[youngho]

Bass traps can help a lot, but moving the speakers and/or your listening position can also make a huge difference.  Modes are a function of your room and are based on math- you can't really change them.  But moving the transducer or the listener can change the way the room is engaged, if that makes sense.

Absolutely. if you put the source in a node of a mode, it will not activate that mode. If you avoid sitting in or too close to a node or antinode of a mode, you'll amelioriate the effects of that mode. By energizing a mode on both sides of a node, you can cancel the mode. The Harman white paper on multiple subwoofers has suggested subwoofer placements, the top two of which rely on node placement and mode cancellation. Earl Geddes describes a cost-effective method using three subwoofers and spatially averaged measurements that can result in remarkably smooth bass in the low bass region. Unfortunately, it does not really address that significant portion of the frequency range above the subwoofers' range but still below the transition region.

These sorts of characteristics of standing waves makes them particularly interesting to me, and they are some of the distinctions that make them different from other interference patterns.

[Ethan Winer]

It's because the sound waves don't stand still. They constructively and destructively interfere. It's the standing wave PATTERN that stands still.

Can you explain the distinction between modal standing waves between two boundaries, and non-modal standing waves from a reflection off a single boundary outdoors? In other words, since both create constructive and destructive interference, what is different in the waves in these two situations? What makes one "standing waves" and the other not?

I assume we're still talking about bass.

Frequency has nothing to do with this. What happens at 40 Hz can also happen at 4 KHz.

If bass comes from two opposite directions and arrive at the listening position in phase, they will constructively interfere. The incident and reflected waves do not stand still.

Again, what is it about modal waves that's different?

this is a semantic argument at this point.

Probably. But I still fail to see why resonance has anything to do with this.

In his book and white papers, Toole shows examples where reducing the amplitude of a bass mode through equalization reduces the decay time of the mode to a level similar to non-modal bass frequencies.

Yes, I've seen that and I believe it. But - and this is a huge one - Floyd never shows that happens a few inches away from the point where the EQ was tweaked. If the EQ is adjusted for minimum ringing with the microphone in one location, but all the ringing comes back two inches away, then it's a technical curiosity but not a practical solution.

you had, in the past, argued very strongly that equalization might reduce the amplitude peak for a single listening position but doesn't affect modal ringing at all.

I understand it is possible to create EQ that is exactly equal and opposite to what the room does at resonance. If you put two equalizers in series, boost one with a high Q which also adds ringing, and cut the second EQ with the same Q, the ringing will indeed be countered. But again, I need to see what happens at nearby locations.

bass ringing ... can occur at modal or nonmodal frequencies

That's a new one on me!

I'll leave it at that. No more from me on the subject.

As you wish.

--Ethan

[Billblake]

To Youngho.....Thanks for your input about certain ear disorders. Due to some of your statements, I did a lot of research about fluid behind the eardrum & I feel more than positive that this is what was causing the feeling of fullness & the feeling of swellness in my ears. This really comes to light when I listen to my music. Last night I decided to give a few LONG & HARD yawns while tilting my head from side to side. This must have opened up my eustachain tubes enough to let some of this fluid to trickle down my throat. It definitely had a funny taste & it definitely worked. The pressure went away. I will see a doctor soon.      Thanks,         Bill

[youngho]

To Youngho.....Thanks for your input about certain ear disorders. Due to some of your statements, I did a lot of research about fluid behind the ear drumb & I feel more than positive that this is what was causing the feeling of fullness & the feeling of swellness in my ears. This really comes to light when I listen to my music. Last night I decided to give a few LONG & HARD yawns while tilting my head from side to side. This must have opened up my eustachain tubes enough to let some of this fluid to trickle down my throat. It definitely had a funny taste & it definitely worked. The pressure went away. I will see a doctor soon.      Thanks,         Bill

You're welcome! Some of what you had said sounded suspicious to me. By yawning or opening your mouth as widely as possible, you can sometimes forcibly open the Eustachian tube, which connects the middle ear with the back of the nose/throat region. If the pressure went away, you actually may not need to see the doctor soon, but if your symptoms recur, that would be a good idea. Happy listening!

[youngho]

It's because the sound waves don't stand still. They constructively and destructively interfere. It's the standing wave PATTERN that stands still.

