Subject:Re: Help: using mics as earphonesFrom:Dan Mapes-Riordan <dmapes(at)luc.edu>Date:Thu, 7 May 1998 13:49:46 -0500Daniel, Yes, in theory reciprocal acoustical transducers (e.g., electromagnetic, electrostatic) are capable of acting as projectors (loudspeakers) and receivers (microphones). A fundamental obstacle to this principle is a 1/frequency dependence in the "reciprocity factor" which means that the inherent frequency response of a reciprocal transducer cannot be flat both as a projector and as a receiver. For example, when operating as a receiver, an electret microphone has a flat frequency response below resonance and a 1/frequency squared response (12dB/octave decrease) above resonance, but as a "loudspeaker" the frequency response "tilts counterclockwise" by 6 dB/octave such that the frequency response peaks at resonance, and decreases by 6 dB/octave below resonance and 6 dB/octave above resonance. Thus, an electret microphone looses it flat frequency response as a "loudspeaker." (Similarly, an analysis of electromagnetic projectors (e.g., your typical loudspeaker) shows that as a projector its inherent frequency response decreases by 12 dB/octave below resonance and is flat above resonance, but as a microphone the response "tilts clockwise" by 6 dB/octave such its inherent frequency response peaks at resonance and falls off by 6 dB/octave below and above resonance.) In practice, this limitation can be overcome by incorporating damping into the system which flattens the peak in the response but also reduces the sensitivity of the device. Therefore, even though some transducers are reciprocal, they still have a preferred direction of operation assuming a flat, wide band response is desired. And, as Dan Levitin pointed out, microphones and loudspeakers are designed to operate over a certain range in terms of frequency, voltage, displacement, etc. to the point where in some cases (as Bill Hartmann mentioned) active electronics are added that essentially eliminate their ability to act as reciprocal devices. In summary I agree with Dan Levitin. There is no need to reinvent the wheel (assuming your application is such that a "wheel" has already been designed). Take a look at some small "acoustic projectors" already on the market like those offered by Etymotic Research. Hope this helps, Dan Mapes-Riordan dmapes(at)luc.edu

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