Sound to electricity: microphones
As invented by Charles Wheatstone in the 1820s, the microphone was a purely acoustical device made of two metal plates clamped to the ears by a springy rod and a length of dressmaker’s ribbon. To use it, one pressed one’s head against the sound source (Wheatstone helpfully suggested a boiling kettle) which, with a bit of luck, could be heard more clearly. To no one’s surprise but Wheatstone’s, it didn’t catch on.
The ancestor of today’s devices—which convert sounds to electricity rather than simply directing them to the ear as Wheatstone’s version did—was the carbon microphone, invented by David Hughes in the 1870s. In this, a thin metal plate (the diaphragm) compresses a vessel filled with carbon granules through which a current is passed, and the compressive force alters the electrical resistance. Although their performance was very poor, carbon microphones were used in telephones for decades.
Although there have been many microphone designs over the years, and several specialist types are available today, only three major ones are now in common use: the dynamic or moving coil microphone, the condenser microphone, and the piezoelectric microphone.
18. Dynamic microphone.
In a dynamic microphone (Figure 18) the diaphragm is attached to a coil of wire that surrounds a stationary magnet. A small voltage is induced in the coil when it moves, which in turn gives rise to a weak current. The low voltage means that the quality of the response is too poor for measurement work, so dynamic microphones are mainly to be found in concerts and recording studios.
In a condenser microphone (Figure 19), the diaphragm forms one plate of a capacitor (formerly called a condenser, hence the name). A capacitor is a pair of parallel metal plates with a thin layer of either air or some other material which does not conduct electricity (known as a dielectric) between them. Attaching one plate to the negative pole of a battery charges it with electrons. All metals contain free electrons, and those in the other plate are repelled by the electrical field arising from the large number now residing in the negative one. The repelled electrons ffow from that plate, to leave it positively charged. The whole capacitor, therefore, now has a voltage (called a polarizing voltage) across it, and the microphone is ready for use: the diaphragm plate moves in and out with the compressions and rarefactions of the sound waves that impinge on it. A large electrical resistance stops the charge from rapidly escaping, so instead the sound waves are transformed to patterns of voltage ripples. Condenser microphones have an excellent frequency response and are used as measurement microphones in laboratories and in SLMs. They also respond more rapidly to sudden sounds than do dynamic microphones.