Listening to TC?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Mddeming-at-aol-dot-com>

Hi All,

        We all know that spark gaps and streamers make some pretty awesome 
sounds, but what about "ultrasounds"? These would be sounds from 15KHz on 
up to the resonant frequency of the coil. This question is prompted by the 
following: Last night, a bat got into the instrument/people section of the 
lab. (The TC section is inside a grounded 10ftx10ftx8ft cage). With the 
coil running, the bat ran into the wall the way a bird or insect will beat 
against a window trying to get out. I turned off the coil and it 
immediately turned around and flew back out the door which was only open 
about 5 inches.  Makes me think maybe the coil was making sonar-jamming 
noises. Of course, it could be pure coincidence.
        Seems about 35-40 years ago, I read about an "ultrasonic listening 
device" that used a very (for that time) high freq. response ceramic mike 
and heterodyned its output with ~38KHz local oscillator which made sounds 
above 20KHz audible but inverted. Example: a 32 Khz "sound" would become a 
6Khz tweet, while a 37KHz one would become a 1 KHz beep, etc. By making the 
local oscillator a VFO, one could listen to a wide range of the audio 
spectrum up to the response limits of the microphone.
        If nothing else, such a gizmo might add to the "Halloween value" of 
a coil. Does anyone know if such devices are still around in a modern form, 
or if there is a microphone/transducer with good well-above audible range 
frequency response for a DIY?

Matt D.