Can you explain the distinction between modal standing waves between two boundaries, and non-modal standing waves from a reflection off a single boundary outdoors? In other words, since both create constructive and destructive interference, what is different in the waves in these two situations? What makes one "standing waves" and the other not?

Ethan, I am not a physicist, either. However, I believe that what makes a standing wave special is the pattern of repeated constructive interference and destructive interference that occurs as the waves get reflected back and forth. It's not the "reflected back" but rather the "and forth" that makes the standing wave special. Without the reflecting back and forth AND BACK AND FORTH (AANNDD BBAACCKK AANNDD FFOORRTTHH etc.), the mathematical description of periodic spacing of nodes and antinodes simply doesn't apply.

That is why, in my opinion, the single boundary is simply an approximation of a standing wave, but there is not the periodic spacing that is characteristic of a standing wave. That is also why other forms of constructive and destructive interference like dipolar radiation or two source interference patterns are not standing waves.

Unless I'm mistaken.

I assume we're still talking about bass.

Frequency has nothing to do with this. What happens at 40 Hz can also happen at 4 KHz.

This is unlikely, given the type of reflections that occur with 4 KHz waves. The wavelength at 4 KHz is significantly less than a foot, and the wavelength at 40 Hz is 28 feet. The kinds of effects that occur when these wavelengths radiate, diffract around obstacles, and reflect off barriers are different. As you know, higher frequencies have more specular reflections, and lower frequencies have more hemispheric reflections. The radiation and reflection patterns are less conducive to standing waves for 4 KHz, unless you're talking about a narrow space or cylinder, like a musical instrument.

If bass comes from two opposite directions and arrive at the listening position in phase, they will constructively interfere. The incident and reflected waves do not stand still.

Again, what is it about modal waves that's different?

Again, for standing waves to occur, there need to be the incident and reflected waves (and the reflections of the reflected waves, and the reflections of THOSE reflections, etc., and the interactions of all these reflections and incident waves) that need to be EXACTLY reversing direction at the 1/2 wavelength mark for that particular frequency, when the original signal is EXACTLY reversing phase. Anything else is simply constructive/destructive interference. Also, the mathematical description of nodes and antinodes is different.

Unless I'm mistaken.

this is a semantic argument at this point.

Probably. But I still fail to see why resonance has anything to do with this.

Ethan, look at two source interference patterns like the one I linked earlier. Ask yourself why this is not called "a standing wave pattern." Use the mathematical description of the spacing of nodes and antinodes for a standing wave pattern to see whether this applies to the two source interference pattern, which actually happens to approximately model the comb filter effects of a reflection, if only for one particular frequency. I believe that you'll ultimately agree that physicists seem to consider standing waves to different from other forms of interference, including two source interference patterns. When it comes to small room acoustics in the bass frequencies, the conditions for standing waves are determined primarily by room boundary measurements and characteristics.

In his book and white papers, Toole shows examples where reducing the amplitude of a bass mode through equalization reduces the decay time of the mode to a level similar to non-modal bass frequencies.

Yes, I've seen that and I believe it. But - and this is a huge one - Floyd never shows that happens a few inches away from the point where the EQ was tweaked. If the EQ is adjusted for minimum ringing with the microphone in one location, but all the ringing comes back two inches away, then it's a technical curiosity but not a practical solution.

Okay, since you seem to want to discuss this. If - and this is a huge if - "all the ringing came back," then the equalization would never have reduced the decay time of the mode to that level in the first place. Instead, this approximates a minimum phase phenomena. Two inches would be unlikely to be sufficient to raise the amplitude perceived at the listening position back to the original level, thus "all the ringing" would not come back two inches away, although the decay time would increase as the amplitude increased. However, if the listening position changed, then the listener would be in a different position to the nodes and antinodes of the various modes, so equalization would be need to be readjusted. I'm talking specifically about bass modes.

you had, in the past, argued very strongly that equalization might reduce the amplitude peak for a single listening position but doesn't affect modal ringing at all.

I understand it is possible to create EQ that is exactly equal and opposite to what the room does at resonance. If you put two equalizers in series, boost one with a high Q which also adds ringing, and cut the second EQ with the same Q, the ringing will indeed be countered. But again, I need to see what happens at nearby locations.

