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Standing waves get their own thread - lucky little blighters!

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Standing waves get their own thread - lucky little blighters!

Postby Anonymous » Thu Dec 16, 2004 2:18 am

Rather than divert the DIY treatment thread with yet another theoretical discussion, I've started this and moved the relevant bits from that thread over here.
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Re: Standing waves get their own thread - lucky little blighters!

Postby Anonymous » Thu Dec 16, 2004 2:20 am

This thread picks up from a bit of a digression on the DIY Acoustic Treatment thread. I've moved to here the bits in that thread which were taking it off topic, as follows:

Ethan Winer:

Paul,

> I hesitate to use the word 'bass trapping' as standing waves can affect lo-mid performance as well. <

In fact, you can and will get standing waves at higher frequences too. Play a 1 KHz or 2 KHz sine wave through your loudspeakers then walk from the front of your room toward the back. You'll hear an obvious tremolo warble as your ears quickly pass through all the peaks and nulls. You don't have to use sine waves either. The exact same effect occurs with sustained flute and clarinet notes.

--Ethan


Paul Woodlock:

Ethan...


Well as standing waves occour at each harmonic above the fundermental, I wouldn't disagree that standing waves occour at much higher freqeuncies. They are definitely there according to theory.

However I would contend that the peaks and dips you are hearing at those freqeuncies are FAR MORE LIKELY to stem from the path length differences of reflections v. direct sound, path length differences of reflections v. other reflections, etc, etc rather than standing waves.

IMO and experience, standing waves from the lowest fundermental freq determined by the dimensions up to around 500Hz are the one's where treatment is necessary in general. Individual rooms of course will have varying requirements.

This is why I think it's good to choose an absorption device for trapping that has good specs not only in the 'bass', but in the lo-mid as well.


Paul


Ethan Winer:
Paul,

> They are definitely there according to theory. <

They are definitely there in practice too! If you take the time to play some 1 and 2 KHz (or even higher) tones as I described you'll immediately see I am correct.

> I would contend that the peaks and dips you are hearing at those freqeuncies are FAR MORE LIKELY to stem from the path length differences of reflections v. direct sound, path length differences of reflections v. other reflections, etc, etc rather than standing waves. <

Absolutely not. They are entirely due to and caused by comb filtering off the various room surfaces. You remember those pesky 1/4 wavelength peaks and nulls, right? In fact, this is further proof that standing waves have nothing at all to do with room modes. Go ahead and try it and I'm sure you'll notice the tremolo effect I described earlier. (Unless you happen to have absorption on the rear wall that the speakers are facing.)

--Ethan


OVU:
Quote:
>this is further proof that standing waves have nothing at all to do with room modes.<


Sorry, you really have lost me now.


Ethan Winer:
0,

> Sorry, you really have lost me now. <

Okay, let's try it this way: If you define standing wave while avoiding the word mode - just describe what it is using plain English - then I'll know better how to explain my point in a way you can understand.

--Ethan


0VU:
E,

Quote:
Okay, let's try it this way: If you define standing wave while avoiding the word mode - just describe what it is using plain English - then I'll know better how to explain my point in a way you can understand.



How about we just try whatever explanation you feel covers it. And don't worry too much about plain English or me understanding it - I'll try to cope.


Ethan Winer:
0,

> How about we just try whatever explanation you feel covers it. <

No, that won't work. I need to know what you think a standing wave is first. Sorry, and I swear I'm not trying to be patronizing or condescending.

--Ethan


Paul Woodlock:

Quote Ethan Winer:
Paul,

> They are definitely there according to theory. <

They are definitely there in practice too! If you take the time to play some 1 and 2 KHz (or even higher) tones as I described you'll immediately see I am correct.

> I would contend that the peaks and dips you are hearing at those freqeuncies are FAR MORE LIKELY to stem from the path length differences of reflections v. direct sound, path length differences of reflections v. other reflections, etc, etc rather than standing waves. <

Absolutely not. They are entirely due to and caused by comb filtering off the various room surfaces. You remember those pesky 1/4 wavelength peaks and nulls, right? In fact, this is further proof that standing waves have nothing at all to do with room modes. Go ahead and try it and I'm sure you'll notice the tremolo effect I described earlier. (Unless you happen to have absorption on the rear wall that the speakers are facing.)

--Ethan



To highlight a speciic part of your post....

Quote:
> Paul says: I would contend that the peaks and dips you are hearing at those freqeuncies are FAR MORE LIKELY to stem from the path length differences of reflections v. direct sound, path length differences of reflections v. other reflections, etc, etc rather than standing waves. <

Ethan Replies: Absolutely not. They are entirely due to and caused by comb filtering off the various room surfaces. You remember those pesky 1/4 wavelength peaks and nulls, right?



