Figure 2-2. Velocity microphone (A) circuit diagram and (B) construction details
g. The voltage output of the velocity microphone across 250 ohms is 0.0002 volt. The
corrugated ribbon has a resistance of only a fraction of an ohm; therefore, the matching transformer is
usually built into the microphone to reduce losses. The frequency response is practically flat from 30 to
15,000 Hz. The low impedance of the velocity microphone permits a long cable connection to the
amplifier, but the cable must be well shielded because of the possibility of AC hum pickup.
5.
Carbon microphone. The carbon microphone is widely used in intercommunications and cueing
systems. In the carbon microphone, a constant direct current is permitted to flow through a mass of
carbon granules. As sound waves vibrate the diaphragm, its resultant motion alternately compresses and
releases pressure on the mass of carbon particles. The changing pressure on the carbon causes the
resistance value of the total mass to change, thus permitting either more or less direct current passage. A
cross section of a typical carbon microphone is shown in Figure 2-3A; A and B are heavy steel rings.
The ridge in one and the groove, D, in the other hold the diaphragm, C, very tightly. The diaphragm is
made of a very tough steel alloy and is generally designed to be from 0.001 to 0.002 inch thick. The
small ring, G, is screwed into the large steel ring, B, to adjust the diaphragm tension, so that its natural
period of vibration is above the desired audio frequency range. The central portion of the diaphragm is
gold-plated on each side to
13
Previous Page
