Types of Microphone
AKA Moving-Coil: the diaphragm is a tightly stretched membrane (usually of very thin plastic) with a thin coil of wire glued to the back; the coil of wire is surrounded by a permanent magnet. When the acoustical energy from a soundwave causes the diaphragm to move, this in turn moves the coil within the magnetic field, generating a small electrical current. (In essence, this operates like a loudspeaker in reverse.) Because the current generated is proportional to the energy of the soundwave, it is an electrical analogue of the acoustical signal. This principle is common to all microphone types and is the basis of how microphones work as transducers. Moving-coil microphones can be either pressure or pressure-gradient types, depending on how they are constructed. While moving-coil microphones can exhibit extremely broad frequency response, their ability to move very rapidly, i.e., their transient response, is limited due to the mass of the diaphragm and coil.
Employing a similar ‘motor-generator’ principle, the ribbon microphone’s diaphragm is a very thin metallic ribbon (usually aluminium) suspended between the poles of a very strong magnet. As described above, when the acoustical energy from the soundwave causes the ribbon to move within the magnetic field, an electrical current is generated. However, because the ribbon has considerably less mass than the moving-coil diaphragm (the ribbon in the classic RCA 44, for example, is only 1.8 microns thick), it is able to respond much more rapidly to the changes in air pressure, and therefore exhibits an exceptionally good transient response. This thinness, however, also makes ribbon microphones more fragile and susceptible to damage from strong air turbulence such as wind, breath blasts, or extremely high Sound Pressure Levels (SPL) at very low frequencies. Most commonly, they have a bidirectional polar pattern, although there are some that are designed to be more unidirectional.
In a condenser microphone, the diaphragm itself is one pole of a capacitor within an electrical circuit. As the acoustical energy stimulates the diaphragm into motion, its distance relative to the other pole changes, thus varying the capacitance in the circuit. These variations, as with the dynamic microphone, create an electrical analogue of the original acoustical signal.
As mentioned earlier, the capacitor element may be charged by an external source of power, or it may bear a permanent charge. The latter is known as an electret condenser. In either case, a preamplifier circuit is contained within the microphone body to increase signal level; this amplifier may derive its power either from an external supply or, in the case of the electret mic, from an internal battery. It is this amplifier circuit that is often the limiting factor for the SPL rating of a condenser microphone, as the preamplifier can be driven into overload from a high SPL signal and result in distortion within the microphone.
A subset of capacitor microphones exists as the RF Capacitor. This uses the capacitor within a radio frequency oscillator circuit, with the FM signal then demodulated to conventional analogue audio. Its prime benefit is to use the capacitor in a low-impedance circuit. Normal DC polarised capacitor capsules, with a very high impedance, are prone to failing under damp conditions, but the low impedance RF capacitor is almost immune to this. Thus this type of microphone has always been greatly favoured for location use.