R/E/P > Acoustics in Motion

Is there a way to check a bass trap?

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bruno putzeys:
Trouble is I've never tried this myself, because this thread is the first time I've thought about it. I might actually be wrong on a few counts, but since acoustics isn't my schtick I'm allowed to be wrong

Anyhow. With one omni you can measure pressure. Take a second omni placed close to it and if you subtract one from the other you measure pressure differential ie. acceleration along the axis of both microphones. Integrate this signal and you get velocity.

Acoustic impedance is pressure over velocity, and at the resonant frequency of the bass trap, acoustic impedance found near it should be real, meaning that pressure and velocity would be in phase.

So if you measure pressure and velocity near the bass trap while some subwoofer elsewhere in the room does a frequency sweep, you should be able to see at which frequency velocity & pressure are in phase, which would be the resonance frequency of the bass trap.

At least I think this should work, I hope some of the real acoustic experts can chime in.

But I understand you're looking for something more practical. Is there anyway you can cover over, seal or otherwise reversibly incapacitate the bass trap?

Ethan Winer:
Blas wrote on Thu, 23 September 2010 07:34
These traps are well attached to the walls

You could cover them with heavy sheet rock to measure the room with and without them being active.


This is starting to get into serious enough efforts that I may have opened a Pandora's Box ...(sitting here, covering my eyes, briefly peeking thru the fingers) maybe I don't need to know.


Bogic Petrovic:
Bruno, your principle is ok theoreticaly, but we possibly have problem if we dont have (very) closely matched microphones ...
I think that microphones matched only within +/-1dB isn't well capable to do this. Or at least we must have well matched compensation files for microphones, or, at least, resources to make this precision compensation files.

bruno putzeys wrote on Thu, 23 September 2010 14:25
But I understand you're looking for something more practical.......

.. and there are something more practical, but very, very expensive... BK sound intensity probe  
and adequate measurement software. This system can measure impedance "in situ" pretty well.

Something other capable to measure acoustic impedance can be Microflown Technologies probe and software.... also very expensive.

That's all of practical solutions AFAIK



bruno putzeys:
Yup, *very* precise matching is key. I had written about intensitity probes before editing it out because I'd rather talk about the principle than start piling up terminology

Talking about Microflown. When they made their first microphone they had an AES booth hoping to sell the actual devices, indicating that it cost below $1. Apparently the demand was too low for this to be economically viable so they had to switch to selling complete measurement equipment. In spite of the low cost they simply cannot afford to sell the mic element separately (they won't, however hard you try). I do not blame them, but this is ecomomics at its most perverse.


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