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Author Topic: Is there a way to check a bass trap?  (Read 7528 times)

Blas

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Is there a way to check a bass trap?
« on: September 22, 2010, 01:02:24 pm »

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.

Blas  
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Conjecture and theory are all well and good...but the proof comes out of the speakers - Fletcher
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Ethan Winer

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Re: Is there a way to check a bass trap?
« Reply #1 on: September 22, 2010, 04:02:49 pm »

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. Very Happy

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

Blas

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Re: Is there a way to check a bass trap?
« Reply #2 on: September 22, 2010, 11:19:04 pm »

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-

Joe
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Conjecture and theory are all well and good...but the proof comes out of the speakers - Fletcher
He who has said the pen is mightier than the sword, has never been in a pen & sword fight!- me

bruno putzeys

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Re: Is there a way to check a bass trap?
« Reply #3 on: September 23, 2010, 03:20:46 am »

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.
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Blas

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Re: Is there a way to check a bass trap?
« Reply #4 on: September 23, 2010, 07:34:50 am »

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!

Blas
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Conjecture and theory are all well and good...but the proof comes out of the speakers - Fletcher
He who has said the pen is mightier than the sword, has never been in a pen & sword fight!- me

bruno putzeys

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Re: Is there a way to check a bass trap?
« Reply #5 on: September 23, 2010, 08:25:47 am »

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 Very Happy

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?
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Ethan Winer

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Re: Is there a way to check a bass trap?
« Reply #6 on: September 23, 2010, 02:59:34 pm »

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.

--Ethan

Blas

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Re: Is there a way to check a bass trap?
« Reply #7 on: September 23, 2010, 04:32:59 pm »

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. Shocked

Blas

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Conjecture and theory are all well and good...but the proof comes out of the speakers - Fletcher
He who has said the pen is mightier than the sword, has never been in a pen & sword fight!- me

Bogic Petrovic

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Re: Is there a way to check a bass trap?
« Reply #8 on: September 24, 2010, 09:26:43 am »

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

regards

Boggy

bruno putzeys

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Re: Is there a way to check a bass trap?
« Reply #9 on: September 24, 2010, 12:28:59 pm »

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 Smile

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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Geoff Emerick de Fake

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Re: Is there a way to check a bass trap?
« Reply #10 on: September 28, 2010, 11:26:09 am »

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.
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Blas

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Re: Is there a way to check a bass trap?
« Reply #11 on: September 28, 2010, 03:15:02 pm »

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!

Joe
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Conjecture and theory are all well and good...but the proof comes out of the speakers - Fletcher
He who has said the pen is mightier than the sword, has never been in a pen & sword fight!- me

DanDan

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Re: Is there a way to check a bass trap?
« Reply #12 on: September 28, 2010, 07:37:28 pm »

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.

DD
www.irishacoustics.com
www.soundsound.ie

Bogic Petrovic

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Re: Is there a way to check a bass trap?
« Reply #13 on: September 29, 2010, 10:32:48 am »

One of methods for measuring acoustic impedance...

Acoustic impedance is defined with:
z=p/u
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:
-Pressure:
p=(p1+p2)/2
-Volume velocity:
index.php/fa/15504/0/
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

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Re: Is there a way to check a bass trap?
« Reply #14 on: September 29, 2010, 01:38:25 pm »

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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