The 'comb filtered' frequency response you describe at the higher frequencies talked about in this thread, is MAINLY caused by reflections at these frequencies. So of course placing absorption on the reflective surfaces will diminish the 'tremolo effect' you describe.

I'm well aware of this tremelo effect btw. you've only got to move your head slightly while listening to music to hear HF fluctuate in volume at various freqs.

You say this is 'further proof' that 'standing waves' are nothing to with room modes. Firstly I have never seen ANY proof of this. So could you kindly explain the initial proof and clarify the 'further proof' above?


The problem is Ethan, is that 0VU, myself and the rest of the world don't agree with your postulation. It's not impossible that you have come up with something entirely new in the science of acoustics.

I would suggest, bearing that in mind, you would make your theories a LOT EASIER to understand if YOU explained to US in DETAIL, jutst exactly what you think a Standing Wave is, and EXACTLY WHY room modes have nothing to do with standing waves.

Myself and 0VU's definition of a standing wave can be found on probably every Website that deals with tutoring basic physics, so there's no point in repeating it here. Google is quicker for everyone.

I have an open mind, and I'm willing to listen to anyone's theory. But in order to listen, you must say something.


Paul


0VU:

E,

Sorry, momentarily distracted there.

Quote:
> No, that won't work. I need to know what you think a standing wave is first. Sorry, and I swear I'm not trying to be patronizing or condescending.<



Rather than risk this getting lost in a discussion of the detail of various definitions of standing waves, I'd prefer you just explained how "standing waves have nothing at all to do with room modes". Perhaps the quickest way to go is if you simply state the definition you prefer and then explain your theory from there.


Incidentally, I have a feeling that we may be in danger of dragging this thread away from it's present real-world, practical basis into another theoretical discussion. I don't want that to happen so if that's likely, it'd be better if this were taken to another thread so as to keep the present one on topic.

0VU



OK - that was where we got in the other thread. Discuss. ;)
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Re: Standing waves get their own thread - lucky little blighters!

Postby thefruitfarmer » Thu Dec 16, 2004 2:33 am

Thanks OVU....

I still don't understand how a "standing wave" can be heard.

:?

Maybe someone can explain how the eardrum is vibrated if the areas of low and high pressure do not actually move in the room. Sound is, after all, a pressure wave of alternating bands of low and high pressure that we perceive from the vibration of the eardrum.

Or is a standing wave a situation where the energy combines to cancel out itself?

I did a search on google and this looks like a good explanation.
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Re: Standing waves get their own thread - lucky little blighters!

Postby Paul Woodlock » Thu Dec 16, 2004 3:49 am

Greetings


think of it this way....


Pluck a guitar string..... Although the string is vibrating, the wave in the string itself doesn't move ( where can it go? ) and is therefore a Standing Wave.

While it still vibrates, lightly touch the string just over the 12th fret. You'll damp the fundermental freq. , leaving the 2nd harmonic and even harmonics above. The 12th fret position, is now a point of no displacement ( node ) for the wave of the 2nd harmonic ( and above ) of the gtr string. That's a Standing Wave.


Now get a Whip and crack it ( don't forget the thigh boots :lol: :lol: ). You'll see the wave travel along the whip to the end. Ouch! That's a Travelling Wave.

In both cases particles in the string and whip do not move location, but are displaced about a fixed point.

Right! I need a coffee. Back later :)

....

Back..

Further to get back to ethan's argument.....


I think Ethan is getting standing waves confused with the fixed areas of cancellation and addition that occour when 2 or more travelling sound waves collide as a result of different path distances due to surfaces in the room.

Play a sine wave a certain distance from a reflective surface and sure you'll get a cancellation at a 1/4wavelength from the wall. But the sound waves causing this are not standing waves, they're 'travelling' to the wall and back.

When you excite the air in a room ( i.e by a speaker ), sound at freqencies which correspond to the resonant freqeuncy of the airspring between two or more surfaces in the room act as standing waves rather than travelling waves.

Now for reasons I don't actually know, the maximum vibration ( anti-node )of these standing waves happens at the boundary of each surface. While the point of minimum or no vibration ( node ) occours halfway between the surfaces ( for the fundermental frequnecy at least ). this is opposite to the stnading wave in the guiter string, as the boundaries ( nut and bridge ) are always the point of no vibration. ( anyone care to exaplin this difference? .... pretty please :) )

This explains why you can hear a lot of bass if you get real close to the wall, and why the very centre of the room usually has the least amount of bass becuase you're at the anti-node of all 3 fundermental axial(*) nodes

Corners are the junction of two or more surfaces, and two or more standing wave's anti-nodes, so the vibrational energy is greatest in a corner. Tri-corners where 3 surfaces meet are even better per se.

