quarter wave vs. room modes and implication for bass traps - AVS Forum
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post #1 of 27 Old 10-16-2009, 06:53 PM - Thread Starter
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I recently noticed an article by Ethan Winer indicating that in small rooms quarter wave cancellation effects may be more significant than room modes. (As a new poster, I can't put up the URL.)


This would seem to suggest that to reduce nulls (which are usually higher than peaks) one should primarily put bass traps directly behind and to the sides of the listening position (at least when walls are close), and to do the same for the speakers. In contrast, the method of putting them at junctions (wall/wall, ceiling/wall, floor/wall), as usually suggested, would seem to be most helpful for reducing peaks and modal ringing.

In my own situation, use of junction-placed bass traps has flattened everything at my listening position except for a significant notch at right around 270 hz. This seems to be a combination of two matching 3/4 wave cancellations: floor and back wall. It's hard to treat the floor beneath one's couch, and, in the configuration of my living room, putting a trap directly behind the couch would block an important thoroughfare. :-) This does seem to be a cancellation effect rather than a mode, as the frequency changes with position (especially, up/down position).

I wonder, is it possible to successfully use EQ for a single dip in this relatively high bass range? Or point a speaker sideways set to _only_ hit this notch?
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post #2 of 27 Old 10-17-2009, 06:46 AM
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Originally Posted by sbratman View Post

I recently noticed an article by Ethan Winer indicating that in small rooms quarter wave cancellation effects may be more significant than room modes.

Well, maybe not more significant, but certainly equal. All peaks and nulls are caused by reflections combining in and out of phase with the direct sound from the speakers, as well as combining with other reflections. Some of the peaks and nulls are modal, and some are not. Modal peaks also ring, where non-modal peaks do not. There's also SBIR (Speaker Boundary Interference Response) which is the same idea, but there the 1/4 wavelength boundary distances are related to the loudspeaker sources rather than the listener. SBIR is improved with bass traps in the vicinity of the speakers.

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This would seem to suggest that to reduce nulls ... one should primarily put bass traps directly behind and to the sides of the listening position (at least when walls are close), and to do the same for the speakers.

Yes, the front and rear (and side) walls are also good places for bass traps. Basically, any surface that can reflect bass waves will cause peaks and nulls. Those boundaries don't have to be very close either, though the closer they are, the stronger the reflections will be.

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In contrast, the method of putting them at junctions (wall/wall, ceiling/wall, floor/wall), as usually suggested, would seem to be most helpful for reducing peaks and modal ringing.

Maybe. I'm not much of a physicist, but my understanding is that low frequency waves tend to spread out when they hit a boundary. Sort of like blowing up a huge balloon in the room. Eventually the balloon hits the wall, then spreads outward toward the corners. Maybe one of the "real" physicists here can clarify.

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I wonder, is it possible to successfully use EQ for a single dip in this relatively high bass range? Or point a speaker sideways set to _only_ hit this notch?

EQ is not useful for nulls. Nulls are often 20 to 30 dB deep or even deeper. Adding that much boost with EQ will blow up your speakers and sound terrible. As for sideways, again, bass waves expand outward and don't follow the "cue ball" analogy as do waves at higher frequencies. So the only solution is more bass traps. (And of course optimizing the speaker and listening positions.)

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post #3 of 27 Old 10-17-2009, 01:43 PM
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Quote:
Originally Posted by Ethan Winer View Post

I'm not much of a physicist, but my understanding is that low frequency waves tend to spread out when they hit a boundary. Sort of like blowing up a huge balloon in the room. Eventually the balloon hits the wall, then spreads outward toward the corners.

If so then it would be interesting in that it doesn't seem to be the same as typical wave behavior, ie implies a bit more of a particle effect being dominant than a wave effect.

Not that this seems entirely out of the question to me since air molecules are fairly large particles inclined to be more likely to be influenced by things such as collisions and gravity than say photons would be. My problem is I'm much more familiar with EM wave theory than acoustics.

