Originally Posted by jfeva0049
could you please explain why the air gap is so important? thanks.
the material we are using is a porous (velocity-based) absorber.
a sound wave is forced through the insulation and energy is converted into heat (which reduces the amplitude, which reduces the energy in the wave as it reflects off the boundary and back into the room). less energy reflected back into the room, less interference it has on the original signal and other reflections/room modes/etc.
therefore, to be most effective at absorbing energy, it needs to be placed at areas of high particle velocity in order for maximum absorption. a sound wave has max particle velocity at 1/4wavelegnth. as a wave approachs a boundary, it compresses (builds pressure) and velocity goes to zero. velocity then = 0 as pressure = maximum --- and the wave reflects off the wall and travels back in the opposite direction. think about a pool/billards table. as a ball is approaching the rail, velocity slows (and pressure builds) until the ball comes to a complete stop (pressure=max and velocity =0) for a very brief moment, and then reverses direction (bounces off the rail).
so, right at the wall/boundary, pressure is max and velocity is zero. that is why porous (velocity-based) bass traps are not effective when placed right against the wall. they need to be placed at areas of high particle velocity. now, for LF waves this is difficult because the wavelengths are so long. this is why the bigger the air gap, the more effective the absorption (as you space the insulation away from the boundary, you're moving it closer to a particular 1/4wavelength point where velocity is higher, and thus the absorber is more effective.
however, for specular energy (eg side-wall early reflections), the wavelengths are much shorter than LF bass waves. for higher frequencies, where wavelength is so short, the 1/4wavelength is usually within the thickness of the panel. but for example 700hz has a wavelength of 19.37". the 1/4wavelength of this (19.37/4) is 4.84". so in order for the absorber to be most effective, it needs to be spaced 4.84" from the wall.
treatments cannot just be blindly installed. they need to be placed at areas of high effectiveness. by adding an air-gap, you're spacing the insulation away from the boundary to areas of higher particle velocity for the lower specular frequencies. and the air-gap is free!
now, there are also pressure-based bass traps, and those work by being placed at areas of high pressure (vs high velocity) - like right up against the wall. however, these are quite difficult to build and have limited effective freq. range. the porous velocity-based bass traps are broadband (absorb a wide range of frequencies), and thus are quite effective for relatively little money/work.
Originally Posted by jfeva0049
also in the future it would not be hard for me to add another 2" to each trap, so that there would be a total of 6". your thoughts ? thanks.
for the corner bass traps (for LF modal issues) - the thicker and the more sq. area you attack the better. there is much more energy in the LF waves so it is more difficult to attenuate.