Condenser Mics: A condenser mic works on an
electrostatic principle, not an electromagnetic one, as in the dynamic mic. The capsule consists of two
plates, separated by a very small distance, that are electrically charged, creating a
capacitor or condenser-like response.
One of the plates, often called the backplate, is fixed;
the other is part of the microphone diaphragm and is usually thinly gold-plated Mylar or similar thin film. As sound waves hit the diaphragm,
the distance between the charged plates changes, producing equivalent
fluctuations in the capacitance, and hence the flow of electrons across the gap. An additional component, a resistor, is
attached across the leads of the two plates. It is the fluctuating voltage across the
resistor that is sent out as the mic's signal. Almost all condenser mics are pressure-gradient, meaning part of the sound wave is let in behind the diaphragm through ports and a time-delaying narrow labyrinth. In addition, most condenser mics have a built-in preamp, though some attach to an external one. When a condenser mic distorts from acoustic overload, it is usually the preamp rather than the diaphragm that causes the issue. For this reason, these mics have attenuators to reduce the voltage into the preamp. However, it is best to remove this attenuation pad when not needed, as it will worsen the mic's signal-to-noise ratio if you need to crank the volume up for softer sounds. Vintage and vintage-based new mics may use an actual vacuum tube rather than an FET transistor in their preamp circuit for a "warmer" old-time sound. The ~$7,000 reissued Neumann U67, used to record Sinatra, the Beatles, etc. is an excellent example of a large-diaphragm tube condenser mic found in many high-end recording studios.
As can be seen on the following page many condener mics feature a second capsule attached to the rear of the backplate. By varying the polarity and strength of the signal from the rear capsule (often with switches on the microphone), multiple patterns can be obtained from the same microphone. If the front and rear capsules are in phase, an omnidirectional pattern is created. If the phase of the rear capsule were inverted, then a variety of patterns, from cardioid to figure-8 can be had by varying the amount of signal from the rear capsule. Some microphones feature an infinitely variable pattern adjustment.
To charge the plates, condenser
mics need a form of power called phantom power. This is
supplied by most professional mixing consoles or digital audio interfaces and is usually lableled +48 volts DC or PWR
— if you need phantom power from the board, look for a switch on
individual channels or channel groups. Phantom power from a console or interface requires a balanced line, since the shield carries part of the charge
(pins 2&3). Some condenser microphones are self-powered with a battery,
designed to go dead at the worst possible moment. Some mics, such as the Røde
NT5's, can run on either phantom power or a battery, and small wireless mics use a smaller charge, often in a belt pack. AKG C414 mics are condenser mics, and they do require phantom power.
Condenser mics normally have superior, flatter frequency response compared dynamic mics but are
much more sensitive to high SPL transients. So, they are good for recording most
instruments, and small bug noises — bad for close-mic'ing bass drums and
A variation of a condenser
mic is called an electret or electret-condenser. This
mic type has a permanently charged element (a special capacitor that acts like a magnet) requiring no phantom power. Better versions will have an internal pre-amp that does require power. Often, these are of back electret design, meaning the electret material is applied to the backplate and thus allows for better frequency response by not impeding the diaphragm. Electrets can, however, lose their charge or efficiency
in high temperature and humidity, also with age.