The definition of a microphone according to the Yamaha Sound Reinforcement handbook, Page 113.
“Microphone is a generic term that is used to refer to any element which transforms acoustic energy (sound) into electrical energy (the audio signal). A microphone is therefore one type from a larger class of elements called transducers – devices which translate energy of one form into energy of another form. The fidelity with which a microphone generates an electrical representation of a sound depends, in part, on the method by which it performs the energy conversion. Historically, a number of different methods have been developed for varying purposes, and today a wide variety of microphone types may be found in everyday use.”
The dynamic microphone is the most common microphone found in modern sound. The element of a dynamic microphone shares the same electro-acoustical properties as a loud speaker. A flexibly mounted diaphragm is connected to a coil of wire. The coil is then attached to the air gap of a magnet so that it is free to move back and forth inside the gap.
When sound interacts with the diaphragm, the diaphragm moves in response to the sound source. The coil attached to the diaphragm moves within the field of the magnet creating a small electric current. The size of that current is directly related to the motion of the coil, so the current is an electrical representation of the sound source
The condenser microphone is another microphone that is very common. The element of a condenser microphone differs greatly from that of a dynamic microphone. A condenser microphone diaphragm is metallic coated, and mounted above a conductive back plate, which is a metallic coated ceramic. The diaphragm and back plate are separated by a small cushion of air to form an electrical component called a capacitor (or condenser)
Condenser microphones need to be powered in order to represent an acoustic signal electronically, and are commonly powered by 48 volts. When the diaphragm vibrates in response to a sound source, it moves toward, then away from the back plate. As it does this the electrical charge that it creates in the back plate is changed. This fluctuation in voltage on the back plate is the electrical interpretation of an acoustic source.
The element of a condenser microphone creates almost no electrical signal, creating very high impedance. Because of this, all condenser microphones use an amplifier what drives the microphone line. It boosts the signal levels and isolates the element from the lower impedance of the input it is connected to.
Electret condenser microphones use a unique plastic diaphragm that retains a static charge indefinitely. The manufacturer charges the diaphragm during construction, so no external voltage is required. Electrets still require an amplifier, and this is normally a transistor unit. The amplifier is commonly powered with a battery – between 1.5v and 9v – housed in the microphone itself. Electrets are becoming more common in both recording and live sound because they are relatively inexpensive and offer unique close micing techniques.
Ribbon microphones use a method of transduction common to that of a dynamic microphone. A corrugated metal ribbon is placed between the air gap a f a powerful magnet. The ribbon is clamped at both ends, but is free to move throughout its length. When sound hits the ribbon is moves in response inside the air gap and creates a voltage inside the ribbon.
The voltage is small, so all ribbon microphones use a built in transformer. The transformer boosts the signal and isolates the ribbon from impedance from the microphones connection. Early models of ribbon microphones were extremely fragile, but have become more durable as technology has advanced. Ribbon microphones usually have very good sonic characteristics with low self-noise and excellent transient response, and are effective on vocal and acoustic instruments.
An early microphone design is the Piezoelectric Microphone. The piezoelectric microphone uses a flexible diaphragm attached to a crystal element with a drive pin. The crystal is mad3e of a material that exhibits the piezoelectric effect. When the crystal is physically deformed, it generates and electrical voltage across its faces. When sound hits the diaphragm it moves, and the crustal is deformed slightly, the crystal generates an electrical current in response to the bending, and this fluctuation of voltage is the electrical representation of the source sound. Piezoelectric microphones, sometimes called crystal or ceramic, are not known for their sound quality, but rather their low price. When used correctly the piezoelectric microphones can perform well, often as a contact microphone or pickup.