Microphones

Author: J.B. Hoag

The construction of a single-button carbon microphone is shown in Fig. 25 I.

Fig. 25 I. A single-button carbon microphone

Some specially prepared carbon granules C are contained in a small fixed insulating cup, closed at one end by a flexible membrane or " piston " P which is fastened to a metal diaphragm D. Sound waves, impinging upon the diaphragm, cause it to vibrate at their frequency and with movements proportional to their intensity. As the piston moves back and forth in the cup, the electrical resistance which the carbon granules offer to the current from the battery is varied in proportionate amounts. The fluctuating currents in the primary of the transformer induce corresponding voltage changes in its secondary. A double-button carbon microphone operates with two buttons on the same diaphragm, in a push-pull circuit. Speech input circuits for single- and double-button microphones are shown in Fig. 25 J.

Fig. 25 J. Speech input circuits for carbon-button microphones

The current through these microphones is usually 50 to 100 ma. The output voltage of a single-button microphone amounts to from 0.1 to 0.3 volt across the 50- to 100-ohm primary of the transformer. A peak voltage of 3 to 10 volts will be developed across a 100,000-ohm load on the transformer secondary. With double-button microphones, 0.02 to 0.07 volt is developed across the 200-ohm primary and 0.4 to 0.5 volt is produced across a 100,000-ohm secondary load. Operating currents are usually from 5 to 50 ma. per button. The double-button type is less noisy than the single-button type.

Crystal microphones contain a pair of Rochelle salt crystals, properly cemented together, and with terminals of metal plated directly on their surfaces. The crystal is fastened directly to a diaphragm in the more sensitive types. When sound waves vibrate the crystal, changing its physical dimensions, small alternating potentials are produced between the electrodes (piezo-electric effect). These are applied to the amplifying tube directly, without the aid of a microphone battery, as shown in Fig. 25 K.

Fig. 25 K. Speech input circuit when a crystal " mik " is used

The output voltage usually ranges from 0.01 to 0.03 volt. High values (1 to 5 megohms) of the grid resistor of the pentode should be used.

A condenser microphone consists of a rigid metal plate in front of which, at a distance of about 0.001 in., is mounted a thin metal diaphragm, everywhere parallel to the plate. A d.c. voltage (180) is applied to this condenser, as in Fig. 25 L.

Fig. 25 L. Amplifier circuit for a condenser " mik "

When sound waves vibrate the thin membrane, the capacity changes and causes a fluctuating charge and discharge current to flow through the coupling resistor. The voltage across this resistor is then amplified by a tube. This tube must be mounted close to the microphone in order to avoid losses in the capacitance of a connecting cable. The better condenser microphones are from 1/100 to 1/50 as sensitive as the double-button type.

Velocity or ribbon microphones have a thin, corrugated metal ribbon suspended between the poles of a magnet. When sound waves vibrate the ribbon back and forth in the magnetic field, cutting its lines of force and generating an e.m.f. in the ribbon, voltages of the order of magnitude of 0.03 to 0.05 volt are generated. An input circuit for this type of microphone is shown in Fig. 25 M.

Fig. 25 M. Amplifier circuit for a velocity or ribbon type " mik "

The dynamic microphone uses a coil of wire, mechanically coupled to the diaphragm, and free to vibrate in a strong magnetic field. The use of several turns of wire in the coil permits this type to deliver a larger output voltage than the ribbon type. For crude work, a small, permanent-magnet dynamic loudspeaker may be used as a microphone. The construction of such a loudspeaker is shown in Fig. 25 N.

Fig. 25 N. A dynamic microphone (or speaker)