Toole readily admits that equalization is only effective for a single listening position. Most listeners only occupy a single position at a time.
*insert "yo mama is so fat..." joke here*

bass ringing ... can occur at modal or nonmodal frequencies

That's a new one on me!

Okay, this is easy. Boost a single bass frequency or a narrow frequency range to +12 dB relative to the other nearby frequencies, or listen to a poorly designed ported loudspeaker or subwoofer. Guess what? They sound boomy, and these frequencies take longer to decay mostly because the amplitude is longer. Go outside and wait for a car to drive by blasting its distorted bass from its crappy "subwoofer," and listen for the "ringing" that occurs at those frequencies, despite the relative absence of modal reinforcement outdoors (or, actually, according to the math, the infinite density of modes that occurs outdoors but without the reinforcement at particular frequencies or frequency ranges that occur indoors). It's mostly because of the initial high amplitudes.

At this point, this is pretty much as much as I know, so I doubt I have much else to offer on the subject.

[youngho]

Sorry, it's clear that with the outdoor single boundary reflection that there is in fact a periodic spacing of the first "antinode", "node," and second "antinode." However, the degree of reinforcement is obviously much less than with a second parallel boundary. So, for me, it's an approximation. However, that's partly because of my definition of standing wave, and it's easy to see how it would be a standing wave differently defined.

[Ethan Winer]

It's not the "reflected back" but rather the "and forth" that makes the standing wave special.

I fail to see a distinction. Whether it happens once or happens repeatedly, the SPL is affected the same at peak or null points.

the single boundary is simply an approximation of a standing wave

As I see it, acoustic interference is the parent effect, and under that are comb filtering and modal resonance, in that order.

This is unlikely, given the type of reflections that occur with 4 KHz waves. The wavelength at 4 KHz is significantly less than a foot, and the wavelength at 40 Hz is 28 feet.

Wavelength makes no difference. Here's an easy experiment you can do to prove this happens at higher frequencies just as readily as at bass frequencies:

Play a 1 KHz tone through your speakers in a fairly live room. Go to the back of the room, then walk slowly toward the speakers in the front. You'll hear the volume go up and down repeatedly. This works at 4 KHz too, but then you have to walk very slowly to clearly hear the peaks and nulls come and go. This is the same "picket fence" effect that happens with FM radio as you slow down at a stop light with a large truck etc nearby.

for standing waves to occur, there need to be the incident and reflected waves (and the reflections of the reflected waves, and the reflections of THOSE reflections, etc.

All you need is one reflection. You do not need "reflection off reflections," and that just adds secondary "parasitic" effects that obfuscate the issue.

Unless I'm mistaken.

I once considered that I was mistaken, but after seeing this perfect quote in Philip Newell's Book Recording Studio Design I knew I was right:

"It should be stressed that standing waves always exist when like waves interfere, whether a resonance situation exists or not, and that the common usage of the term 'standing wave' to describe only resonant conditions is both erroneous and misleading."

I believe that you'll ultimately agree that physicists seem to consider standing waves to different from other forms of interference

Again, standing waves can occur in electrical wires at the point of termination when an impedance mismatch causes the wave to reflect back into the oncoming source. This is an important principle in radio engineering and, clearly, there is no resonance. The very same impedance mismatch that causes standing waves at the end of an RG58 wire happens when sound waves hit a rigid boundary and reflect back.

If - and this is a huge if - "all the ringing came back," then the equalization would never have reduced the decay time of the mode to that level in the first place. Instead, this approximates a minimum phase phenomena. Two inches would be unlikely to be sufficient to raise the amplitude perceived at the listening position back to the original level

What I think you're missing is that the absolute SPL at any given cubic centimeter in a typical room is the sum of the direct sound, plus many competing reflections. Moving even a few inches can make a large change in the response even at very low frequencies. This graph shows the response in a 16 by 11.5 by 8 foot room at two locations only four inches away:



Note that the peak at 42 Hz varies by 3 dB for these two nearby locations, and there's still a 1 dB difference even as low as 27 Hz. The null at 71 Hz in one location becomes a peak at the other! More here:

A common-sense explanation of audiophile beliefs

Toole readily admits that equalization is only effective for a single listening position. Most listeners only occupy a single position at a time.

