quarter wave vs. room modes and implication for bass traps

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?

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

Steve

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?

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

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.

Steve

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.)