall plot, or a jiggly line plotted over an X axis with ticks for every frequency per single Hz does nothing to increase the statistical significance [accuracy] of the measurement.

To "zoom in" on the peak would require a statistically significant sample to be taken at each discreet frequency one desired to measure, and this would need to take place in an acoustics lab's reverb chamber. This process would be expensive, but the RTA's used at labs could do this even if ETF didn't exist. Once calibrated, I see no reason that one couldn't use ETF for this job, but it would do nothing to reduce the enormity of the task of taking several dozen "shots" at every frequency across the desired band. Moreover, I am not aware of any reaon to think this particular piece of software would be desired as opposed to using the lab grade systems that are in place and already calibrated and validated by the lab's previous work.

ETF is a neat product, and will help the DIY’er do all kinds of measurements in a particular room… but it can’t defy the laws of physics, or make statistical uncertainty disappear. You mislead yourself [and others] by pretending that because you have a made a graph with greater resolution… you have measured with greater resolution.

Please stop spreading this misinformation.

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http://readyacoustics.com
http://forum.studiotips.com

Kevin,

> but the Mondo trap at 80Hz and 125Hz <

I would take that with a grain of salt. If we measured the MondoTraps a second time the result might well have had the same shape as a MiniTrap only showing more absorption. This is the point I've made here repeatedly. These tests give different results every time you run them! I have even seen labs report negative values for absorption. Think about that. One problem is that the measuring microphone in a lab is constantly in motion in all three planes, so some times it picks up one "pattern" and other times another pattern. The lower in frequency you measure, the larger the potential deviation between tests.

> i was also thinking along the lines that surface area, or face width like you suggest, could have something to do with it. <

I doubt it, but the only way to know for sure is to measure different size samples and in different size labs. The data also needs to be resolved to much finer than 1/3 octave so if a change in sample or lab size really is a factor, you'll be better able to spot that.

--Ethan

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The acoustic treatment experts

I don't want to sound patronising, but these are great posts guys. Nice to see the science being discussed sensibly without all that baggage. Keep it up (please).

hugh

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Technical Editor, Sound On Sound

Ethan,

Just one simple question.
The MondoTrap curve suggests that possibly there are 2 membranes at a distance at work.
Does the Mondo have a board inside with an own membrane?

So, not to misunderstand: are there two membranes seperated from one another with a certain thickness of glassfiber or whatever comparable in-between?

It doesn't matter that this absorption is a seperate board or not.


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

Scott,

> You are wrong to think that ETF will measure absorption properties in finer granularity than RTA tests done in a real lab <

If labs have the ability to measure to a finer resolution than they could give results as accurate as ETF. I don't know if the IBM lab we use can resolve to better than 1/3 octave, but as I recall it cannot. More to the point, having the microphone constantly moving might help in the frequency ranges that labs are certified to test, but I'm pretty sure it harms accuracy at lower frequencies.

> you are lost in the idea that because you can measure a sine wave in your room [or any room], and plot the results with a $100 piece of software <

What does the cost of the program have to do with anything?

> [ETF] does nothing to increase the statistical significance ... You mislead yourself [and others] ...
Please stop spreading this misinformation. <

You fail to understand the difference between ETF decay tests done with a microphone in a single position versus the moving microphone tests labs use. Apparently you've never been in an acoustics lab and seen how these tests are run, so I'll explain it to you. When a lab tests absorbers they run the same test multiple times. IBM's lab runs each test 100 times in succession. While the tests are running the measuring microphone is in constant motion in three planes. The averaging is therefore to combine the results from all those different locations. The labs also require a reverberant field, which does not exist in those rooms at very low frequencies. Since ETF measures the decaying energy directly, and to a very fine frequency resolution, this "$150 program" does a great job of showing the effects of adding absorption even in small rooms and even at very low frequencies.

To be perfectly clear, I absolutely agree that reverb room tests using pink noise could give similar results with the appropriate software and appropriate techniques.

==========

I just got off the phone with Bob Boyes, a senior engineer at IBM's acoustics lab we use and he had a lot to offer on all of this. Some of what Bob told me is off-topic from what we're now discussing, but I'll include it because it's interesting.

