Microphone

MICROPHONE, a device used to convert acoustic air vibrations into electrical current fluctuations. The first microphone, invented by Hughes (1878), was based on contact resistance variation. It consisted of a carbon rod A clamped between two carbon plates C connected to a battery circuit E supplying a direct current (see figure). The slightest change in contact between the carbons alters the strength of the direct current passing through the device. Therefore, if sound waves act on plate B connected to carbon posts C, the current strength changes in accordance with the pressure changes in the incoming sound waves. Carbon microphones are the most common and come in two designs: microphones with carbon powder and microphones with carbon granules or small balls. In the first design, current is supplied to a fixed frame and a carbon membrane, between which fine carbon powder is poured, whereas in the second design, current is supplied to the membrane and a frame containing a large number of recesses in which large carbon granules or small balls lie, which cannot leave the recesses they are placed in when the membrane moves. When sound waves act on the membrane, the contact resistance in the carbon powder or granules changes in the rhythm of the sound waves and causes identical fluctuations in the strength of the direct current passing through the microphone. Current fluctuations, acting on a telephone receiver connected to the microphone circuit (directly or after amplification), reproduce the sounds that reached the microphone (in radio practice, after amplification, the microphone current enters the radio transmitter devices). The carbon powder microphone design is more sensitive to sound waves, but is unstable in operation and produces greater distortion than the second, less sensitive design. When the sound source is moved away from the microphone, the sensitivity of the carbon microphone drops sharply, which is why microphones used for the hard of hearing give a good effect when amplifying close sounds (conversation) but perform poorly when amplifying sounds from long distances (speech in an auditorium). At high sound intensities, the sensitivity of the microphone drops because carbon granules at large deflections no longer return to their initial positions, which causes sound distortion; another cause of distortion is the resonance of the microphone membrane at certain frequencies of sound vibrations. Another common type is the electrodynamic microphone. Their principle of operation is the generation of an induced current in a conductor moving under the influence of sound waves in a constant magnetic field. The advantage of this type of microphone is that the movable membrane in the magnetic field, carrying the conductor in which currents are induced, can be made very light, which makes it possible to transmit the highest tones (up to 10,000 vibrations per 1 second) that determine speech intelligibility. For acoustic measurements, a condenser microphone is used, in which one of two very close thin plates of a capacitor serves as a membrane vibrating under the action of sound waves. Periodic changes in the capacitance of such a capacitor, to which a high potential difference is applied, create an alternating electromotive force in the circuit where this capacitor is included; the magnitude of this electromotive force can be used to judge the strength of the sound reaching the microphone. Microphones find application not only in wire telephony and radiotelephony, but also in amplifying speeches in an auditorium, demonstrating quiet sounds in an auditorium (heartbeat, etc.), and as an amplifying device for the hard of hearing.