Adult ears are about 6 inches apart. So based on my graph above, EQ cannot even make the response flat, or the same, for both ears at the same time! It is this critical balance between the direct sound and multiple reflections that prevents EQ from reducing ringing for more than a single small location. These multiple reflections also cause a room not to behave as a single-pole system, making it even more difficult for simple EQ to reduce ringing.

Boost a single bass frequency or a narrow frequency range to +12 dB

Sure, but in this case the ringing is added electrically by the EQ.

or listen to a poorly designed ported loudspeaker or subwoofer.

And in this case it's the speaker that rings. But that's not the same as a room adding ringing at non-modal frequencies which is what I thought you said earlier.

At this point, this is pretty much as much as I know, so I doubt I have much else to offer on the subject.

Aw c'mon, why should that stop either of us?

--Ethan

[Ethan Winer]

the degree of reinforcement is obviously much less than with a second parallel boundary.

Not so! A reflection off a single rigid boundary can (in theory) create a null infinitely deep, and a peak of 6dB (true doubling).

--Ethan

[youngho]

As I see it, acoustic interference is the parent effect, and under that are comb filtering and modal resonance, in that order.
...
Play a 1 KHz tone through your speakers in a fairly live room. Go to the back of the room, then walk slowly toward the speakers in the front. You'll hear the volume go up and down repeatedly. This works at 4 KHz too, but then you have to walk very slowly to clearly hear the peaks and nulls come and go. This is the same "picket fence" effect that happens with FM radio as you slow down at a stop light with a large truck etc nearby.
...
All you need is one reflection. You do not need "reflection off reflections," and that just adds secondary "parasitic" effects that obfuscate the issue.
...
I once considered that I was mistaken, but after seeing this perfect quote in Philip Newell's Book Recording Studio Design I knew I was right:
...
"It should be stressed that standing waves always exist when like waves interfere, whether a resonance situation exists or not, and that the common usage of the term 'standing wave' to describe only resonant conditions is both erroneous and misleading."

Ethan, I've already asked that we agree to disagree on this issue. I've already discussed the radial distribution of peaks and nulls with the two source interference pattern, which is similar with the comb filter effect off a single boundary. I've already asked you REPEATEDLY to provide your definition of standing wave, which you REPEATEDLY failed to do, so I've basically ended up with "We must have different definitions of standing waves, and so let's leave it at that."

There's a book called "The Master Handbook of Acoustics written by F. Alton Everest. From that book: "The concept of standing waves is directly dependent on the reflection of sound as emphasized in Chapter 15. Assume two flat, solid parallel walls separated a given distance. A sound source between them radiates sound of a specific frequency. The wavefront striking the right wall is reflected back toward the source, striking the left wall where it is again reflected back toward the right wall, and so on. One wave travels to the right, the other toward the left. The two traveling waves interact to form a standing wave. Only the standing wave, the interaction of the two, is stationary. The frequency of the radiated sound is such as to establish this resonant condition between the wavelength of the sound and the distance between the two surfaces. The pertinent point at this moment is that this phenomenon is entirely dependent on the reflection of sound at the two parallel surfaces." Obviously, this is an oversimplification, but you get the point.

I'm sure your library of acoustics textbooks is vastly larger than mine, and your apparent inability to admit even the possibility of error (much less actual errors, like when I clearly demonstrated that the modal sound waves don't "stand still" but actually do travel when they form a standing wave pattern) makes it clear to me that further discussion is not likely to be fruitful for either of us, so can we agree to disagree and stop beating this dead horse?

Again, standing waves can occur in electrical wires at the point of termination when an impedance mismatch causes the wave to reflect back into the oncoming source. This is an important principle in radio engineering and, clearly, there is no resonance. The very same impedance mismatch that causes standing waves at the end of an RG58 wire happens when sound waves hit a rigid boundary and reflect back.

I don't know anything about electrical engineering. My extremely limited understanding is that the phenomena are a little different in that the impedance mismatch causes a net loss of energy transfer at the standing wave frequency but that the acoustic phenomenon of standing waves maximizes transfer of energy at the standing wave frequencies, like for listeners outside the room, assuming that the boundaries are not perfectly rigid, hugely massive, and hermetically sealed. Furthermore, since a pure standing wave is not achieved in these engineering/electrical transmission lines, they are referred to as "partial standing waves," hence the existence of the term "standing wave ratio" to reflect this approximation. Again, I'm not a physicist (or an engineer), so I really can't comment further.