The wave model I have learned to use while working with EM systems (which I had assumed was analogous to acoustics, etc.) suggests that whenever an impedance difference is encountered by a wave there is potentially transmission and reflection. The model I have used suggests that the direction these reflections and transmissions take is predictable based on angle of incidence and material impedance. The interesting thing to note is everything is a straight line except for at the impedance boundaries. The squashing balloon effect you describe seems to imply this model I've described doesn't apply.
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post #4 of 27 Old 10-18-2009, 11:05 AM
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Originally Posted by Ugly1 View Post

The squashing balloon effect you describe seems to imply this model I've described doesn't apply.

I'm sure the impedance mismatch model applies too. Bass waves are definitely reflected by a rigid boundary. But they must always spread outward because putting bass traps in the room corners has a huge effect.

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post #5 of 27 Old 10-19-2009, 12:37 PM
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Quote:
Originally Posted by Ethan Winer View Post

putting bass traps in the room corners has a huge effect.

I always figured this is due to the corners being where the longest room dimensions are and so the location of where the largest percentage of standing waves (and especially the most annoying IMO long wavelength standing waves) could be affected by the traps. Isn't it true that traps also have a positive effect in other locations too but just to a lesser extent?
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post #6 of 27 Old 10-19-2009, 12:45 PM
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Acoustic sound wave span 1000:1 ratio, so reflection, transmission, scattering, diffraction, interference are all in play simultaneously. BTW EM are transverse waves and sound waves are longitudinal in a fluid.

At 270 Hz, the problem null has wavelength of 4 feet, small enough to be reflected. It could be a first reflection problem. Have you treated all the first reflection points on the walls and ceiling?
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post #7 of 27 Old 10-20-2009, 01:15 AM
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Originally Posted by gsmollin View Post

BTW EM are transverse waves and sound waves are longitudinal in a fluid.

Ding ding ding. Lights go on. You have created a moment of clarity for me. Thanks.
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post #8 of 27 Old 10-20-2009, 05:27 AM
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Originally Posted by gsmollin View Post

BTW EM are transverse waves and sound waves are longitudinal in a fluid.

for those of us who are following along and trying to learn, can someone explain what this means and what the resultant effect is?

in small words, as i have a small brain...

thanks.

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post #9 of 27 Old 10-20-2009, 06:48 AM
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EM, electromagnetic waves are composed of magnetic and electric fields that oscillate in a complimentary manner, and are at right angles to each other and the direction of travel. Sound waves in a fluid are composed of pressure waves and displacement waves that oscillate in a complimentary manner, but these waves lie in the direction of travel, so are called longitudinal. In a solid, sound waves may also be transverse because the medium can support shear forces.

The know-it-all of the internet has the details prepared for you:
EM, electromagnetic waves: http://en.wikipedia.org/wiki/Electromagnetic_waves

Sound waves: http://en.wikipedia.org/wiki/Sound_waves#Sound_waves

Note: In the second reference, I think the 2-D representation of the longitudinal wave appears to be a transverse wave. It could be an eye-perception issue.
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post #10 of 27 Old 10-20-2009, 09:38 AM
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Originally Posted by Ugly1 View Post

Isn't it true that traps also have a positive effect in other locations too but just to a lesser extent?

Yes.

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post #11 of 27 Old 10-20-2009, 02:14 PM
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@gsmollin... thanks....

this stuff makes my head hurt... i understand a lot of the concepts, but piecing them together can be a bit of a challenge sometimes...

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post #12 of 27 Old 10-21-2009, 05:17 AM
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Quote:
Originally Posted by gsmollin View Post

EM, electromagnetic waves are composed of magnetic and electric fields that oscillate in a complimentary manner, and are at right angles to each other and the direction of travel. Sound waves in a fluid are composed of pressure waves and displacement waves that oscillate in a complimentary manner, but these waves lie in the direction of travel, so are called longitudinal. In a solid, sound waves may also be transverse because the medium can support shear forces.

