Microphone

A microphone is an electromechanical device that uses vibration to create an electrical signal proportional to the vibration, which is usually an air pressure wave. There are many different types of microphone, ranging from the old condensers to the modern piezoelectrics.

Microphone History

Alexander Graham Bell invented the first microphone in 1876 as part of the telephone. Thomas Edison invented the first carbon microphone in 1886, a significant improvement on Bell’s impractical liquid microphone, and the forerunner of the modern microphone.

Microphone Types

Condenser microphones are a capacitor that has a fixed plate and a moving plate plate connected to a diaphragm. Air vibrations cause the diaphragm plate to move slightly and change the voltage between the plates. The electret microphone is a modern improvement on the old condenser design, and uses a dielectric material that has a permanently static charge, eliminating the need for a power supply to maintain the charge. This allows electrets to be made very small and cheap.

Dynamic microphones have a coil connected to a diaphragm that moves between a fixed permanent magnet. Vibration causes the diaphragm and coil to move, inducing a current in the coil proportional to the vibration. It is the opposite process of creating sound with a speaker, and while speakers can be used as microphones, their signal quality is poor.

Carbon microphones have a fixed plate and a moving plate connected to a diaphragm. Between the plates are tiny carbon grains that move when the diaphragm is vibrated. This movement changes the total contact surface area of the carbon, which also changes the resistance between the plates. The changing resistance results in voltage changes proportional to the vibration.

Ribbon microphones use the movement of a thin metal foil suspended in a magnetic field to create a signal. Piezoelectric microphones convert vibration into mechanical stress to create a charge from the piezoelectric crystal.

Sound Direction

Microphones are also categorized according to how well they pick up sound from certain directions. Omni-directionals detect sound equally well from all angles, bi-directionals pickup from the front and back but not the sides, and uni-directionals only pick up sound from the front.

Microphone Applications

Microphones are commonly used in television, radio, concerts, telephones, and public address systems, but also in other unusual applications. They have been used by rescuers to find survivors after disasters, and by police to conduct surveillance. They are used for feedback in noise cancellation systems, and used to detect the vibrations that precede volcanoes and earthquakes.