How to measure it

Measuring this mic took a little effort, let me tell you. I had no idea how to measure a -22 dB SPL noise floor when I started! The answer turned out to be thermal. You reduce the air temperature to the point just above where it liquefies (around -260° F. - you wanna be careful doing this, because you can wreck the lattice, not to mention break off a finger!). At that temperature, there is no significant molecular motion, hence no sound detected by from the mic, in theory at least. So, using a 600 hp supercharged gas engine hooked up to a 30-ton apartment-building AC compressor and some over-sized cooling coils I borrowed, I took the mic to a local cold storage warehouse, set up and hooked up everything, and working inside a modified small freezer room (inside the cold-storage warehouse, yet!) I actually got the temperature down to -237° F. long enough to get the test done. With the mic output set at most sensitive, I actually got an output that toggled back and forth between 0 and 3 dB, referenced to -22 dB SPL. Awesome! The compressor failed before I could go any further with this test, thank God! As it was, it took 8 hours for the optical cable to thaw to a point where it could be moved without shattering. The guys at the cold-storage plant were not too pleased.

Measuring peak levels turned out to be comparatively easy. As you probably know, the sonic output of an air-bag opening in a car is about 165 dB SPL (if you think of those things as speaker diaphragms, it all begins to make sense!). So, the sound of two airbags is going to be 6 dB greater, or about 171 dB SPL, just about maximum level for the Mikrodigi 360. What I did was borrow my neighbor's new Lincoln (he's in Florida for the winter, and he left me the keys, just in case, you know), mounted the 360 in the back seat, hooked it up to a portable DAT, closed all the doors and windows, bypassed the failsafes on the secondary safety stage of the sensor system and triggered the airbags by hitting the front bumper with a small sledge-hammer. HAWHUMPFF! Wow, those suckers really pack a punch! I'm not sure I ever wanna be in there when one goes off! Anyway, once the gas cleared out (the airbags gave off a pungent white cloud of gas that really fogged up the interior for about half an hour), the Mikrodigi didn't seem to be much the worse for wear and the peak-hold level indicator on the DAT read -1 dB. Pow! Right on spec!

Frequency response was flat as far as I could tell - it matched my B&K to 24 kHz. at the 96 kHz. sampling rate, while falling off steeply beginning at 21 kHz. at the 44 kHz. rate.

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