Re: Help: using mics as earphones

[Dan Mapes-Riordan <dmapes@xxxxxxx>]

Daniel,

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@luc.edu