c. The dynamic microphone elements are normally housed in a metal shell and covered with a
metal grill and silk cloth to prevent damage from foreign particles and to minimize dust collection. The
improvement in frequency response of this type of microphone over other types lies in the inclusion of
an echo compensation circuit, which consists of an air chamber between the housing element and an air-
vent tube; the length and diameter of the air-vent tube control the echo compensating action of the air
chamber.
d. The impedance of the moving coil in the dynamic microphone is approximately 50 ohms;
therefore, the coil may be connected to an amplifier by means of long cables. :There are microphones
available with built-in matching transformers to match low impedance of 30, 50, and 250 ohms or high
impedance up to 50 kilohms. A switch is built in to select the various impedances. The frequency
response of this type of microphone is reasonably flat over the range from 40 Hz to 10,000 Hz. Since
the output voltage level is only about 0.00004 volt, a preamplifier must be used for adequate
amplification. The circuit diagram of the dynamic (moving coil) microphone is illustrated in Figure 2-
2B.
e. The ribbon microphone, a variation of the dynamic microphone using the moving coil
principle, is widely used in studio operations. It has no real diaphragm, and its operation depends upon
the velocity of air. Therefore, it is sometimes termed a "velocity" or "pressure gradient" microphone.
The microphone, as shown by diagram in Figure 2-2A, consists of a powerful horseshoe-shaped
electromagnet or permanent magnet, M, with special pole pieces between which a thin corrugated metal
ribbon, R, is suspended. The ends of this ribbon are connected to the primary of a special step-up
transformer.
f. The construction details of the ribbon microphone are illustrated in Figure 2-2B. The
microphone should be so placed that the incoming sound strikes it at right angles, as those from the side
have practically no effect. The sound striking the ribbon causes it to vibrate and thus cuts some of the
magnetic lines of force between the pole piece. This action generates a voltage in the ribbon that is
coupled to the grid of an amplifier via a special step-up transformer. Since the ribbon microphone is
sensitive to velocity, it should be covered or otherwise shielded when used outdoors or in drafty areas,
where air tends to produce undesirable ribbon vibrations.
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