Ethan seems to be spread pretty thin these days (wonder when he sleeps :=) but I'll see if I can clarify a bit -
First, the way bass traps work is that they present an "acoustic resistance" to sound waves. This is the function of the rigid fiberglas inulation board. Moving air (sound wave) can't get thru the insulation board as easily as it can move in free air, so the energy it uses to pass thru is partially converted to heat. Less sound energy, less sound.
Every sound has its own wavelength, which is expressed in (normally) either feet or meters; in feet, the formula is Wavelength=1130/f, with 1130 being the speed of sound in feet per second (approximate) at sea level, and f being the frequency in Hertz.
A wall, floor or ceiling is referred to as a "boundary" in acoustics - if we consider these boundaries to be impenetrable for sake of discussion, then at any boundary the air velocity from a sound wave striking it perpendicularly, will be ZERO, while its sound PRESSURE will be maximum.
If you move away from the wall a distance of 1/4 wavelength of the sound (just one frequency for this example) at that point, the sound VELOCITY will be maximum, and the sound PRESSURE will be zero. This is important, because
The only place it does any good to place an acoustic resistance in order to trap sound, is where there is air movement, or velocity. The more velocity, the more sound energy is converted to heat.
This is why absorbent traps need air space behind them - the more distance included in the trap cavity, with all other things equal, the lower frequency the trap will attenuate. The closer to 1/4 wavelength the absorbent is placed from the boundary, the better that frequency is absorbed.
Using the wavelength formula, if we plug in 100 hZ we find that the wavelength is 11.3 FEET ?!?!! sooo, 1/4 wavelength would be 2.825 feet, or nearly 34 inches. That would be the optimum depth of a trap that would work at 100 hZ - just one example, so you can see some of the problems of making low bass traps.
One of the benefits of a corner trap is that the depth is varied, so the trap works at a broader range of frequencies.
Keeping in mind that in order for the trap to work, the sound wave must pass THROUGH it, not AROUND it, then it makes sense that one important criteria is that the absorbent material is SEALED around the edges so the sound must pass THROUGH it.
The frame is mainly there to support the absorbent, and give you a place to tack a cloth cover, and for looks. The important part is the type and thickness of absorbent, the depth of the cavity behind the absorbent, the thoroughness of seal,and somewhat the angle of incidence that the sound strikes the absorbent. Perpendicular gives more absorbency than a glancing blow, for example.
Most if not all this information can be found in F. Alton Everest's Master Handbook of Acoustics, available at Amazon.com for about $25. Well worth the $, if you want even more detail... Steve
Hehehehheee