| Tomas Danko wrote on Sat, 18 December 2004 13:20 |
Dude, be very careful whenever being anal anywhere near long and sleek measuring microphones ok? I sure hope there won't be an unfortunate mishap regarding that straight up and down axis thing.
|
Is that why they put a MALE connector on the thing? Thanks for the warning, Thomas.
I received this excellent response from David Josephson on the Pro Audio List and I'd like to cross pollinate it here:
Bob Katz wrote:
>Hi, everyone. Santa bought me a Chinese measurement microphone, 1/2"
>capsule, from BSWA Technology Co. Inc. I hope Santa is just as good for you!
>
>BSWA seem pretty sophisticated, so it's not nice to say "it's a B&K 4006
>knockoff", but since the model number includes 4006 there is no coincidence
>there.
>
>
We have been looking at BSWA for some time as an alternative for
measurement mic capsules. At the moment they don't meet our needs but
they are improving and are certainly better than the mics based on
jellybean plastic diaphragm capsules. They are a commercial spinoff of a
research academy in Beijing and have some interesting ideas -- whether
this can translate into a reliable commercial product remains to be seen.
>The microphone is individually calibrated, and the curve supplied labelled
>"0 degree free-field response" is ruler flat to 1 kHz (awfully suspicious)
>and has a rise in a slightly peaky shape to +0.75 db at about 9 kHz. Flat at
>15K, and -2 dB at 20K (strange).
>
>
That's not suspicious or strange at all, that curve is believable for a
low-tension 1/2" capsule resonant at 9 kHz, except that the response at
20 kHz may in fact drop lower than -2 dB.
>Do you think this was taken with the Protective grid on the capsule or not?
>Should I measure with or without the grid? Should I point the microphone at
>the loudspeaker or straight up and down? My guess is straight up and down
>(90 degrees to the source) because of the rising response on axis.
>
>
You should ask BSWA, but I would expect that, as in most measurement
mics on the market, the curve is made with an electrostatic coupler and
"corrected" to actual freefield response with the grid on. You don't
want to use it at 90 degrees to the sound source unless you know what
the response is at 90 degrees -- use it on-axis and correct for the
curve that you have.
The only true calibration for a free-field measurement microphone is
free-field reciprocity, and it costs several thousand dollars to have
NIST do that for you to an accuracy of about +/- 0.2 dB mid band, to
about 8 kHz. Some makers (Microtech Gefell for instance, which we sell
in North America) use a differential coupler which produces a
measurement referenced to a calibrated standard mic, but most (including
B&K) use an electrostatic actuator, the data from which is fudged to
match the data that would be produced in freefield conditions.
Fortunately, absent a design or manufacturing defect, small
omnidirectional (pressure) mics are inherently flat within a small
fraction of a dB from their low frequency rolloff point to about 2/3 of
the resonant frequency. The low frequency rolloff point is set by the
vent behind the diaphragm and is probably not shown on your curve.
>My next question is do you think this is a "free field microphone" or a
>"pressure response capsule"? There is a correction needed of -0.3 dB for
>1/2" free field microphones with my ACO Pacific Calibrator. Hey, as long as
>I'm getting anal, I might well get that SPL RIGHT!
>
>
There is no such thing as a "free field microphone" as such. That term
is used to claim "flat" response in free-field conditions, which the
curve you have asserts. The Microtech Gefell measurement mics we sell
come with calibration curves that show the actual actuator response, and
what that translates to in both free-field and pressure conditions.
Microphones are available that are tuned for mostly flat response in
either free-field or pressure conditions. You would ordinarily use a
pressure mic only in a pressure coupler, for instance measuring the
output of hearing aids, or in a diffuse-field measurement, which
requires spatial averaging to be accurate.
You should confirm this with ACO Pacific, but their -0.3 dB correction
for "free field microphones" refers to *their* free-field microphones,
and may be different for those of BSWA, for instance. The compliance and
equivalent volume of the BSWA capsule may be different, yielding a
different result.
It's not "getting anal" to be careful about your measurements. What good
is a measurement if you don't know how accurate it is? The basis of all
the military, ISO, IEC etc. calibration standards is simply to make you
think about how sure you are about the measurement you're making. At the
end of the day you should be able to arrive at a number for the
measurement, and a number for how accurate it is, and how you know that
these two numbers are true in the worst case of drift and uncertainty.
--
David Josephson / Josephson Engineering Inc /
www.josephson.com
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Topic: Measurement microphones (Read 5235 times)