Project #2 - Speaker as a Microphone

The idea of using a loudspeaker to record low- frequency sounds has rumbled around in the back of my head as an interesting project. Since most (even "professional") dynamic microphones have a low-frequency roll-off beginning at 100Hz or more, a loudspeaker of sufficient size should be able to do justice to recording very low frequencies.

I decided a "large-diaphragm microphone," using a 15" speaker intended for commercial use, should do the trick. First, I had to build the cabinet to house it.

The real treat of my recording session at the Austin Bergstrom International Airport was watching and listening (with headphones and ear protectors) to a couple of the Texas Air National Guard F16s take-off. At 109db from my location about 150 yards from the runway, it was a real experience. True, there was some distortion on the digital tape due to overload of the DAT recorder microphone preamplifier channel. I also recorded on the second channel with a Sennheiser MD421 microphone; that channel just touched zero. The typical commercial jet take-off and landing registered 85-95db.

For my presentation to the EFH class, I took my new low-frequency cabinet, a Frazier CAT-40 2-way commercial speaker, and a QSC RMX850 (200-watts rms per channel) amplifier. I connected the DAT recorder directly to the amplifier, the low frequency channel to the new cabinet, and the MD421 recording to the Frazier speaker. I also took my sound pressure level meter to adjust the playback level to something close to what I had recorded.

Needless to say, I asked the class to stuff their fingers in their ears when I played back the F16s.