Mounting acoustic panels with a variable air gap

I have been progressively “deadening” my project studio. This article provides an easy solution to a recent challenge: mounting acoustic panels to the wall with a variable air gap behind each panel. A secondary goal is to minimally destroy the walls in the process.
Why would I want a variable air gap? I will answer that in two parts: why have any air gap, and why make it variable.

Why would we want an air gap at all? Why not mount the panels flush with the wall? The reason is that low-frequency absorption is improved when the panel is away from the wall.
RealTraps
When mounted on a wall with a spacer, sound gets behind [an acoustic panel] so its rear surface can also absorb. This air gap increases absorption as much as fifty percent, and also extends absorption to lower frequencies when compared with flat wall mounting. When placed straddling a corner the absorption is even greater, especially at low frequencies.

Ethan Winer of RealTraps gave me another data point: spacing a MicroTrap 3'' off the wall extends low-frequency absorption by one octave.
My next problem is that the wall in question is opposite a 9-foot sliding glass door — a perfect environment for standing waves.
“Standing waves” happen when a soundwave bounces back and forth between two parallel surfaces. The waveforms of the direct and reflected signals combine, causing peaks and dips that can vary by tens of decibels. In practice, the presence of standing waves means that moving a microphone a few inches creates an effective EQ boost or cut greater than can be corrected in the recorded track. This problem marred the results of the sax mic session of the $60,000 ribbon mic test, in which we later found up to 7dB difference at 400Hz, between two mics recorded simultaneously.
Therefore I was interested in mounting these panels at an angle to the opposite wall, in hopes of reducing the area of parallel surfaces in the room.
The importance of acoustic treatment
Before we go farther, I want to step back to cover the idea of acoustic treatment from a more basic perspective. If you’re new to home recording, and you’re recording acoustic instruments in a home office or bedroom, you should strongly consider investing in acoustic treatment. Untreated small rooms usually sound awful. Room reflections can kill a track.
Further, it is a myth that room reflections can be corrected by equalization. Check out Randy Coppinger’s notes from an AES session on low-frequency damping:
Randy Coppinger
Standing waves linger past the end of the original sound, a la reverb. My take away: Trying to control acoustical room response w/ EQ ignores the time delay (“after ring”) of the resonance. Treat acoustics not signal!

In other words, even if you know you have a dip or boost at a particular frequency, chances are the reflections causing that dip or boost are hurting your recorded tracks in other ways.
In short, small rooms are a problem. The smaller the room, the bigger the problem. As a gag, I once recorded some vocals in a toilet stall. No pun intended, but the results really did sound like crap.
Therefore my bias when recording in a small room is to treat it. The room need not be lined wall-to-wall in acoustic foam, although if it were, it would probably produce better sounds than an untreated drywalled bedroom with a hard floor. I think the ideal is somewhere in the middle, with lots of corner bass trapping and enough wall trapping to nuke flutter echo.
The Variable Air Gap mounting solution
My idea for mounting rigid acoustic panels with a variable air gap is pretty low-tech: it’s a fat wooden shim. It acts as a spacing block that both holds the panel away from the wall, and creates a variable air gap. This particular design is perhaps unique to the panels I was using (RealTraps Microtraps), but the general principles should apply to any rigid acoustic panel.
First I made a cardboard form. It measured 18'' wide, with one long side cut at an angle so that the short sides measured 3'' and 5''.
Using the form as a guide, I cut three matching boards from scrap wood (9/16'' redwood), then notched the angled edge (with a Dremel tool) to accommodate the wire-hanging hardware on the back of the MicroTrap. I painted the blocks to match the wall, and mounted some self-adhesive silicone feet on the straight edge of each board to prevent scratches on the wall.
The panels hang from heavy-duty picture hangers, rated at 10lbs capacity. (RealTraps’ MicroTraps weigh 8lbs.) This approach does no more damage to the wall than hanging a framed photo, thus being vastly superior to hanging foam panels with glue.
The standoff boards simply sit behind each MicroTrap. No special mounting is necessary; the boards are held in place by the weight of the acoustic panel. It would be easy to use Velcro to attach the spacer boards to the metal rail across the back of the MicroTraps, but there does not seem to be any value in doing so. In fact, the passive nature of this installation means it is trivial to remove these spacer blocks altogether. They can be inserted within seconds prior to a tracking session.
Audio tests
I hung the panels primarily for use during tracking, so the best test is a quick before-and-after recording. Before we get to the audio, I should mention that this room is already well-treated, so the “before” track is really not bad. You’ll hear a difference when the three new MicroTraps go up, but it’s more subtle than it would be if I was starting from bare walls.
First up is a loop: 4 bars without the new panels, 4 bars with the new panels, and a repeat. Listen to the space around the kick and snare; you can hear how it gets smaller when the panels are in place.
(Update: These audio tracks are mono overheads — one condenser hanging above the middle of the drum kit.)
To be clear, I do not at all intend to suggest that having room sound in a drum track is a bad thing. But I’d prefer to have the room sound in a room mic, not in my overheads. Otherwise, it’s like printing reverb to every track.
The second test is even more revealing. There are two clips below, one with the panels and one without. I won’t label them, but if you can’t tell the difference, then — good news — you don’t need to spend any money on acoustic treatment! But I think you’ll hear how much tighter and cleaner the “with panels” clip is.
And again, remember that even the “before” clip has RealTraps MiniTraps at all the wall/ceiling junctions, most of the wall/wall junctions, and two of the upper tricorners, plus a 16-square-foot cloud of 4'' Auralex Studiofoam above the drum kit.