Microphone Types
From IBS_FAQ
Contents |
Microphone Types
Ribbon
The ribbon is the simplest type of electrodynamic Microphone; the diaphragm is a narrow ribbon of very thin conducting metal suspended between the poles of a strong magnet. When the ribbon is moved by the vibrations in the air, a current which is proportional to the sound causing the vibration is induced in it. This “single turn coil” has a very low Impedance and even with a very powerful magnetic field it produces a very low output voltage. It is usual to fit a transformer (or in some cases nowadays an electronic amplifier) in the microphone casing to raise both the output level and Impedance to values in the same range as other types. Due to the delicate nature of the ribbon these microphones are not as robust as Moving Coil ones. See also Ribbon Microphone in the Compendium.
Moving Coil
The moving coil microphone, as its name suggests, has a circular diaphragm attached to a coil of wire which is suspended in a magnetic field, usually in a sealed housing. Moving coil microphones tend to be very robust and can handle very high SPLs. Since the coil is very sensitive to external electromagnetic fields it is common to add a second (hum bucking) coil close by which is identical but does not move. This second coil is connected with opposite polarity and, as both coils will be affected similarly, any induced hum is "bucked" or cancelled out.
NOTE: Although it is common practice to use the term “Dynamic Microphone” to refer only to Moving Coil types, this is incorrect and misleading. “Dynamic” is an abbreviation of “Electrodynamic” and refers to both Moving Coil and Ribbon Microphones.
Condenser
The words Condenser and Capacitor are interchangeable, and are both used to describe microphones which use Electrostatic principles. The capsule of an electrostatic microphone has a conductive diaphragm held at a fixed distance from a backplate, forming a capacitor connected in an electronic circuit. Movement of the diaphragm caused by sound waves changes the Capacitance and results in an output proportional to the sound waves.
Condenser microphones may be either DC or RF operation.
In the DC type the capacitor has a fixed DC bias (often about 50V) applied across it via a very high resistance. This bias supply feeds charge into the Capacitance of the capsule until the voltage across the capsule rises to be equal with the supply voltage - the charge then stored in the capsule is given by the equation Q = CV, where C is the Capacitance in Farads, Q is the charge in Coulombs and V is the voltage. When sound falls on the capsule, the diaphagm moves causing a change in Capacitance. Because of the very high impedence of the circuit surrounding the capsule, the charge can only change very slowly, even compared to the lowest audio frequency likely to be encountered, and so can be regarded as being constant to a very close approximation. It can be seen from the equation above that if the charge is held constant while the Capacitance varies, the voltage across the capacitor will vary in the opposite direction to the change of Capacitance - in other words, if the Capacitance increases the voltage will decrease and vice-versa. The voltage change is picked up by an amplifier connected across the capsule. In order to maintain the very high impedance needed to prevent the charge form changing, this amplifier usually employs a valve or a FET, and is built into the microphone very close to the capsule. The output from the amplifier is at a much lower impedance, suitable for feeding low impedance cables.
In the RF biased type the Capacitance formed by the diaphragm and the backplate is part of a tuned circuit which determines the frequency of an RF oscillator. Movement of the diaphragm causes a change in frequency which is converted to a varying voltage by a frequency discriminator built into the body of the microphone. The advantage of this system is the the impedence of the circuitry around the capsule is much lower than in the DC case, making the microphone much less susceptible to noise caused by condensation inside the capsule - this means that it is likely to perform better than a DC microphone in damp environments. The system is also said to be more linear than the DC type. However, a problem can occasionally arise when several RF microphones are used in close proximity, since each must have its oscillator on a different frequency to avoid mutual interference, which may not always be the case with a randomly-selected group of microphones.
The necessary electronics for these types of microphone consumes a significant amount of power – especially if the amplifier uses valve (vacuum tube) circuitry. Battery power is rarely adequate, so T Power, Phantom Power or a mains power unit are needed for these designs.
Electret
The Electret microphone (sometimes called Pre-Polarised) is a variation of the DC type of capacitor microphone, the difference being that the capsule employs a material known as an "Electret", a type of material which has a permanent charge, thus needing no DC bias supply. The most commonly met ones are small personal microphones but the electret principle is not limited in terms of size; "large diaphragm" and even dual diaphragm designs exist. The two electrodes of the capacitor are the flexible diaphragm and a fixed backplate. The charge can be on either, though the latter "back electret" version allows a thinner diaphragm and is preferred. With no biasing requirement the only essential electronics are a simple FET Impedance converting amplifier. This needs minimal power so many electrets can be powered by a battery, wireless transmitter, Plug-in Power or by a Phantom Power adapter. The "permanent" charge on the element will eventually decay over time causing some changes in output. However this is a very, very slow process with modern electret materials.
