tional patterns of microphones; the term field pattern is interchangeable with the
term polar pattern. Basically, there are four types of polar patterns (fig 31).
Figure 31. Basic microphone field patterns
a. The omnidirectional is a circular, or nondirectional field pattern,
representing the crystal, dynamic, capacitor, carbon, electronic frequency
modulated, and inductor type microphones.
b. The semidirectional pattern obtained with an adjustable field pattern
microphone is illustrated in Figure 31. As the pattern shows, this type of
microphone is directional at high frequencies but nondirectional at low frequencies.
c. The bidirectional pattern obtained with a ribbon microphone is also
illustrated by figure 31. The microphone is essentially dead to pickup at the
sides. This pattern is generally referred to as the figureeight field pattern.
d. Cardioid. Microphones are available that will fit the field pattern to
be varied to fit almost any situation and include all of the foregoing patterns in
some form or the other.
5. Carbon Microphones. How is a carbon microphone constructed and what is its
principle of operation? Several hundred small carbon granules are held in close
contact in a brass cup called a button, which is attached to the center of a
metallic diaphragm. Sound waves striking the surface of the diaphragm disturb the
carbon granules, changing the contact resistance between their surfaces.
a. The change in contact causes a current from a battery connected in series
with the carbon button and a primary of a transformer to vary in amplitude,
resulting in a current waveform similar to the acoustic waveform striking the
diaphragm.
b. After leaving the secondary of the transformer, the minute changes
current through the transformer primary are amplified and reproduced in a
conventional manner. Figure 32 shows the circuit diaphragm and construction of a
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