fredk, I learned that having space was ideal behind the trap, perhaps contrary to what's been said here (?). That the bass waves get attenuated three times before reaching your ears: once thru the trap, a bit of absorption at the actual bare wall, and the third time coming back thru the trap before arrival at the ears.
This statement is half true, but I feel it could use some clarification, so that the casual observer it not led astray.
Let’s break it down simply by just looking at panels on a flat wall first. Say that you hang a 2in panel on the wall and compare it to a 2in panel that has a 2in air gap between it and the wall. The panel with the air gap will perform better based on the basic principal that you’ve stated.
Now if you take the 2in panel with air gap and compare to a 4in panel flush on the wall you will find that 4in flush panel will out perform “2in+ gap” below 315 Hz. The OP is concerned with bass so this is an important point.
Even though there isn’t an “air gap” with the 4in panel the same principals apply. The sound is still traveling through the material and hitting the wall and then returning to the room through the material again. In the lower frequency range the extra 2in of material is providing more “resistance” to the sound’s movement that than the equivalent amount of free air used in the gap model.
In the bass region, more material is king.
But when is come to the increased cost of more material, the increased cost of shipping more material, and the extra effort in mounting the extra weight of more material one often finds that the gains in absorptive performance do not always warrant the extra cost.
In short, an air gap extends and enhances a panel’s low end performance but a panel equivalent in thickness to panel+air gap will perform slightly better when it comes to bass.
