c. The body of the microphone (C) consists of a molded spherical housing
containing a permanent magnet (D) with a center pole piece over which the voice
coil is centered. The edge of the diaphragm is hinged and supported at the edges
by the housing. The outer surface of the diaphragm is protected from damage by
perforated grid (E).
d. A twolayer circular mesh screen baffle, (F), with layers of silk between
the screens, is placed in front of the diaphragm.
e. The perforated grid and screen act as acoustical equalizers to improve
the omnidirectional characteristics.
f. At the lower left is a small metal tube (G) termed an acoustical
equalizer. Its function is to release air pressure behind the diaphragm to prevent
distortion of the diaphragm during its inward travel.
g. Working in conjunction with this tube are two airrelease vents (H) under
the voice coil to provide acoustical resistance. External pins (I) provide
connections to the voice coil lead (J).
h. The (L) baffle is used to assist in achieving a directional
characteristic.
Figure 37. Crosssectional view of Western Electric 620B
3. Microphones of the above type do not employ an output transformer. The output
voltage is taken directly from the voice coil winding. The frequency of the
microphone is little affected by the angle of incidence up to 120 degrees (fig 3
8). The impedance of this microphone is 20 ohms, but is operated into a 30 to 50
ohm preamplifier.
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