[bpape]

Let's clarify a few things here.

First of all, a soft absorber like 703 or mineral wool is a velocity absorber, not a pressure absorber.

Second, the air behind it means zero - sorry. When you move it out, you're getting the leading edge of the panel farther from the hard surface where velocity is higher (so a % reduction is more significant) and you're getting closer to the optimal place to absorb a wave which is at it's quarter wavelength. THAT's what matters. You can actually take a very thin piece of material and hang it at the 1/4 wave of a specific frquency and filter that frequency pretty narrowly.

That said, the amount of material it must pass through also has an impact as that puts more of a specific wave inside the absorbtion at one time. The only 'pressure' component of what's happening here is the natural destructive wavefronts coming toward and away from the wall at the same time and colliding.

To get a true pressure absorber, you need a cavity that's sealed air tight. Yes, you can do tubes with caps on them and force a VERY VERY SMALL amount of additional pressure due to the resistance of air leaving the tube but it's really inconsequential as far as science can prove today.

Bryan