Ethan claims standing waves are nothing to do with standing waves. Which he could be forgiven for, if he's also confused between travelling waves causing cancellations v. and standing waves caused by MSM ( wall-air-wall ) resonances.

Room modes, As I see it, is the term for the collection of various resonant wall-air-wall frequencies in the room itself. I know of 3 different types of room modes.

(*)1. Axial modes are the standing wave freqeuncies caused by air being excited between two surfaces

i.e front to back, left to right and up and down

2. Tangential Modes are the standing wave freqeuncies caused by air being excited between 4 surfaces.

i.e 4 walls, 2 walls and floor and ceiling, etc

3. Oblique Modes are the same but for 6 surfaces.

Tangential Modes, I have repeatedly heard, can have more than a little bearing on the acosutic performance of the room, and it's been advised to use a room mode calculator that shows ALL mode types.


Anyway, that is how I see things. I beg any corrections as I'm here to learn more about this stuff :)



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Re: Standing waves get their own thread - lucky little blighters!

Postby cc. » Thu Dec 16, 2004 9:56 am


Here's the way I look at it: all the problems you get are caused by refections - you hear one sound straight from the speakers and the same sound delayed bounced off a wall (or 2 or 3 or more) and you get the potential for cancellations.

The 'standing wave' is just a special case of this where the dimensions of the room mean that the 2nd and subsequent refelections reinforce the 1st refection giving the potential for a greater null or peak.

Simple eh?

(too simple :? :headbang:)
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Re: Standing waves get their own thread - lucky little blighters!

Postby thefruitfarmer » Thu Dec 16, 2004 11:10 am

This site provides an explanation that makes sense to me.

"In conclusion, standing wave patterns are produced as the result of the repeated interference of two waves of identical frequency while moving in opposite directions along the same medium. All standing wave patterns consist of nodes and anti-nodes. The nodes are points of no displacement caused by the destructive interference of the two waves. The anti-nodes result from the constructive interference of the two waves and thus undergo maximum dispacement from the rest position."

Not quite as amusing as Paul's Madam Whiplash example... :)
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Re: Standing waves get their own thread - lucky little blighters!

Postby cc. » Thu Dec 16, 2004 12:47 pm


Yeah, that's exactly right fruity - a standing wave is caused by two travelling waves going in opposite directions.
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Re: Standing waves get their own thread - lucky little blighters!

Postby cc. » Thu Dec 16, 2004 1:04 pm

Paul Woodlock wrote:
Now for reasons I don't actually know, the maximum vibration ( anti-node )of these standing waves happens at the boundary of each surface. While the point of minimum or no vibration ( node ) occours halfway between the surfaces ( for the fundermental frequnecy at least ). this is opposite to the stnading wave in the guiter string, as the boundaries ( nut and bridge ) are always the point of no vibration. ( anyone care to exaplin this difference? .... pretty please :) )


There isn't really a difference, but you've got a bit confused!

Starting with the guitar string - the classic way to think about this is a load of balls (!) attached by springs. If you pluck the string the balls in the middle move up and down the most. The balls at the end move the least - obviously because they are fixed! But there's something else going on too : the springs. If you look at how the tension in the springs varies you'll see that in the middle it varies the least because two balls in the middle are at the top of the 'sine shape' where it's almost flat, so they move very little relative to each other, and hardly stretch the spring at all.

On the other hand if you look at the springs right at the end you'll see that the tension varies the most because one ball is fixed and the next moves up and down in the steep part of the 'sine shape'.

So, going back to air - the movement of the air right next to the wall is zero just like the guitar string. But the pressure variation right next to the wall is maximum which is why you can hear the bass there (your ears are measuring the pressure variation, not the air movement).
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Re: Standing waves get their own thread - lucky little blighters!

Postby Tequila Slammer » Thu Dec 16, 2004 3:16 pm

Ah, the joys of comparing transverse and longitudinal waves!
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Re: Standing waves get their own thread - lucky little blighters!

Postby Paul Woodlock » Thu Dec 16, 2004 3:43 pm

cc. wrote:
Paul Woodlock wrote:
Now for reasons I don't actually know, the maximum vibration ( anti-node )of these standing waves happens at the boundary of each surface. While the point of minimum or no vibration ( node ) occours halfway between the surfaces ( for the fundermental frequnecy at least ). this is opposite to the stnading wave in the guiter string, as the boundaries ( nut and bridge ) are always the point of no vibration. ( anyone care to exaplin this difference? .... pretty please :) )

There isn't really a difference, but you've got a bit confused!

