frequencies about 2000 Hz. Special construction of the microphone housing or case may produce
additional directivity at frequencies below 2000 Hz. Examples of specially constructed microphones are
the "shotgun" and "reflector" types. Nondirectional microphones also require special design
considerations. For nondirectivity, it is necessary to so design the housing that signals from all
directions exert uniform pressure on the diaphragm. The microphone which is designed for general
purpose use is polydirectional; this type of microphone is usually mechanically adjustable to achieve the
desired pattern of pickup.
Dynamic microphone. The dynamic or so-called moving-coil type is the most widely used.
Because of its low (and adjustable) impedance, it can be installed with long cables without serious
adverse effect to the overall audio system; it is not easily damaged by rough handling and is not
particularly sensitive to blasts (instantaneous sound peaks).
a. A dynamic microphone contains a coil made of a large number of turns of extremely thin
metal ribbon attached to the diaphragm. This coil is insulated from the diaphragm by a thin coat of
insulating varnish. The coil extends from the diaphragm to a point between the poles of a powerful
permanent magnet. When sound waves strike the diaphragm, the coil moves back and forth within the
magnetic field between the poles of the permanent magnet and cuts the magnetic lines of force.
b. An illustration of a microphone designed to operate on this principle is shown in Figure 2-1a.
This action induces a current in the coil in direct proportion to the sound pressure exerted on the
Figure 2-1. Dynamic microphone (A) mechanical details and (B) circuit diagram