John,
Welcome to the arcane world of sound measurement, which is only exceeded in
arcanity by light level measurement.

As Bob pointed out: The "official" method of calibrating consists of
generating a known sound pressure level (SPL) with a piston/cylinder tightly
coupled to the sound meter microphone. A converted weed whacker motor would
likely turn the sound meter into an air-propelled projectile, however,
rather than calibrating it. The old B&K (Bruel and Kjaer, not the cheesy TV
service equip manufacturer)"Gold Standard" calibrator consisted of a roughly
1" diameter piston with about a 1/6" stroke powered by a little battery
fired Mabuchi motor. The "cylinder" is about 1/2" long. You could probably
fake the mechanical stuff by using a little speaker coupled to a little tube
driven by a multivibrator. Make sure the sound level meter mike is tightly
sealed to the "cylinder" The calibrating of the calibrator is the hard part.
I suggest that you do a transfer calibration from a "known good" meter to
avoid all the mathematical intricacies.

Edward Gisske, P.E.
Gisske Engineering
608-523-1900
gisske@offex.com


> > I use a Radio Shack DB Meter for sound measurement of competition model
> > airplanes. It works pretty well for relative measurements but seems
> > occasionally be off by as much as 3db from the "official sources". A
> > commercial calibration device is available, but is beyond what I am
> > willing to spend.
>
> Using a mechanical piston/cylinder is the only official method I know of
> for microphone calibration, that shouldn't be too hard to come up with.
> You could use an old weed-eater motor as a basis, subtitute something else
> for the cylinder head, use an electric motor(speed controlled by a PIC!)
> to spin it.
>
> Cheers,
>
> Bob
>
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