Starting with the guitar string - the classic way to think about this is a load of balls (!) attached by springs. If you pluck the string the balls in the middle move up and down the most. The balls at the end move the least - obviously because they are fixed! But there's something else going on too : the springs. If you look at how the tension in the springs varies you'll see that in the middle it varies the least because two balls in the middle are at the top of the 'sine shape' where it's almost flat, so they move very little relative to each other, and hardly stretch the spring at all.

On the other hand if you look at the springs right at the end you'll see that the tension varies the most because one ball is fixed and the next moves up and down in the steep part of the 'sine shape'.

So, going back to air - the movement of the air right next to the wall is zero just like the guitar string. But the pressure variation right next to the wall is maximum which is why you can hear the bass there (your ears are measuring the pressure variation, not the air movement).


Cheers for that CC. :)

Gotcha. Well explained.


Paul :)
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Re: Standing waves get their own thread - lucky little blighters!

Postby Ethan Winer » Thu Dec 16, 2004 9:03 pm

0VU,

> Perhaps the quickest way to go is if you simply state the definition you prefer and then explain your theory from there. <

It's very simple: Standing waves are waves that are standing still. Where does that happen? Only at node points! Nodes are caused by equal-level waves traveling in opposite directions. You can have node points outdoors caused by reflections off a wall, where there obviously are no room modes.

I realize the term "standing wave" has been misappropriated by some acousticians, but that doesn't make it correct. Even if you can find it in a book. The only book that can claim to define a word or term is the dictionary. My dictionary says "...where the amplitude varies from place to place [and] is constantly zero at fixed points and has a maxima at other points."

You can have zero and maximum wave pressure at fixed points outdoors, and you can have them at non-modal frequencies inside a room. Standing waves also occur in electrical wires when an impedance mismatch causes energy to be reflected back down the wire toward the source. They can even occur at a junction of two water pipes if the pipes do not fit properly and some water is reflected back into the oncoming stream. Indeed, I first learned about standing waves 40+ years ago in an electrical context. To my way of thinking standing waves have nothing to do with room modes, and they have everything to do with reflected waves.

--Ethan
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Re: Standing waves get their own thread - lucky little blighters!

Postby Digipenguin » Fri Dec 17, 2004 1:51 pm

cc. wrote:
Yeah, that's exactly right fruity - a standing wave is caused by two travelling waves going in opposite directions.


Ethan Winer. wrote:Standing waves are waves that are standing still. Where does that happen? Only at node points! Nodes are caused by equal-level waves traveling in opposite directions.


Just to clarify and avoid future confusion. Standing waves are caused by equal frequency waves traveling in different directions, including opposite. The waves can be different levels travelling perpendicular and still cause standing waves as long as the frequency at the node center is equal in both waves.

A standing wave has no direction, it is a point in space.
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Re: Standing waves get their own thread - lucky little blighters!

Postby Digipenguin » Fri Dec 17, 2004 2:25 pm

This is potentially off topic but I think it's important for those new to acoustics or those looking for greater understanding of acoustics to understand that a graphical representation of a wave is, in most cases, not a picture of the wave. It is a graph of two and sometimes three descriptive properties of a wave drawn relative to each other. Since these properties are often not legended in the graph* it is important to know which properties are being graphed.

In particular, a standing wave is most often shown in a two dimensional line graph plotting Amplitude vs. Time. The waveform created in this graph is not a physical picture of the pressure wave. It is a graph of pressure measurements made at a single point in space over time.

* - forumees often cut and paste a picture from a website or do a screen shot and crop out the desired graph with the assumption that everyone reading the graph knows what the axis titles are. This isn't as bad here as on some other forums but worth mentioning nonetheless.
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Re: Standing waves get their own thread - lucky little blighters!

Postby Ethan Winer » Fri Dec 17, 2004 8:54 pm

John,

> Standing waves are caused by equal frequency waves traveling in different directions, including opposite <

Yes, and I have a litte more to offer here too. The more I think about it, the more I realize that "misappropriate" is the correct word to use when discussing standing waves as described by acousticians. In a true standing wave situation the waves literally are standing still for their entire length. Using the water example I gave earlier because it's simple to understand, water does not compress very much under pressure. So the entire length of the back flow can be said to be standing still, starting at the point of the reflection created by the mismatched junction.

Waves in the air are very different because air is extremely springy. When a standing wave is created in the air the waves are in fact travelling toward each other from opposite directions. It's only at the precise null center that anything is standing still. So there really is no such thing as an acoustic standing wave except for at the exact null point where the deepest cancellation occurs.

--Ethan
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Re: Standing waves get their own thread - lucky little blighters!

Postby Scott R. Foster » Sat Dec 18, 2004 3:05 am

"misappropriate"


Ethan:

I think you are right, that word does seem to fit the circumstances... though I suggest you might want to look it up in a dictionary... the word may be more "appropriate" than you intended.

:headbang:
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