A microphone is a device that converts sound into electrical energy; as such, a microphone is a type of transducer, converting energy from one form into another. In general, a microphone contains a flexible diaphragm that responds to the changes in air pressure that constitute sound waves and produces an electrical signal whose amplitude follows the variations in air pressure of a sound.
There are various types of microphones, each converting sound energy into electric energy by a different mechanism. The principal types are carbon, moving-coil, ribbon, condenser, and crystal microphones. The choice of microphone for a given application is influenced by cost, size, sensitivity, directionality (ability to select sounds from specific directions), and frequency response.
Carbon microphones
The sound detector of a carbon microphone is a chamber that contains carbon granules between a flexible diaphragm and a rigid cup. The diaphragm and cup are both conductors, but they are electrically isolated from one another. One terminal of a direct-current supply is attached to the diaphragm, the other to the cup.
When sound waves strike the diaphragm, their pressure variations cause it to vibrate, thereby imposing a fluctuating pressure on the carbon granules. Since the conductivity of the mass of carbon granules increases when the granules are forced together, the fluctuating pressure causes variations in electrical resistance (resistance is the inverse of conductivity). As a result, the current that flows between the cup and the diaphragm oscillates about a mean value in step with the pressure variations of the sound waves. This varying current is in effect the sum of an alternating current and a direct current, and the component that alternates is extracted by a transformer.
Since the carbon microphones respond to pressure variations on only one side of their diaphragms, they are called pressure-operated devices. In contrast, pressure-gradient-operated devices respond to differences in pressure between the two sides of a diaphragm.
Carbon microphones are sturdy and cheap to produce, making them the microphone of choice for telephones for many decades. Their frequency response is limited, however, and the granules are prone to clogging, which causes noisy signals. As a result, they have been largely superseded by superior and more compact condenser microphones in recent years.
Moving-coil and ribbon microphones
When a springlike conducting coil expands and contracts repeatedly in a magnetic field, an alternating electric current flows in the coil. Moving-coil microphones use this effect to create signals from the oscillations of a flexible diaphragm. The coil is attached to the diaphragm at one end and a permanent magnet at the other, causing the coil to expand and contract in the magnetic field in time with the vibrations of the diaphragm. The current thereby induced in the coil is directly related to the incident sound wave.
Ribbon microphones also use electromagnetic effects to produce electrical signals from sound. A corrugated metal ribbon strung between the poles of a permanent magnet acts as the diaphragm, and receives an induced current related to the incident sound wave. A ribbon microphone is an example of a pressure-gradient-operated device, since it responds to differences in pressure between the two faces of a ribbon.
