646 Chapter 22
22.2.5 Piezoelectric
Piezoelectric microphones depend on a structural property possessed by certain dielectric
crystals and especially prepared ceramics. The nature of this property is that if the crystal
or ceramic is subjected to a mechanical stress, its shape will be distorted. When this
occurs, an electric fi eld appears in the substance as a result of shifted ion positions within
the structure. A capacitor can be formed employing such a dielectric that will generate a
voltage that is proportional to the mechanical stress. The mechanical stress can be made
to result from the motion of a diaphragm exposed to acoustic pressure. In this fashion it
is possible to construct a relatively simple, inexpensive pressure-sensitive microphone.
Piezoelectric microphones have very high capacitive output impedances. In the past the
high voltage sensitivity of such microphones made them popular for recorders and simple
public address applications where quite short connecting cables were possible. They
are still employed in some sound level meters but other professional application is quite
restricted.
22.2.6 Matching Talker to Microphone
Distant or bashful talkers require microphones of higher voltage sensitivity in order
to produce voltage levels matching those required by microphone input amplifi ers.
Nearby and professional talkers require microphones of less sensitivity in order to match
amplifi er input requirements without the use of pads in the input circuitry. Rock singers
are an extreme case requiring the least input sensitivity and further requiring both breath
blast and pop fi lters particularly when pressure gradient microphones are employed.
Table 22.1 lists representative voltage sensitivity ranges typical of microphones classifi ed
according to the mechanism.
Table 22.1 : Microphone Sensitivity Comparison
Microphone mechanism SV in dBV/Pa range
Carbon 20 to 0
Capacitor 50 to 2 5
Dynamic 60 to 5 0
Piezoelectric 40 to 2 0