Bass Trap Testing, The Sense and Nonsense

For some time now I have been developing a new series of bass traps that will ultimately result in (15) new models that will cover a wide variety ofBanges and Q's to satisfy most acousticians requirements. This test regimen began a little over a year ago at NWAA Labs in Elma, Washington.

Some might ask why we are doing this at NWAA instead of our own reverberation chamber at RealAcoustix. The reason is simple and complex at the same time. First of all, it's important to understand how absorption tests are done, what standards are used and what works and, more importantly, what doesn't work.

Due to size i.e. real estate, limitations, many manufacturers test their bass traps in impedance tubs using ASTM E384 or the ISO equivalent. I won't go into how this is done other than to say I do not believe in impedance tube testing except (possibly) for frequencies above 150Hz. It does not mimic real world use and has proven to be very inaccurate at low frequencies.

Our lab at RealAcoustix is a standard reverberation chamber designed to operate accurately down to 100Hz. Below that, the modes become too difficult to obtain a truly accurate measurement. Therefore, we may experiment in our lab for reference, but the actual testing that you see is done at NWAA Labs, which chamber is verified down to 40Hz. It is the largest commercial reverberation chamber in the U.S.A.

Reverberation chambers more accurately reflect real room responses than an impedance tube could ever hope to do. Additionally, many manufacturers modify their designs to accommodate the impedance tubes responses making the test data, at best, questionable. One manufacturer even modifies the diaphragmatic plate in testing to, ostensibly, even the field of response of the units. The question is, if it's not the model being sold, why are you testing a different model or models?

Another questionable design theory is that you can obtain lower frequency response out of a unit design that is less than the physical size of the frequency itself. I question that theory inasmuch as the test methods used, again, do not replicate real world usage.

Indicating that a 2' x 2' bass trap can achieve a 40Hz absorption coefficient by itself is just a bit disingenuous when you are stacking 18 units together to get the result. That is, of course, unless you plan to use 18 units stacked together in your room.

The idea is to be smart about how we test these units in order to exact real results that can be replicated in the applications in which they are used. For instance, when we test our bass traps, we use what is referred to in ASTM c423 (the standard absorption test) as the "J-Mount. They are generally placed in the corners of the reverberation chamber, separated from each other, in order to more accurately represent real world use.

By doing this, we imitate the way bass traps are commonly used in rooms. We also don't artificially take advantage of the coupling of multiple units to achieve a result that does not tell us what the individual bass trap actually looks like in reality.

Our clients and customers respect the "Real" in RealAcoustix. It's why I named the company that. The reality is that we live in a world where acoustics has a lot of practitioners that bend the truth to their own agenda or perceived product application.

Acoustics is physics. Physics is a science. When we forget that, or don't apply it logically, we might as well be selling snake oil out of the back of a covered wagon.

Richard L. Lenz/President ASA/ASTM/AES

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