So: SierraMikeBravo, I thought the bass traps were triangular or put across corners so they could interact with the higher velocity movement. And, I thought as the energy of the sound was absorbed and converted to heat, the wavelength no longer mattered for the portion absorbed because the pressure goes away with the absorption of the energy. This, of course, is only true for the portion of the energy that the trap can absorb.
All of my comments below refer only to absorption of sound energy in air.
There are two commonly used mechanisms to absorb sound energy: (1) velocity type absorption; and, (2) diaphragmatic or pressure absorbers. For a velocity type absorber to function effectively, that device must be placed in a location where the velocity of the air molecules are at their maximum velocity. For a pressure type absorber to function effectively, that surface must be placed at a location where air molecule velocity is zero (or near zero) and pressure is at its maximum.
If we imagine an air molecule striking a wall, at the instant it strikes the wall, its velocity is zero and pressure is high. However, sound propagation is cyclic in nature and cycles through rarefication and compression. At maximum rarefication and maximum compression, velocity is zero and pressure at its maximum.
To achieve maximum effectiveness in a velocity absorber, that absorber should be placed such that it is located at the 1/4 wavelength of the frequency(ies) for which we wish to reduce energy. Velocity absorbers are characterized by fiberglass batts or fiberglass panels. At 80Hz, the wavelength is 14' and the quarter wavelength is therefore 3.5'. If a dimension in your room is any multiple of 14', then a velocity absorber would need to be placed 3.5' away from a wall surface to be effective at 80Hz.
A simple form of pressure absorber is a fiberglass batt with paper backing. If you imagine a trampoline, the paper backing is the surface of the trampoline and the fiberglass is the spring. Now, imagine jumping off the roof of your house onto your trampoline. The trampoline will indeed absorb some of the energy from your fall; but, the spring is going to release some of that energy back throwing you back into the air. The spring action is an issue in the design of diaphragmatic type absorbers since they can become "speakers".
One of the characteristics of modal frequencies is they, by definition, are always at their minimum velocity and at maximum pressure at the wall surface. In this situation, a pressure type absorber is more effective at the wall surface. A velocity type absorber would need to be placed at a 1/4 wave length distance from the wall surface. (At 80Hz, that's 3.5'.)
In the low frequency arena, the bigger (not only) sound quality problems are with modal frequencies (and some would argue that in a small room, it's the first three axial modes which are the real killers). [Note: in small rooms modal problems dominate from approximately 300Hz downward but generally present the biggest audible problems below 100Hz.]
The reason we typically find vendors suggesting their bass control devices be placed in room corners is because it is at the room corners where all the axial modes are at their highest pressure/lowest velocity.
The statement made by SMB is true in that general purpose bass traps/absorbers are not very effective below 100Hz. For them to work at the velocity level, they'd have to be 14' from a wall (assuming a 20Hz modal frequency). For them to be effective as pressure absorbers, they'd need to be at the wall surface (for modal frequencies). Neither is very practical. When placed any distance from a wall, their effectiveness would vary by frequency based almost entirely on their placement (a 6" difference can radically change the frequencies upon which you'd see the greatest effectiveness).
Multiple tools need be utilized to resolve low frequency sound quality issues in small rooms. These tools would include electrical as well as mechanical.
As to the question about nulls/peaks existing in multiple areas of the room, a couple of points. First, we don't give a tinker's damn about sound quality in areas of the room where no one is sitting. Secondly, when it comes to LF and modal issues, it is the amplitude of the peaks/nulls which are audible and cause us grief. As the energy to that frequency increases, the delta between the peak and null gets larger and more audible. The converse is true as well. As energy is absorbed, that delta decreases and you have more consistent response (I didn't say good, I said "more consistent"). The use of differential parametric EQ, is one means be which modal frequencies can be 'resolved' without sucking the life or energy out of a room.
Among the challenges for a well performing room is to have all frequencies not only decay at the same rate (in the seating locations), but all frequencies to have the same relative SPL. This is a significant challenge since any form of pressure, or velocity type absorber will affect different frequencies differently just based on their position in the room. Adding bass trapping devices to a small room is very helpful. It cannot, however, be a helter skelter placement nor simply putting fuzzy stuff in the corners. There will be a point at which the treatment ceases to be helpful and begins to work against your objective.