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?
Originally Posted by Ethan Winer
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!