R/E/P > Acoustics in Motion

Is there a way to check a bass trap?

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Geoff Emerick de Fake:
What about putting a speaker in the bass-trap and exciting it with a sinewave?
It should allow verifying the Helmholtz tuning.
I know it can be totally impractical, though.

I don't want to 'undo' a group of well constructed bass traps.  I was just hoping there was an easy method of checking without getting into big bucks, special gadgetry, or heartbreak to answer my wonderment.
Keep those idea's coming...someone may hit one outta the park!


The notion of in situ measurement is very attractive IMHO. Clearly the intensity probe is the way forward. I wonder do the elements need to be that accurately matched?
The decay methods don't seem to be that accurate so I don't think the competition is strong here. Bruno, I might be able to do this experiment. I have omnis and fig8's.
I also have an undamped Kick drum and a Signal Generator. Can you suggest how I should proceed?

Another approach. I use Labmeter from rustykat.com for many purposes.
It can help get a snare drum or tom to an actual note. When I thump the end wall in my CR I can read the primary axial 34Hz quite reliably. So Joe, if you have a Mac I recommend that software for all sorts of jobs. Apart from thumping traps to determine their resonant frequency it can listen to a bass line and in real time show the frequency of the booms or notes that stick out. Remarkably useful for a few bucks.


Bogic Petrovic:
One of methods for measuring acoustic impedance...

Acoustic impedance is defined with:
Where is:
z – Acoustic impedance
p – Air pressure
u – Air volume velocity

Although pressure measuring are simple and straightforward (directly, using microphones)...
measuring volume velocity to get relevant results, aren't simple.
With two microphones, we can estimate:
-Volume velocity:

Where ρ is density of air and d is distance between microphones.

Estimating volume velocity is difficult if we have loosely calibrated and different microphones because
we will do integral of pressure difference that come from different microphones itself,
not only from measurement, and that is the reason why we need very closely calibrated microphones.

Geoff Emerick de Fake:
So, what about using a loudspeaker and measuring its impedance in real-time? In order to make this sensitive enough, the loudspeaker should have a very low Fs, be mounted in a large closed box and its membrane put as close as possible to the trap.
It may be the most practical solution...


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