Bob told me they wrote the software they use. The actual reverb room measurements come from a hardware analyzer that has 27 bands of 1/3 octave hardware analog filters with RMS amplitude averaging. (Not to be confused with "averaging" multiple tests.) Their custom software reads the hardware analyzer's output for each band, and computes absorption based on the difference between when the room was empty and after the test samples were placed. Bob said their software can display a waterfall plot, but only with all of the frequencies within each 1/3 octave band averaged together.

Bob also confirmed my assumption that most US labs are not only similar in volume but also have similar dimensions.

Bob was not aware of ETF but he told me what matters more than anything else in acoustics testing is repeatability. He said if ETF gives the same reading every time you run it then it can be counted on. Maybe not for absolute Sabins, but for comparative tests made in even a very small room at very low frequencies. This is key, and is the defining difference between a standard lab test and ETF. Labs give different results at low frequencies every time you run a test. ETF gives the same result whether you run the test once or two dozen times.

I also asked Bob about the effects of temperature and humidity on decay tests, and also the importance of sample size. As I suspected, temperature and humidity affect readings at mid and high frequencies much more than at low frequencies. Bob also said that having a sufficient sample size is far more important at low frequencies than at higher frequencies. He could not tell me precisely how small a sample can be tested reliably below 100 Hz, but he is certain the requirements for a large sample are more important below 100 Hz than above.

Finally, Bob told me IBM recently participated in a "round robin" test with three other labs, where they all tested the same physical absorber samples FedEx'd among them. He said another lab sponsored the test so he doesn't know exactly how much deviation there was between labs, but he said the deviation was "substantial." The sponsoring lab called him several times to ask about placement and other test issues, because the deviation was so much larger than they had expected.

I also wanted to speak with Dr. Matt Nobile, IBM's chief acoustician, but he was at lunch. If anyone here wants more information or detail on any of this let me know and I'll call Dr. Matt and ask him to clarify.

--Ethan

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The acoustic treatment experts

Eric,

> are there two membranes seperated from one another with a certain thickness of glassfiber or whatever comparable in-between? <

No.

--Ethan

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The acoustic treatment experts

Quote Ethan Winer:

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... Some of what Bob told me is off-topic from what we're now discussing, but I'll include it because it's interesting ...

... I also asked Bob about the effects of temperature and humidity on decay tests, and also the importance of sample size. ...

... Bob also said that having a sufficient sample size is far more important at low frequencies than at higher frequencies. He could not tell me precisely how small a sample can be tested reliably below 100 Hz, but he is certain the requirements for a large sample are more important below 100 Hz than above ...

---

Ethan thanks for the answer on the 2 membranes.

You referred here to the off-topic part. What about the on-topic part??
I'm sure you asked confirmation about the 64 sft you claim to be the minimum for valid testing (or whatever there about)....
What did he tell about the minimum he knew about?....

If he spoke about sample size, did he refer to the size of the individual objects or the total sample size? Did he distinguish between both?

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

Eric,

> What did he tell about the minimum he knew about? <

All Bob had to say is exactly what I wrote. In fact, when I asked him about small sample sizes he said they'd normally do that in their impedance tube which can reliably measure a hand towel.

In order to determine the minimum size needed to obtain reliable data in a reverb room at a given low frequency requires testing a range of different sizes and assessing the results. I was convinced of the need for sufficient sample size two years ago when I saw negative numbers at very low frequencies in a test of only two panels.

--Ethan

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The acoustic treatment experts

Ethan,

Quote Ethan Winer:

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Yes, I understand that pink noise can be analyzed to very narrow bandwidths. But do labs actually have the tools to do that? ETF can analyze to about 3/4 Hz resolution. Can any/most labs do that with the tools they have?

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No offense, but perhaps finding these things out would be a prerequisite to commenting on laboratory testing methods?

AFAIK, labs can do just about whatever is requested of them...for a price.

Best regards,

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

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All the best,
Savant

Edited by Savant (14/12/04 08:18 PM)