644 Chapter 22
or decrease the compaction of carbon granules contained in a brass cup so as to decrease
or increase the resistance of the assembly. The impinging sound thus modulated the
direct current in a circuit containing a battery and the microphone element. Carbon
microphones are quite sensitive and inexpensive to construct. In addition to the normal
thermal noise, such microphones suffer from fl uctuations in contact resistance between
carbon granules even in the absence of acoustic excitation. The high noise fl oor and
restricted frequency response limit the application of such microphones in sound
reinforcement systems.
22.2.2 Capacitor
Capacitor microphones exist in two basic forms. In one form a capacitor has a front
plate formed by a fl exible low-mass, metallic, or metal fi lm diaphragm separated by an
air gap from an insulated, rigid metallic perforated back plate. Air motion through the
perforations in the back plate serves to damp the mechanical resonance of the diaphragm.
This resonance occurs at a high frequency as a result of a stiff, low mass diaphragm. The
diaphragm is operated at ground potential while the back plate is charged through a very
high resistance by a DC voltage source ranging up to 200 V.
In a second form, a permanently polarized dielectric or electret is positioned on the
surface of the back plate removing the necessity for an external polarizing voltage source.
In both instances the capacitor circuit is completed through a resistance of the order of
109 Ω and the charge on the capacitor remains approximately constant. Pressure variations
on the fl exible diaphragm produce changes in the air gap dimension, thus raising or
lowering the capacitance by a small amount depending on the degree of diaphragm
displacement. With a constant charge on the variable capacitor, the voltage variations
track the diaphragm displacement variations.
The capacitor circuitry itself is of high impedance and requires that a fi eld effect
transistor (FET) source follower be contained within the microphone housing. The source
follower may be energized by a local battery in the case of the electret form or may
derive its power from the polarizing voltage source in the pure air capacitor form. These
microphones, although not the most rugged, can be of extremely high quality with regard
to frequency response. As discussed later, the construction details of the microphone
capsule may be varied to make the microphone capsule sensitive to either acoustic
pressure or acoustic pressure gradient.