As I understand it, some the implications of the complimentary variation of pressure and displacement of sound waves for bass and bass traps are rather interesting. It's not simply a matter of sound waves travelling around the room and spreading out. Areas of maximal pressure fluctuations for frequencies of room modes will be those where air particle velocity are minimal, like at room boundaries.

Purely porous or resistive absorption converts air particle motion to heat through friction, and this requires moving air particles. Thus, this kind of absorption benefits from moving it out from the wall a little, rather than mounting it directly on the wall, as illustrated by absorption data like the crude graph showing similar absorption of directly mounted 2" fiberglass compared with 1" fiberglass with 3" air space behind it in Alton Everest's book. For drapes hung straight, absorption is maximal at odd multiples and minimal at even multiples of quarter wavelengths relative to the distance of the drape from the wall, again having to do with the inverse relationship of air particle velocity and and pressure fluctuations near the boundaries. By hanging drapes to half of their width, one increases the absorption (the sound strikes the drape material at an oblique angle and consequently passes through more of it) and introduces variation in the depth of the space from the wall. Some commercial absorption panels incorporate an air space into their design, one even has a variable depth air space that wall mounted. Corner-mounting automatically means a variable depth air space.

Certain kinds of bass traps (it's helpful to specify the design) actually work by absorbing through damped resonance, so these should be placed at areas of maximal pressure fluctuations, particularly junctions of room boundaries. Often the room boundaries themselves have some effect along these lines. Some commercial absorption panels incorporate a membrane, which has some of this effect and also reduces mid- and high-frequency absorption.

At least, this is my limited understanding.
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post #13 of 27 Old 10-22-2009, 02:26 PM - Thread Starter
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I don't think I quite got my question across. Let me try again:

I don't doubt that putting bass traps in corners damps room modes wonderfully, as all modes are active in tricorners and lots of modes are active in ordinary corners. But the quarter wave reflections aren't room modes, are they? Aren't room modes types of standing waves? And aren't quarter wave cancellation be more of a local effect?

Eg: The sound hits, bounces, and cancels itself at 1/4 wave, 3/4 wave etc. distances from a surface. If the frequency matches the length (width, height, oblique distance) of a room, then a standing wave can appear. In such cases, the nodes and peaks are of fixed frequency, and recur in various positions through the room. But if the frequency is not a mode, there is still a cancellation locally, only one of a different kind: In this type of effect (as per Ethan's article) the _frequency_ of the node changes as one moves to different distances from the surface. These nodes are at 1/4 wave from a surface; 3/4 wave, etc. (As a perhaps important aside, there should also be an oncoming wave from the opposite wall, and since the frequency is not one that forms a standing wave -- by definition for the example-- this means that the oncoming wave is out of phase in such a way that no standing wave can form. Thus, there must be partial cancellations and reinforcements in various changing places as the waves move. Nonetheless, particle velocity is always zero at the wall surface, so it would seem that there would have to be a fixed cancellation at 1/4 the wavelength from the surface. Perhaps this becomes more mixed up further out.)

In any case, this type of variable frequency cancellation has nothing to do with room resonances or standing waves, but rather has to do with the nearest wall -- and, as per Ethan, most prominently the front and back walls along the line of fire of the speakers. And, it is this type of cancellation that would seem not to be addressed by putting traps in corners. Rather, traps would seem to be best placed on the wall behind the listening position, and on the wall behind the speakers.

If, further, this type of cancellation is a bigger effect than room modes (again, as per that article), then it would seem that the benefit of putting bass traps in corners would be less significant than it seems. Such placement would reduce modal ringing, and reduce nodes and peaks caused by standing waves, but would not directly affect the 1/4 wave cancellation issue. Or am I missing something?

Steve

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post #14 of 27 Old 10-22-2009, 02:41 PM - Thread Starter
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Here's a different question, though somewhat related: Measurement of bass trap absorption generally shows much better absorption with corner placement. However, when bass traps (or, at least, bass trap material) is tested at, say 16 inches from the wall, absorption measurements seem to approach, or match corner placement. (This is seen on Bob Gold's page of data, and also on the GIK website.) This would tend to suggest that part of the raw increase in absorption usually seen in corner placement has to do with the distance from the wall. If so, the main advantage of the corner placement is that two walls are affected for the price of one. In addition, the presence of two walls at a corner might doubly concentrate bass, and so give the trap more sound to damp. Is this correct?

