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Author Topic: Who measures the measurement mic?  (Read 5344 times)

bruno putzeys

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Who measures the measurement mic?
« on: July 31, 2010, 03:08:14 pm »

I have been looking round the net for information on how to measure the free-field response of a measurement mic. My old Earthworks came with a chart but it was done using a B&K reference mic. So I went to look for information on how to do a "primary calibration" myself. Well. Thousands of posts from people saying "use a spark gap", not one from someone who'd actually done it (a few say they did but did not post the result which makes one wonder).

So I had to work out something for myself and I hope posting it here will, some time in the future, save someone else a few hours of work.

Step 1: Construct a spark gap. On the interwebz they say you need one great whopping spark (some even advocate a gun, oblivious of the fact that guns are bigger than a quarter wavelength of 20kHz), but that sounds odd to me because how likely is the microphone going to remain linear under those conditions?
So, make a triggerable circuit that can produce small but precisely timed sparks. The circuit I used is hardly elegant, but it just had to work and I was too lazy to get up to fetch anything that wasn't already floating about the bench. Anyone else would probably use a thyristor.
Step 2: Synchronise a trigger with the sampling rate you're going to use to record the signal. I used the sync output of an AP but you can do it with a sound card too. Make a square wave signal which you play through a squarer to trigger the spark gap and record the result synchronously.
Step 3: Record and average a lot of impulses.
Step 4: Correct 6dB/octave. A spark creates a succession of an explosion and an implosion, so you get a 6dB/oct rising response.
Step 5: I got a DC shift after the impulse, indicating that the mic or the preamp became non-linear. This confirmed my suspicion that loud sparks will play havoc with most mics. A minor non-linear correction between steps 3 and 4 remedied that. Since the mic is fairly flat any discrepancy between the correction and the actual error source will not significantly affect the result.
Step 6: Use just the anechoic portion. Since we're calibrating a measurement mic, errors will only affect the top end of the frequency band, so the impulse response is short. You do not need a long impulse response to be accurate. This is why I could do my test just above the lab table.
index.php/fa/15173/0/
All in all the whole exercise took 4 hours from idea to result. About as much as the time I wasted looking round the web.
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johnR

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Re: Who measures the measurement mic?
« Reply #1 on: August 01, 2010, 03:23:54 pm »

Useful info. Thanks for posting.
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Thomas Jouanjean

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Re: Who measures the measurement mic?
« Reply #2 on: August 02, 2010, 10:38:06 am »

So, when can I drop by and get my mics calibrated at Mr Putzey's (tm) lab  Very Happy

Cool tool!
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Thomas Jouanjean
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bruno putzeys

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Re: Who measures the measurement mic?
« Reply #3 on: August 02, 2010, 11:27:10 am »

Well do warn me in advance when you're on your way because I don't always have all the necessary items in the house.

By which I mean beer or course. Technical stuff is always available.
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Thomas Jouanjean

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Re: Who measures the measurement mic?
« Reply #4 on: August 02, 2010, 01:33:41 pm »

Deal.
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Thomas Jouanjean
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Larrchild

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Re: Who measures the measurement mic?
« Reply #5 on: August 20, 2010, 01:24:14 am »

The gated pulse I use for FFT tests seems to like about 1 microsecond to get the highs and lows flat. Is this similar?
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bruno putzeys

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Re: Who measures the measurement mic?
« Reply #6 on: August 20, 2010, 06:01:14 am »

It's shorter than that, which may explain why the electronics got nonlinear. 6dB per octave out to a couple of hundred kHz = loud...
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Larrchild

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Re: Who measures the measurement mic?
« Reply #7 on: August 20, 2010, 07:27:45 pm »

I notice constant directivity horns require a 6dB/ oct boost. Stick it in the throat of a CD Horn, lol.
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Bogic Petrovic

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Re: Who measures the measurement mic?
« Reply #8 on: September 24, 2010, 11:06:26 am »

lol... i post reply in different thread about needs for precise compensation files for measurement microphones... but you already find a way  Very Happy
Thanks for a recipe Smile

This is a way to measure relative frequency response, but how to  accurate measure microphone sensitivity?

For low end response measurement (below 500Hz) there is a method described by Alex Khenkin from Earthworks and practicaly released by Dr. Ivo Mateljan in   http://www.fesb.hr/~mateljan/arta/AppNotes/AP5_MikroMeasCham ber-Rev03Eng.pdf

regards

boggy

bruno putzeys

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Re: Who measures the measurement mic?
« Reply #9 on: September 24, 2010, 12:32:23 pm »

The most reliable way of calibrating a microphone's absolute sensitivity is a pistonphone. It's pretty much what Mateljan does, except that the piston is driven by a rotor with a precisely defined ellipse-shaped actuator attached. This insures that the exact excursion is directly determined by the construction, instead of calculated electroacoustically.
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Bogic Petrovic

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Re: Who measures the measurement mic?
« Reply #10 on: September 24, 2010, 01:03:46 pm »

bruno putzeys wrote on Fri, 24 September 2010 18:32

The most reliable way of calibrating a microphone's absolute sensitivity is a pistonphone. It's pretty much what Mateljan does, except that the piston is driven by a rotor with a precisely defined ellipse-shaped actuator attached. ....


Thanks! Smile

franman

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Re: Who measures the measurement mic?
« Reply #11 on: February 08, 2011, 08:08:51 pm »

Great stuff Bruno.. wish I was in your neighborhood.. I'd be by tonite with the brews... Thanks!

FM
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