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40KHz microphone bias tone

 Drive microphones with a moderately strong 40HKz signal to avoid hysteresis
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One mechanical difficulty with constructing microphones is that it's hard to eliminate mechanical friction. Because mechanical friction which isn't eliminated can cause distortion, many inexpensive microphones don't work terribly well.

I would suggest that driving a microphone with a 40HKz biasing signal (perhaps using an internal transducer, or by adding a 40KHz transducer nearby) could help reduce the hysteresis effects caused by mechanical friction. The amplitude of the 40KHz signal would have to be controlled to ensure that it didn't ever contribute to driving the microphone into saturation, but otherwise I would expect the biasing would be a simple means of improving microphone performance quite inexpensively.

—supercat, Jan 17 2004

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       The goal would be to reduce hysteresis effects due to sticking. The tone would likely produce harmonics of 40KHz but they'd be at 80KHz, 120KHz, and above--not at 20KHz and 13KHz.   

       The mass of the microphone would also affect the sound, but those effects would take the form of reduced frequency response rather than distortion.   

       While I am well aware that filters aren't perfect, modern digital recorders already require a filter that takes out everything above 22KHz and absolutely everything above about 29KHz. The 40KHz bias tone would be well above the filter's cutoff.   

       As for demodulation, that shouldn't be necessary--simple filtering would suffice.
—supercat, Jan 17 2004
   

       [admin: renamed idea title microhphone -> microphone]
—st3f, Jan 18 2004
   

       If the input signal were clipped prior to filtering, then the 40KHz wave would be modulated rather than summed, but the idea would be to avoid having the total signal strength get loud enough for that to occur.   

       You do bring up an interesting point, though, which is that if distortion does cause any intermodulation (which would be likely in a cheap microphone) that could produce sum and difference tones. Thus if there were an input signal present at 19KHz, that could produce a 21KHz 'garbage' signal in the produced waveform. The necessity of keeping any garbage signals above 22KHz would suggest that the bias tone should be at least twice the target audio rate. Perhaps it would be best to use 44.1KHz synchronized with the audio sampling rate since this would also minimize any beating effects.
—supercat, Jan 18 2004
   

       Can one of you guys fix my Mr. Microphone?
—dbsousa, Jan 18 2004
   

       I can see a pack of labradors coming over the horizon.
—suctionpad, Jan 18 2004
   


 

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