MICROPHONE or TELEPHONE TRANSMITTER, a device for converting mechanical energy of sound waves into electrical energy with similar vibrational characteristics. The first instrument used for this purpose was the telephone receiver in vented by Alexander Graham Bell in 1876, the same type of in strument being then used in a telephone circuit for transmitting and receiving. (See TELEPHONE.) The term microphone, how ever, was first used by D. E. Hughes in connection with his dis covery in 1878 that a loose contact in a circuit containing a battery and a telephone receiver may give rise to sounds in the telephone corresponding to the vibrations to which the contact is subject.
Hughes constructed his microphone in the form of a horizontal carbon rod resting in grooves in two carbon blocks, the battery and telephone receiver being connected in series with these blocks.
Microphones have in general been developed on two main lines. First, as instruments (similar to the Hughes type) where the action is that of an amplifying relay, the impact of sound waves on the microphone causing a variation of energy supplied by a local battery. In this case efficiency is the primary object.
Secondly, as instruments where the electrical energy is mainly derived through the microphone from the mechanical energy of the sound waves, the first considerations being the perfection of frequency characteristic and a linear relation between air pressure and consequent electric current.
The former type has been developed for commercial purposes such as telephone communication systems ; the latter for scientific work such as sound measurement (see SOUND) and for purposes where high quality is essential, as, for instance, in broadcasting and gramophone recording installations.
The conversion of pressure vibrations in the air into corre sponding electrical vibrations in an electrical circuit is generally carried out in two operations, which take place simultaneously.
First of all the sound wave impinges on a surface in the in strument (usually known as the diaphragm), which is capable of slight movement. The variation of air pressure on this diaphragm
causes it to move to and fro in a manner corresponding to the backwards and forwards movement of the particles in the air— that is corresponding to the original sound.
The second operation is that in which the diaphragm by its movement causes a corresponding change in some property of an electrical circuit. Thus the diaphragm may be one plate of a condenser producing variations in capacity, or it may be part of an inductive electromagnetic system, or it may cause variations of electrical resistance. In each case variation of current takes place by diaphragm movement.
Thus, in studying the performance of microphones, two things must be considered: first, the mechanical movement of the dia phragm; secondly, the nature of the method in which this move ment sets up the changes which we desire.
The change of resistance with pressure varies greatly with materials used, and it has been found that particles of carbon in light contact with each other produce the greatest change in re sistance when subjected to small variations of pressure.
The electrical A.C. energy produced at the contact, since the power is taken from a local battery, may be many times the energy of the accoustic input, and this may be increased by in creasing the current from the battery passing through the contact. But there always exists a slight variation of current at the contact even when no sound waves are impinging on it, which is heard as a hiss in a telephone receiver, and if the current through the con tact is increased too much, the hiss becomes a loud crackling noise, due to overheating at the contact points.
Another disadvantage is that the contact or contacts may get into a very insensitive state known as "packing," the remedy for which is an occasional shaking.