As an aside, one would think that where the diagonally placed trap touches the wall, the absorption is nill regarding reflections from that wall, though still quite adequate regarding reflections from the other wall. Would a diagonally placed bass trap would work better if it were displaced outward from both walls by four inches or so?
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post #15 of 27 Old 10-23-2009, 07:06 AM
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Quote:
Originally Posted by sbratman View Post

But the quarter wave reflections aren't room modes, are they? Aren't room modes types of standing waves? And aren't quarter wave cancellation be more of a local effect?

LOL, this was the subject of much contention a few years ago among the "Internet" acousticians. Here's the way I see it, and I believe this is now pretty well accepted by all. Nay-sayers are welcome to pipe up again.

Acoustic interference is the parent property. Modes, standing waves, and resonances are a subset of this basic interference. However, I distinguish between modes, which are a propensity to resonate, and standing waves which are caused by waves colliding and interfering. A reflection off a single boundary outdoors can create a standing wave. So the only difference between a resonant (modal) standing wave and a non-resonant standing wave is that one happens to fit evenly between two parallel (or effectively parallel) boundaries. So modal and non-modal cancellations are not a "different kind," but are in fact exactly the same.

This quote is from the book Recording Studio Design by acoustician Philip Newell, who makes the case clearly:

Standing waves occur whenever two or more waves having the same frequency and type pass through the same point. The resultant spatial interference pattern, which consists of regions of high and low amplitude, is 'fixed' in space, even though the waves themselves are travelling. ... 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.

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it is this type of cancellation that would seem not to be addressed by putting traps in corners. Rather, traps would seem to be best placed on the wall behind the listening position, and on the wall behind the speakers.

Yes, mostly, but bass waves propagate all around a room. So wherever bass builds up the most is a candidate location for bass traps. It is well-known that putting bass traps on the walls is useful, in addition to corners. And your basic premise of SBIR and LBIR affecting the response independently of modes is 100 percent correct. The frequency response at any given cubic centimeter location in a room is the sum of the direct sound plus many different and competing reflections, all arriving at different volume levels with different amounts of time delay / phase shift.

BTW, my idea of an "ideal" room is a free-standing shed in the back yard having walls made of cardboard. Then you'll have no bass reflections at all!

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post #16 of 27 Old 10-23-2009, 07:13 AM
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This would tend to suggest that part of the raw increase in absorption usually seen in corner placement has to do with the distance from the wall.

Yes and no. Distance from a boundary increases the effective absorption because the wave velocity is higher away from a boundary. But bass tends to collect in corners, so putting traps there also increases absorption. A bass trap in the middle of a wall appears to absorb less than a bass trap at one end of the wall near to a corner.

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Would a diagonally placed bass trap would work better if it were displaced outward from both walls by four inches or so?

This is an excellent question, and a proper test is on my to-do list. Hopefully some time in 2010. I have read reports that bass traps work better when "sealed" into a corner, and I have read other reports that say four inches out is better. I don't believe anything unless I duplicate it myself.

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post #17 of 27 Old 10-23-2009, 12:59 PM
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Quote:
Originally Posted by Ethan Winer View Post


BTW, my idea of an "ideal" room is a free-standing shed in the back yard having walls made of cardboard. Then you'll have no bass reflections at all!

--Ethan

Or an open backyard home theatre??
Just a front wall and floor - grassy floor


I moved the sides/rear speker closer to seating zone upon testing, and was very happy with the 6.1 sound with the 16' x 9' image.

btw;
great educational thread
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post #18 of 27 Old 10-23-2009, 02:29 PM - Thread Starter
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Well, we're getting into a bit of quibbling here, though it's interesting quibbling.

