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Condenser microphones

Description and application of the condenser microphone.


The capacitor mic, formerly known as the ‘condenser mic’, works in a completely different way to the dynamic. Here, the diaphragm is paralleled by a ‘backplate’. Together they form the plates of a capacitor. A capacitor, of any type, works by storing electrical charge. Electrical charge can be thought of as quantity of electrons (or the quantity of electrons that normally would be present, but aren't). The greater the disparity in number of electrons present – i.e. the amount of charge – the higher will be the voltage across the terminals of the capacitor. There is the equation:

Q = C x V


charge = capacitance x voltage

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Note that charge is abbreviated as ‘Q’, because ‘C’ is already taken by capacitance.

Putting this another way round:

V = Q/C


voltage = charge / capacitance

Now the tricky part: capacitance varies according to the distance between the plates of the capacitor. The charge, as long as it is either continuously topped up or not allowed to leak away, stays constant. Therefore as the distance between the plates is changed by the action of acoustic vibration, the capacitance will change and so must the voltage between the plates. Tap off this voltage and you have a signal that represents the sound hitting the diaphragm of the mic.

The great advantage of the capacitor mic is that the diaphragm is unburdened by a coil of any sort. It is light and very responsive to the most delicate sound. The capacitor mic is therefore much more accurate and faithful to the original sound than the dynamic. Of course there is a downside too. This is that the impedance of the capsule (the part of any mic that collects the sound) is very high. Not just high - very high. It also requires continually topping up with charge to replace that which naturally leaks away to the atmosphere. A capacitor mic therefore needs power for these two reasons: firstly to power an integral amplifier, and secondly to charge the diaphragm and backplate.

Old capacitor mics used to have bulky and inconvenient power supplies. These mics are still in widespread use so you would expect to come across them from time to time. Modern capacitor mics use phantom power. Phantom power places +48 V on both of the signal carrying conductors of the microphone cable actually within the mixing console or remote preamplifier, and 0 V on the earth conductor. So, simply by connecting a normal mic cable, phantom power is connected automatically. That's why it is called ‘phantom’ – because you don't see it! In practice this is no inconvenience at all. You have to remember to switch in on at the mixing console but that's pretty much all there is to it. Dynamic mics of professional quality are not bothered by the presence of phantom power in any way, One operational point that is important however is that the fader must be all the way down when a mic is connected to an input providing phantom power, or when phantom power is switched on. Otherwise a sharp crack of speaker-blowing proportions is produced.

A capacitor microphone often incorporates a switched -10 dB or -20 dB pad, which is an attenuator placed between the capsule and the amplifier to prevent clipping on loud signals.


The electret mic is a form of capacitor microphone. However the charge is permanently locked into the diaphragm and backplate, just as magnetic energy is locked into a magnet. Not all materials are suited to forming electrets, so it is usually considered that the compromises involved in manufacture compromise sound quality. However, it has to be said that there are some very good electret mics available, most of which are back-electrets, meaning that only the backplate of the capacitor is an electret therefore the diaphragm can be made of any suitable material. Electret mics do still need power for the internal amplifier. However, this can take the form of a small internal battery, which is sometimes convenient. Electret mics that have the facility for battery power can also usually be phantom powered, in case the battery runs down or isn’t fitted.

By David Mellor Sunday April 13, 2003