What I think you're missing is that the absolute SPL at any given cubic centimeter in a typical room is the sum of the direct sound, plus many competing reflections. Moving even a few inches can make a large change in the response even at very low frequencies. This graph shows the response in a 16 by 11.5 by 8 foot room at two locations only four inches away:

Note that the peak at 42 Hz varies by 3 dB for these two nearby locations, and there's still a 1 dB difference even as low as 27 Hz. The null at 71 Hz in one location becomes a peak at the other! More here:

This is very interesting, but it's not like "all the ringing comes back" at the equalized modal frequency, which is what you originally wrote. Some of the ringing comes back as the amplitude increases, but hardly "all the ringing comes back." There can even be new ringing at different frequencies compared to the old position.

Adult ears are about 6 inches apart. So based on my graph above, EQ cannot even make the response flat, or the same, for both ears at the same time! It is this critical balance between the direct sound and multiple reflections that prevents EQ from reducing ringing for more than a single small location. These multiple reflections also cause a room not to behave as a single-pole system, making it even more difficult for simple EQ to reduce ringing.

I never claimed that EQ could make the response flat or the same for both ears at the same time. These are straw men arguments (So, obviously, EQ cannot create world peace or eliminate poverty). Again, the "ringing" is not modal but rather seems to be primarily a function of amplitude. For your reference, what I wrote is "Reducing the amplitude of a bass mode through equalization reduces the decay time [perceived by a single listener] of the mode to a level similar to non-modal bass frequencies."

Sure, but in this case the ringing is added electrically by the EQ.
...
And in this case it's the speaker that rings. But that's not the same as a room adding ringing at non-modal frequencies which is what I thought you said earlier.

You can always scroll back to see what I said. What I wrote was that "modal ringing is controversial" and "In other words, bass ringing is bass ringing, it's not particularly modal, it can occur at modal or nonmodal frequencies (hence, it's not modal ringing but rather bass ringing), etc." The ringing may be added electrically, it may be the speaker, it may be the room, whatever. Ringing is not controversial, it's the question of whether there is additional ringing at modal frequencies that is not predominantly accounted for by amplitude alone that is controversial.

Aw c'mon, why should that stop either of us?

Because I'm tired of discussing this. I will leave the last word to you, unless you specify that you would like me to comment further. For everyone else, peace, and happy listening.

[Billblake]

Man oh man oh man oh man......You guys have said a mouthfull!  Now I need to decipher it!   But thanks for all the info.    Cheers,           Bill

[Ethan Winer]

I've already asked you REPEATEDLY to provide your definition of standing wave, which you REPEATEDLY failed to do

Not intentionally. I thought it was clear that my "definition" of a standing wave is where waves remain stationary (Everest's word) at the point where they collide in mid-air.

There's a book called "The Master Handbook of Acoustics written by F. Alton Everest.

Yes, I have that book too and of course I've seen that. Like some other acousticians, Everest considers standing waves to be resonant. This is exactly why I raised the issue in the acoustics newsgroup I linked to. Not all acousticians agree on this, though I believe the current prevailing wisdom agrees with my view for all the reason's I've stated. If you prefer to consider resonance a requirement, that's fine. I still like you a lot.

I'm sure your library of acoustics textbooks is vastly larger than mine

Probably not. My main two books are Everest's and Newell's that I already mentioned. I'm not a math guy, so I save a lot of money by not buying books that are full of math and undecipherable to me.

I don't know anything about electrical engineering.

Trust me, it's exactly the same. Audio, radio, and light waves all behave similarly but at different frequency ranges. Here's a mechanical analogy that might be more intuitive:

Let's say you have two water pipes in series, each one inch in diameter, with a pipe coupler like this joining them together:



This type of coupler goes around the outside of the pipes, so the pipes butt against each other perfectly and water flows smoothly from one pipe into the other with no interference. Let's say you put a washer inside the coupler, partially blocking the pipes, and the washer has a hole 1/2 inch in diameter. Now, some of the water hits the washer's hole and goes through, but some of the water hits the flat of the washer and is reflected back into the oncoming stream. This too creates standing waves, but the waves are in the water rather than electrical or acoustic. Clearly these are standing waves, and clearly there is no resonance.