Room modes are spatial resonances between parallel walls, occurring at integer multiples of the freguency whose wavelength is the distance between the walls. This is the type of resonant frequency that occurs in such things as flutes (though in two dimensions instead of one dimension), and so deserves to be called a resonance. It also has a high energy character, as described in a moment. There are additionally materials resonances, etc., and, yes, there are standing waves with any frequency. However, the standing waves that occur at the spatial resonance frequencies known as room modes have a special character.

Modal frequency reflections become "synchronized" in the form of a sustained standing wave between two opposite walls, with the phase relationships established by the walls, not the position of the speaker. Such reflections have high energy efficiency because at the point where the standing wave hits the wall it is exactly at the maximum pressure/zero velocity point; since the reflection from the wall must always begin in that same state, there is maximum energy efficiency in the bounce. In contrast, non modal frequencies do not "synchronize" via resonance between the two walls, and therefore hit the walls at varying points in the wave, where the bounce is necessarily less efficient (except at those moments where the wave happens to hit with the right phase.)

As a consequence, this type of standing wave characteristically and specifically involves two walls. Therefore, it can be "treated" by damping the bass energy bouncing between those walls at any point in the two-wall system. Furthermore, _any_ bass energy in the room at a modal frequency is steered into the wall-wall system with a spatial resonance at that frequency. Therefore, putting traps in any corner is helpful, because a lot of the room's bass energy is present there (and, therefore, damping has a greater effect.)

Non-modal 1/4 wave reflections of the type I've been talking about have a different character. Yes, they involve standing waves, but not of the room mode type; the wall-wall distance is not an integer fraction of the wave length involved, and the reflection phase has to do with the distance between the listener or the speaker and the near wall, not the distance between walls.

Therefore, the deep cancellation caused by 1/4 waves is a local effect: the bass from the speaker hits the wall behind the listener, is "reset" to a high pressure/zero velocity state reversed in phase from the incoming wave, and cancels the incoming wave at 1/4, 3/4 wave etc. (Ditto for the wall behind the speaker.) The bounced wave is exactly out of phase with the incoming wave, and will produce deep nulls. However, this effect has nothing to do with the wall/wall distance, or the general bass energy in the room. True, the distant wall (and the ceiling, etc.,) will also be sending out a wave, and it will interfere with the other signals, but it will not be in proper phase with them by definition: we are talking about frequencies that are not a modal frequency. Thus, the distant wall reflection could as well reduce the 1/4 wave cancellation as increase it. In any case, it will not be lined up properly to produce a truly deep null, as it will not be in exact opposite phase. In this situation, treating the distant wall will not generally be useful: the problem is not the general bass energy bouncing around the room; it is specifically the bass energy between the back wall and the listening position. (Or between the speaker and its near wall.)

Again, I am not denying that treating various corners is good for improving room acoustics. My suggestion, though, is that it only treats modes, and not quarter wave cancellations. Thus, the type of null that varies in frequency with distance (as described in your article) will not be improved, except accidentally, by treating distant parts of the room. In general, only the type of null that varies in strength with position but remains at fixed frequency -- a room mode, or spatial resonance -- will be improved by distant trapping.

Obviously, these are important nulls! In the usual analysis of rooms, they are the most important nulls. But your testing article suggested that 1/4 wave cancellations are perhaps more important; at least, they are equally important. Therefore, it would seem that a high priority should be set on trapping the walls near the listening position and the speakers; that's the _only_ way to treat 1/4 wave cancellations. Trapping corners would still be good, of course, for treating modes.

Or am I missing something?

Steve

Quote:
Originally Posted by Ethan Winer View Post

LOL, this was the subject of much contention a few years ago among the "Internet" acousticians. Here's the way I see it, and I believe this is now pretty well accepted by all. Nay-sayers are welcome to pipe up again.

Acoustic interference is the parent property. Modes, standing waves, and resonances are a subset of this basic interference. However, I distinguish between modes, which are a propensity to resonate, and standing waves which are caused by waves colliding and interfering. A reflection off a single boundary outdoors can create a standing wave. So the only difference between a resonant (modal) standing wave and a non-resonant standing wave is that one happens to fit evenly between two parallel (or effectively parallel) boundaries. So modal and non-modal cancellations are not a "different kind," but are in fact exactly the same.

