Creating an air gap between the 703 and a wall does not "trap" anything. The reason people do it, is to increase the distance from the face of the 703 and the wall, as the reasoning is this. 703 absorbs because it is a fiber material which has close nit "intersticies", which creates a RESISTANCE to the movement of air molecules. It is this resistance that converts the sound wave energy to heat, hence absorption. Since the velocity of the wave is greatest at 1/4 wavelength, the distance from the face of the absorber to the wall, determines the lowest frequency which will be absorbed. Since velocity is zero at a wall, there is no need for the absorber to be against the wall. Spacing it 1/2" to an 1" is recommended here often. A 100 hz sound has a wavelength of 11.3 feet. Therefore, 1/4 wavelength equals about 2.825 feet, which explains why resistance absorbers absorption drops off dramatically. If you only have 4", with 1" airgap, figure it out. What 1/4 wavelength equals 4.5". The the real eyeopener is doing the math. This will tell you the truth, instead of guessing. The equation is:the speed of sound
1130 feet per second(close enough)
divided by the frequency equals the wavelength.
Divide that by 4 and it will tell you the 1/4 wavelength.
But then, how do you know how MUCH you are absorbing in SABINES? How much absorption do you need? 1 square foot of open window equals 1 sabine.
fitZ