Why Should Bass Traps be Mounted 2-4 Inches off the Wall?
One of the things that we recommend our customers do with their Bass Traps / Acoustical Panels is to mount them 2-4" off the wall. The absorption is 4 times higher at 500hz and below.
This happens because the sound goes through the panel, travels to the wall bounces back and passes through the panel again offering 2 opportunities for absorption. We can't take the credit for that great nugget of knowledge, it comes directly from Bob Hodas (the best in the biz).