This quote is from the book Recording Studio Design by acoustician Philip Newell, who makes the case clearly:

Standing waves occur whenever two or more waves having the same frequency and type pass through the same point. The resultant spatial interference pattern, which consists of regions of high and low amplitude, is 'fixed' in space, even though the waves themselves are travelling. ... 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.



Yes, mostly, but bass waves propagate all around a room. So wherever bass builds up the most is a candidate location for bass traps. It is well-known that putting bass traps on the walls is useful, in addition to corners. And your basic premise of SBIR and LBIR affecting the response independently of modes is 100 percent correct. The frequency response at any given cubic centimeter location in a room is the sum of the direct sound plus many different and competing reflections, all arriving at different volume levels with different amounts of time delay / phase shift.

BTW, my idea of an "ideal" room is a free-standing shed in the back yard having walls made of cardboard. Then you'll have no bass reflections at all!

--Ethan

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post #19 of 27 Old 10-23-2009, 02:35 PM - Thread Starter
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Ethan,

If one listened to music in a cardboard room in an open space, wouldn't one lose most of the sound energy? Despite the problematic nature of reflections, they are also the source of most of the sound one hears. Sound is radiated spherically (actually at all frequencies, but there are issues at higher frequencies that produce direction-like effects); and so, without reflection, one would only hear a tiny, tiny bit of the sound from a sound source: the proportion of the sphere one's ears subtend. That's miniscule! One would need very loud speakers.

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Presumably, the cardboard would reflect some high frequencies (to avoid the "dull room" effect.) This would mean that higher frequencies would be differentially increased in power, because they're collected, compared to the lower frequencies that pass through the walls. This would require marked adjustment to the loudspeaker output to create balanced sound.

I wonder, is there a "dead room" effect when using headphones? There are certainly no reflections. (I know that there are big stereo problems with headphones, but that's a different subject.)
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post #21 of 27 Old 10-24-2009, 05:45 AM
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Quote:
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Or am I missing something?

What you're missing is that the waves "reset" at a boundary. This is why I mentioned being able to create peaks and nulls outdoors against a single boundary. If that boundary is 100 percent reflective at a given frequency, then the null will be infinitely deep at 1/4 wavelength away. More here:

Bass Waves in the Control Room

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post #22 of 27 Old 10-24-2009, 05:51 AM
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Presumably, the cardboard would reflect some high frequencies (to avoid the "dull room" effect.)

Yes, exactly, which is why said I'd rather have a room like that than be outdoors. Though I love the photo mtbdudex posted. I bet the bass is perfectly even and not at all boomy!

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I wonder, is there a "dead room" effect when using headphones? There are certainly no reflections.

This is a great point, and it shows the flawed reasoning of those who say early reflections are helpful. I think most hi-fi types will agree that good music heard through headphones can sound huge and spacious. Yet there are no room reflections! This is why absorbing all early reflections makes music sound larger. Early reflections impart a "small-room" sound onto the music that drowns out the reverb and ambience already present in the recording. When those reflections are absorbed the sound stage becomes larger and more natural, not smaller and less natural.

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post #23 of 27 Old 10-24-2009, 09:14 AM
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I think most hi-fi types will agree that good music heard through headphones can sound huge and spacious.

Not all of us. It is hard to get the sound out of my head.

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post #24 of 27 Old 10-24-2009, 09:27 AM
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I recently noticed an article by Ethan Winer indicating that in small rooms quarter wave cancellation effects may be more significant than room modes. (As a new poster, I can't put up the URL.)

Since you've been around for awhile now, perhaps you could post the link.
Obviously both can be important, but I think that ringing is usually more of a problem than a local null caused by a single reflection.

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This would seem to suggest that to reduce nulls (which are usually higher than peaks) one should primarily put bass traps directly behind and to the sides of the listening position (at least when walls are close), and to do the same for the speakers. In contrast, the method of putting them at junctions (wall/wall, ceiling/wall, floor/wall), as usually suggested, would seem to be most helpful for reducing peaks and modal ringing.

