R/E/P > Acoustics in Motion

Is there a way to check a bass trap?

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It has been a number of years (nearly ten)since completion and I can't run into the layout papers of our control room. There's a total of 6 floor-to-ceiling traps (mixed, some 3 inch, some 4 inch deep) and 3 six inch wide cotton batting burlap covered absorbers in between them. We incorporated Ethan Winter's different freq. designs, but was wonder of anyone knowing a way to actual measure what a given trap is really helping mop up?

This is beyond my scope of knowledge, but hope someone has a creative way of testing or checking the traps response.


Ethan Winer:
That's a tough question, and I don't have a good answer. Maybe one of the experts here can suggest something. I can tell you what I have tried that didn't work.

I once placed a front-firing subwoofer right in front of a wood panel trap, pointing at the trap. I figured whatever frequencies make the front wood panel vibrate, then that's where the trap is working. Alas, the panel vibrated readily at whatever frequency we played! What actually matters is the resistance the wood panel offers to the sound waves. This type of trap acts as a sort of "shock absorber" for sound waves.


Ethan, My friend...good to hear/see from you again.  I realized (too late) I added a "t" in your name.  Please forgive me.

Anyway, I've wondered for some time if there was a 'reasonable' method to check what each one is impacting, short of hiring 6 or 8 big beer drinkers to hug each one and then take room pink noise prints!  A little more science please...

I hope your business is going well, and if you're ever in ole' St. Louis on a given Monday, I'll request/invite you to speak at our local AES meeting.

So for now I'll continue my endless search-


bruno putzeys:
I've never tried this and this is the first time I find myself wondering about how to do this, but I should think that if one were to measure the phase angle of the acoustic impedance as close as possible to the bass trap, the resonance frequency should show up as transitions through zero degrees. So for instance you'd put an omni and an 8 in front of it and plot the phase shift between the outputs of both as you sweep the frequency of the source.

Both mics would need to have very good bass extension though. Fine for an omni, not so for an eight unless you were to use an intensity probe (2 spaced, matched omnis with an integrator).

Anything more practical than that probably entails measuring the room, carrying the thing out and measuring again. Taking it into an echo chamber is even better but at least you have a direct indication of whether it was effective at combating whatever it was supposed to fight.

Bruno, These traps are well attached to the walls, so removal would take an earthquake (no thank you, please). Regarding the mic array,and the number of in-place traps, I have to say I don't full understand the method you speak of.

But feel free to educate me better in this area!



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