Audio Engineering

Microphone Technology 643

result 94 dBm lower than that of Equation (22.5). If we symbolize this rating by
GAIP , then

GSAIP (V 10 logRo70 dBm.) (22.6)

In this rating system the example microphone would produce  53 dBm at the threshold
of hearing. The advantage of this system is that the power level supplied by a given
talker’s microphone is obtained by simply adding GAIP to the pressure level of the talker’s
voice at the microphone’s position. GAIP as defi ned here is very similar to the EIA
rating for microphones. The EIA rating system differs in that rather than employing the
actual output resistance of the microphone, a nominal microphone impedance rating is
employed instead.

22.2 Microphone Selection ...........................................................................................

Microphones are usually selected on the basis of mechanism, sensitivity, nature of response,
polar response pattern, and handling characteristics. Mechanism refers to the physical
nature of the transducing element of the microphone. Sensitivity in current practice refers
to the voltage sensitivity, SV. The nature of response refers to whether the microphone
output is proportional to acoustic pressure, acoustic pressure gradient, or acoustic particle
velocity. Polar response patterns summarize a microphone’s directional characteristics.
Handling characteristics are a result of whether the structure of the microphone housing is
mechanically isolated from the transducing structure of the microphone. The following is a
list of popular microphones according to the transducing mechanism:

1. Carbon.


2. Capacitor.


3. Moving coil.


4. Ribbon.


5. Piezoelectric.

22.2.1 Carbon

Carbon microphones made their advent as transmitters in early telephones. Pressure
variations on a metallic diaphragm actuated a metallic button contact to either increase