Both would be better Corners to absorb as much of the room mode energy as possible, and near the listener to help calm local effects.

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In my own situation, use of junction-placed bass traps has flattened everything at my listening position except for a significant notch at right around 270 hz. This seems to be a combination of two matching 3/4 wave cancellations: floor and back wall. It's hard to treat the floor beneath one's couch, and, in the configuration of my living room, putting a trap directly behind the couch would block an important thoroughfare. :-) This does seem to be a cancellation effect rather than a mode, as the frequency changes with position (especially, up/down position).

I think the first thing is to determine if this is a modal response or a local problem. One easy way to do this is to select a location where the 270hz signal is strong, then look at the waterfall plots - If it is modal it will ring, if not it will not. Fortunately 270 is pretty easy to deal with. 6" of 703 type stuff should help alot. You say you cannot treat the wall behind you, can you move your chair? Moving the chair will not cure the 1/4 (or 3/4 and so on) cancellation, but it will change the frequency. Are you treating first reflections? Using 6" thick panels for this might help your situation (don't forget the ceiling).

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I wonder, is it possible to successfully use EQ for a single dip in this relatively high bass range? Or point a speaker sideways set to _only_ hit this notch?

I would not recommend using EQ to try to fill in a notch. I would also not suggest using another speaker to just fill in the notch (it would cause problems elsewhere in the room if nothing else)
The human mind is an amazing thing, it will often fill in missing information (using harmonics I suppose), but it has a harder time eliminating extra information - i.e. typically peaks are more of a problem than nulls. Especially model ringing peaks.

The bottom line is you'll never get your room perfect (especially if you are restricted on how it can be treated) so you just have to do the best you can. And feel free to experiment - treating under your couch may help, blocking the passageway behind the couch might be acceptable for a short term trial.

disclaimer - I am NOT an expert of room acoustics, just an interested amateur.

Everything I say here is my opinion. It is not my employers opinion, it is not my wife's opinion, it is not my neighbors opinion, it is My Opinion.
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post #25 of 27 Old 10-24-2009, 09:31 AM
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Not all of us. It is hard to get the sound out of my head.

A good binaural recording designed to be played from headphones can be absolutely amazing. But I still miss the part of the sound that is sensed from body parts other than ears - i.e. a good chest thump when called for I also find that not having the sound change when I move around somewhat odd.

Everything I say here is my opinion. It is not my employers opinion, it is not my wife's opinion, it is not my neighbors opinion, it is My Opinion.
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post #26 of 27 Old 10-24-2009, 12:35 PM
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A good binaural recording designed to be played from headphones can be absolutely amazing.

Yes but they are rarer than hen's teeth. I was speaking about mono and stereo sources.

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post #27 of 27 Old 10-24-2009, 05:51 PM
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I think the first thing is to determine if this is a modal response or a local problem. One easy way to do this is to select a location where the 270hz signal is strong, then look at the waterfall plots - If it is modal it will ring, if not it will not.

Waterfall plots provide a good way to distinguish between a 1/4 wave notch and a modal notch. If it's a 1/4 wave notch, it will only be visible in the first portion (perhaps 100 ms, depending on frequency) of the plot. It will rise back up after this. This is because a 1/4 wave notch is due to phase cancellation between the direct and first reflected sound only. After that relatively short period of time, it is gone. A modal notch, on the other hand, will build up over a few cycles and hang on to the end.

When looking for 1/4 wave cancellation notches, set the waterfall time duration to a small number. Say, no more than 200-300 ms. Set the FFT window size to something relatively small -- no longer than the time range for the whole waterfall display. This reduces frequency resolution, but that can't be helped. Greater resolution in time necessarily means less resolution in frequency. This is just a fact of signal processing called the Time-Frequency Uncertainty Principle (not the same beast as the quantum mechanical Uncertainty Principle! ). You may have to play with the 3D display angle to get a good view of the back -- the earliest time.

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