Editor’s Note: The author, Ethan Winer, is a principal of RealTraps, which designs and manufactures a variety of acoustic treatment products.
It’s well known that an air gap behind an absorbing panel extends its effectiveness to a lower frequency. Most acoustic panels are velocity absorbers, which means they act on the wave velocity as sound waves pass through them and are converted to heat.
A car traveling 100 miles per hour toward a brick wall has plenty of velocity, but once it hits the wall the velocity is zero. Likewise, there’s little wave motion at a room boundary, so velocity absorbers are more efficient when spaced a few inches away from a wall or ceiling. Bass traps are often mounted straddling a corner, where an even larger triangle shaped air gap forms behind them.
Years ago someone in an acoustic forum claimed that bass traps straddling a corner need to be placed tight into the corner, to prevent “leakage” around the edges. More recently someone else claimed that air gaps larger than one inch are not useful unless the panel edges have “skirts” around them to, again, create an airtight seal. So I decided to test these claims and also test another bass trap placement method that people have asked about many times.
The official way to assess the performance of acoustic products is by measuring them in the reverb room of an accredited acoustics lab, as explained in this article that shows how RealTraps products are tested. As absorbers are added or changed, the reverb decay times are measured at multiple frequencies, and from that the amount of absorption is calculated. But such tests are extremely expensive, and I wanted to compare seven different mounting methods.
For comparative tests – which is better, A or B? – these tests can be done successfully in a home setting. You can’t measure absolute absorption amounts accurately, but with a large enough sample size you can definitely see performance differences and trends. To do acoustic tests with confidence at home requires an empty room large enough to have plenty of reverb, and resonant mode frequencies low enough to assess bass traps. The room must also be very quiet, without the sound of nearby passing traffic or the noise of air conditioning blowers.
I have an extra room in my home that’s 19.5 feet long (29 Hz), 13 feet wide (43 Hz), and 8 feet high (71 Hz), so that’s what I used for the tests described following. The photo below shows the basic setup with a calibrated microphone feeding a Windows 10 laptop running the Room EQ Wizard software. You can hear this room’s natural reverb when empty in this MP3 file.
The first thing I tested was how an air gap of varying depth affects absorption at low frequencies. I tested five 2×4 foot RealTraps MiniTraps, one slightly larger MondoTrap, plus three 2×2 MiniTraps, for a total of 61.5 square feet of absorbing surface. I tested these nine panels first flat on the floor, then raised up 1-1/2 inches off the floor, then raised up 4-1/2 inches. Cat food cans served as perfect spacers, and with three hungry kitties to feed I always have several cases on hand!
One big feature of testing in a home-sized room is to obtain data for frequencies much lower than most professional labs accommodate. In this case, rather than try to compare absolute absorption amounts, we simply assess how quickly the room’s resonant modes decay using a waterfall plot. In this type of graph the “mountains” come forward over time, and the height of their peaks show the response peaks of the room. The more sound that’s absorbed by the panels, the faster the ringing decays.
The animated sequence of waterfalls that follows show how absorption increases as 1) the nine panels are placed in the room flat on the floor, then 2) the panels are raised to create a 1-1/2 inch gap, then 3) raised further to 4-1/2 inches. With each reduction in decay times the peaks at most frequencies are reduced in level, and the trails don’t come forward as far because they decay more quickly. So clearly, each increase in the gap size improves absorption at most frequencies.
The next thing I wanted to compare is one trap straddling a corner versus two traps flat against the wall, parallel to the wall. It’s common to have a door in a corner, making it impossible to straddle the corner which would have been ideal. So we often recommend using two traps in such corners, with one on the wall and the other on the door. If you can space the traps off the wall three inches using Post Base Mounting Kits, all the better. In order to obtain enough of a difference three such sets of traps were used for the next set of tests.
Since it’s common and recommended to space bass traps and other absorbers off the wall, I also compared three traps straddling versus six having three inches of air gap. Here’s the setup showing six traps parallel to the wall or floor with a gap. The vertical traps are also spaced out from the wall behind them:
With a 3-inch air gap behind them, the paired traps are now much better than one trap straddling:
The last test compares straddling bass traps tight into the corner versus allowing some air “leakage” around the edges:
I wasn’t able to prop the traps up on cat food cans, so I used single-gang plastic electrical boxes below and behind the three traps shown above. Nails pre-attached to the boxes conveniently stuck into the carpet and kept the boxes from sliding around. In this case it turns out that allowing air to leak around the trap edges does reduce their effectiveness a little at most frequencies.
So there you have it — I believe these tests debunk the myth that air gaps larger than one inch aren’t helpful for panels mounted flat on a wall or hung below a ceiling. But the other claim that bass traps straddling a corner work better when sealed air-tight is proved mostly true. Of course, even with a gap, bass traps straddling a corner are still very effective.