Capacitor mics have been around for several decades, and although modern capacitor mics do incorporate a few small technical improvements, the sound character has actually changed very little -- some of the best-sounding models were designed over 20 years ago. Basically, the heart of any capacitor mic is a pair of conducting plates, one fixed and the other in the form of a moving diaphragm. When the spacing between the plates changes (as it does when the diaphragm vibrates) the capacitance varies, and if a fixed electrical charge is applied to the capacitor, an electrical signal is produced which faithfully represents the diaphragm vibration.
To keep the weight down, the diaphragm is often made from gold-coated plastic film. As a result, the diaphragm assembly is very light compared to that of a dynamic mic, so the system is much more efficient, and is capable of capturing harmonics right up to the range of human hearing and beyond. The size of the diaphragm also has an effect on the tonal quality of the mic -- large-diameter models are chosen for vocal work because of their warm, flattering sound. Small-diaphragm models tend to be chosen where high accuracy is required.
Even though they are relatively efficient, capacitor microphones still produce such a small electrical signal that they require a special type of built-in preamplifier to bring the signal up to usable levels, and this is one factor that contributes to the higher cost when compared to dynamic mics. Additionally, all capacitor mics need a polarizing voltage in order to work. The most common source of polarizing voltage is the 48V 'phantom' power source, which is why many mixing consoles have a phantom power supply built in. The term 'phantom power' came about because the polarizing voltage is supplied via the signal leads of the microphone -- no additional cabling is needed. Because of the way phantom power is supplied, all phantom-powered microphones must be balanced, and must employ the same wiring configuration. Budget mixers or cassette multitrackers with unbalanced mic inputs cannot be used with conventional capacitor microphones unless an external mic preamp (with phantom power) is used.
power is supplied, all phantom-powered microphones must be balanced, and must employ the same wiring configuration. Budget mixers or cassette multitrackers with unbalanced mic inputs cannot be used with conventional capacitor microphones unless an external mic preamp (with phantom power) is used.
Broadly speaking, capacitor microphones are more expensive than their dynamic counterparts, but they are also much more sensitive, and can capture high-frequency detail much more accurately. Furthermore, the capacitor principle, unlike the dynamic principle, lends itself easily to the production of mics with switchable pickup patterns (see the box elsewhere in this article for more information on these), although the cheaper models tend to offer just a fixed cardioid pattern. Currently, there are some real bargains to be had in the capacitor mic market -- for example the AKG C3000 (currently retailing at £351) and the Russian-made Oktava MK219 (£311).
CAPACITOR MICROPHONES
ELECTRET MICROPHONES
There is another type of capacitor microphone, known as the electret mic. Despite inauspicious beginnings, these have now been developed to the point where they can rival true capacitor quality for a much lower price. Instead of applying an electrical charge to the microphone capsule via an external power source, electret mics use a diaphragm made from an insulating material that has a permanent electrical charge. A preamplifier is still needed, but this can be built very cheaply, and will run from a battery in some cases.
Electret mics made in this way don't offer any real advantage over dynamic mics, because the diaphragms have to be quite heavy in order to carry the permanent electrical charge -- but what if the permanently-charged material was fixed not to the diaphragm, but instead to the stationary back-plate?