Microphone

Microphone is a device that changes sound into electric energy. This energy instantly travels over wires or through the air to a loudspeaker or some other instrument that changes it back into sound. Microphones, often called "mikes," are also used in recording sound.

The first microphone was the telephone transmitter, which the American inventor Alexander Graham Bell developed in 1876. Today, microphones are used 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. Citizens band (CB) radios and ham (amateur) radios also have mikes.

Microphones of various designs are used for different purposes. CB operators and 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 some microphones, sound produces variations in an electric current. In others, sound generates a current. In all microphones, 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) capacitor. Moving-coil, ribbon, and capacitor 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. Carbon microphones are used chiefly in telephones.

Crystal 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 used in ham radios, home tape recorders, and public-address systems.

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. Many CB radios have moving-coil microphones.

Ribbon microphones 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.

Capacitor microphones, often called condenser 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. Capacitor microphones are classified as electrostatic because they require an electric charge to change sound into electric energy.

A capacitor microphone produces only a low amount of electrical energy. Therefore, it must have a device called a preamplifier, which boosts the mike's signal to a usable level. In most capacitor microphones, called electrets, the capacitor is permanently charged and a small battery powers the preamplifier. Electret capacitors are used in hearing aids.

Contributor: Stanley R. Alten, Ph.D., Prof., School of Public Communications, Syracuse Univ.; Author, Audio in Media.

Microphone

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