Radio and television stations make use of microphones. They are also used in public address systems and in motion pictures and phonograph records. The mouth piece of a telephone is a simple type of microphone. Let us see what exactly a microphone is.
A microphone is a device which converts sound waves into electrical signals. These signals can then be broadcast through the air or sent over to distant points, where they can again be converted back into sound.
Microphones can be divided into two groups depending upon how they respond to sound waves. These are: the pressure type and the velocity type.
The pressure type microphones contain a thin metal plate called a diaphragm. This is stretched like a drumhead inside a rigid frame. The diaphragm is a part of the electrical circuit. When the sound waves strike the diaphragm, it starts vibrations at the same rate as the sound waves. These vibrations produce corresponding electric signals by changing the electric current that flows through the circuit.
The pressure microphones are of several types, such as condenser microphone, moving coil or dynamic microphone, the crystal microphone and the carbon microphone.
In the condenser microphone, the vibrating diaphragm changes the capacitance of a condenser. A moving coil microphone consists of a coil of wire attached to a diaphragm. As the diaphragm vibrates in response to the sound, the coil slides up and down the centerpiece of an M – shaped permanent magnet. The coil thus cuts through the magnetic lines of force which induce a fluctuating voltage in it. This fluctuating voltage represents the variations in sound pressure.
Crystal microphones make use of piezoelectric effect. Whenever certain crystals such as quartz are bent or twisted, they generate an electric voltage. Such crystals are called piezoelectric crystals. In a crystal microphone, a piezoelectric crystal is firmly clamped at one end and attached at the other to a flexible diaphragm. When someone speaks before it, the sound waves make the diaphragm vibrate. Due to this, a pressure acts on the crystal, and as a result, an oscillating voltage is produced. This is picked up by the heads attached to the crystal surface. This is how sound waves are converted into electric signals and transmitted.
A carbon microphone works like a telephone transmitter. In this, a carbon granule varies the current to create a signal. Ribbon microphones have a metal ribbon instead of coils.
The velocity microphone has a light ribbon of aluminium foil loosely held in a strong magnetic field. The sound waves vibrate the ribbon. The movement of the ribbon in the magnetic field induces a current along its length which is picked up by the connecting leads.
Several people including the American inventor Thomas Alva Edison have been credited with the invention of the microphone. The first practical microphone however was invented in 1878 by David Edward Hughes of United States. Other inventors who have contributed to the invention of the microphone are Emile Berliner, Philip Reis, Franus Blake and Henry Hunnings.
