Microphone is a device that changes sound into electric energy. This energy travels over wires or through the air, ultimately reaching a loudspeaker, which changes it back into sound. Microphones, often called “mikes,” are also used in recording sound.
The first microphone was the telephone transmitter, which the Scottish-born inventor Alexander Graham Bell developed in 1876. Today, microphones are used to pick up the voice in all telephones, in announcing over public-address systems, and in broadcasting radio and television shows. They are also used in recording the sound for motion pictures and in making compact discs and tape recordings.
Microphones of various designs are used for different purposes. Some entertainers use mikes held in the hand. Other mikes are attached to stands. Still others have an arm, or boom, which holds the microphone above a TV or movie performer. The boom and microphone can follow the performer in any direction, but they stay out of view of the camera. Lavalier microphones, also called lapel microphones and tie-tack microphones, are fastened to clothing.
Some microphones pick up sound from all directions, but others are sensitive to sound from only certain directions. An omnidirectional microphone picks up sound from all around. A bidirectional microphone is used for sound coming from the front or from behind, but not from the sides. A unidirectional microphone picks up sound from only one direction.
In most microphones, the energy in sound waves is transduced (converted) into an electric current. The current corresponds to the pattern of the sound waves.
Microphones may be classified according to how they change sound into electric energy. The five main types, in order of increasing complexity, are (1) carbon, (2) crystal and ceramic, (3) moving-coil, (4) ribbon, and (5) condenser. Moving-coil, ribbon, and condenser mikes can reproduce sound much more accurately than the other types and are used by the movie, radio, recording, and television industries.
Carbon microphones
have a small container called a button, which is filled with particles of carbon. An electric current from a generator or battery flows through the carbon. A metal disk called a diaphragm presses against the button and vibrates when struck by sound waves. The vibrations cause variations in the current running through the carbon.
Crystal microphones and ceramic microphones
contain substances called piezoelectric crystals. Pressure on these crystals makes them generate an electric current (see Piezoelectricity ). Crystal and ceramic microphones may or may not have a diaphragm touching the crystals. An electric current is produced by pressure from sound waves that hit the diaphragm or strike the crystals directly. These microphones are sometimes used in home tape recorders.
Moving-coil microphones
have a wire coil attached to a diaphragm. The coil and diaphragm are suspended in a magnetic field. When sound waves hit the diaphragm, the coil moves across the field. This movement produces an electric current in the coil. Moving-coil microphones are rugged, and they are often used on stage at live concerts.
Ribbon microphones
typically have a metal ribbon suspended in a magnetic field. An electric current is generated when sound waves hit the ribbon and move it across the field. Both ribbon and moving-coil microphones are called dynamic microphones. These mikes are classified as electromagnetic because they use a magnetic field to change sound into electric energy.
Condenser microphones,
sometimes called capacitor microphones, have two metal plates set slightly apart. The plates are electrically charged and serve as a capacitor, a device that stores a charge. The front plate is flexible and acts as a diaphragm. The back plate cannot move. Sound waves make the front plate vibrate, which causes variations in the electric current from the capacitor. Condenser microphones are classified as electrostatic because they require an electric charge to change sound into electric energy. A condenser microphone produces little electric energy. A device called a preamplifier boosts the mike’s signal to a usable level. In some inexpensive condenser microphones, called electrets, the capacitor is permanently charged. Electret microphones are used in hearing aids, camcorders, telephones, and some computer sound cards, which convert sound to digital (numeric) data and back again.