This is very interesting, but it's not like "all the ringing comes back" at the equalized modal frequency, which is what you originally wrote. Some of the ringing comes back as the amplitude increases, but hardly "all the ringing comes back." There can even be new ringing at different frequencies compared to the old position.

How could you know that unless you've tried it?

I never claimed that EQ could make the response flat or the same for both ears at the same time. These are straw men arguments

Not straw man! The claim is that EQ can reduce ringing in a usable manner. If the ringing is reduced for only one cubic inch, it's not usable. Now, I admit I have not done this test either, but based on having done hundreds of room measurements I don't see how it could play out any differently.

Again, the "ringing" is not modal but rather seems to be primarily a function of amplitude.

I have no idea what that means. If a room rings, and it's the room, then it's modal. Or maybe flutter echo which is also modal because the frequency is related to the boundary spacing.

Because I'm tired of discussing this. I will leave the last word to you, unless you specify that you would like me to comment further. For everyone else, peace, and happy listening.

Okay.

--Ethan

[youngho]

As I promised, I left the last words to Ethan, so this is not a response. Rather, I wanted to point out that there is an extremely interesting discussion that touches on topics, some directly and others peripherally related, mentioned earlier in this thread. The participants are so vastly qualified (in no particularly order: James Johnston, Robert Reams, Earl Geddes, Todd Welti, Sean Olive, Dennis Erskine, etc) that I suggest browsing the second half of http://www.avsforum.com/avs-vb/showthread.php?t=874883&page=22 and then http://www.avsforum.com/avs-vb/showthread.php?t=1113793, but ignoring the personal attacks in the former and the posts by JBLsound4565 in the latter. Cheers, and happy listening.

[Ethan Winer]

This is exactly why I raised the issue in the acoustics newsgroup I linked to. Not all acousticians agree on this, though I believe the current prevailing wisdom agrees with my view for all the reason's I've stated.

I just came across this post by former Auralex engineer Jeff Szymanski:

http://forums.audioholics.com/forums/showthread.php?p=523029#post523029

I link this not to stir up trouble, but because Jeff is a perfect example of a professional acoustician who used to consider resonance a requirement for standing waves, then changed his mind after being part of the many forum discussions.

Jeff elaborates further in his blog entry 09 October 2007, A room mode by any other name (scroll most of the way down the page):

http://10xtheblog.blogspot.com/2007_10_01_archive.html

--Ethan

[youngho]

"Standing waves that do not correspond to room modes can occur in small rooms if circumstances are just right. This is sometimes the case when a low frequency wave from a loudspeaker at one end of a room bounces off the opposite wall and interferes with the wave coming from the loudspeaker. No resonance need be present for wave interference to take place, thus creating a standing wave pattern in the room. Or is this a standing wave? That is the question!

Philip Newell, in his magnificent text, Recording Studio Design, is careful to differentiate resonant standing waves (room modes) from the more broadly defined standing waves. The following is excerpted from the definition of Standing waves and resonances in Newell's Glossary of terms:

"It should be stressed that standing waves always exist when like waves interfere, whether a resonance situation occurs or not, and that the common usage of the term 'standing wave' to describe only resonant conditions is both erroneous and misleading."

This perspective is debatable. Some respected thinkers in the field of acoustics would argue that the sort of non-resonant behavior being described does not conform to the definition of a standing wave. I cannot say I agree. I cannot say I disagree. What would be best, IMHO, is if those of us who consider ourselves "experts" could better delineate the exact type of behavior they are referring to when using the term standing wave.

As a parting thought, a fair amount of heated discussion has taken place on the Internet concerning this (seemingly minor) subject. I grow increasingly convinced that it may never be resolved. (It certainly would be nice to resolve the whole thing right here and now...but I am not unrealistic.) What is unfortunate is that some of the discussions, I feel, have irritated people in the myriad acoustics forums; people who are mostly novices, who only seek simple advice for making their room(s) sound better, and who do not wish to bear witness to what amounts to a debate of semantics between a few members of the acoustical literati. IMHO, acoustics forums are not the best place for this debate. I feel the issue can be resolved easily and then left alone. I hope the above will help us "experts" work towards that goal."