What is a condenser microphone?

Description of the condenser microphone and explanation of its relationship to the capacitor microphone

 

Microphones can be separated into two basic types - those that work on magnetic principles and those that work on electrostatic principles. The former are known as dynamic microphones, the latter as capacitor - or condenser - microphones.

Internal view of the Neumann BCM 104

A capacitor is a device that stores energy by separating positive and negative electrical charges. 'Condenser' is a old word for 'capacitor', so a condenser is a capacitor, hence a condenser microphone is a capacitor microphone. In common usage however, it is not uncommon to call a large-diaphragm, vintage vacuum tube microphone (or its modern retro-designed replica) a condenser microphone. It is however rare for a small-diaphragm capacitor microphone to be referred to as a condenser microphone.

In technical terms...

The capacitor in a capacitor microphone (or condenser microphone) consists of the diaphragm, which picks up sound vibrations, and a fixed back plate separated by an air gap. A 48-volt 'phantom' power supply (or a power supply unit dedicated to the microphone) separates positive and negative charge between the diaphragm and the back plate, thus charging the capacitor.

Sound vibrations picked up by the diaphragm cause the distance between the diaphragm and the back plate to change in proportion to the movement of the air molecules in contact with the diaphragm. Since the value of the capacitance of the diaphragm/back plate system is related to the distance between the diaphragm and back plate, then the capacitance of the system will change in proportion to the incoming sound wave. Also, since the voltage between the diaphragm and back plate changes in proportion (actually inverse proportion) to the value of capacitance, the system will produce a voltage in response to the incoming sound wave.

In practical terms...

The essential difference between the capacitor microphone and the dynamic microphone is that the diaphragm of a dynamic microphone has to be attached to a coil of wire that moves within the field of a magnet (or it could alternatively, in principle, be attached to a magnet, the field of which passes through a coil) in order for the microphone to function. The diaphragm of the capacitor mic has no such burden and is therefore much more free to move in response to the incoming sound wave.
A capacitor microphone, or condenser microphone, therefore has a sound texture that in most cases has a higher degree of clarity than a dynamic microphone. Capacitor microphones will pick up the overtones of metallic percussion instruments very much more clearly than a dynamic microphone would. They are also noticeably demonstrate crystal-clarity on most other acoustic instruments. The differences between the capacitor microphone and dynamic microphone are less marked on membrane percussion instruments (drums) and electric instruments such as the electric guitar.
One small additional point is that because a dynamic microphone works on magnetic principles, it can pick up hum through magnetic coupling when positioned close to the transformer of an electric guitar combination amplifier-loudspeaker cabinet. Moving the microphone away from the transformer will usually reduce the hum to near-inaudibility. However, a capacitor microphone works on electrostatic principles and is not sensitive to this source of hum at all.

Phantom power

Capacitor microphones need electrical power to function, firstly to charge the diaphragm and back plate, secondly to power the essential internal amplifier. The signal from the diaphragm is extremely weak and has to be amplified within the microphone, before sending it down the cable to the preamplifier, audio interface or mixing console.

Some types of capacitor microphone, known as electret, or back electret, have a permanent electric charge, but they still require power for the amplifier. Many electret microphones can however operate from an internal battery.

Power is normally supplied directly from the microphone preamplifier, audio interface or mixing console. Power in this form is known as phantom power, because no power supply is visible.

However, vacuum-tube microphones, of the type that may archaically be referred to as condenser microphones, require higher voltages and thus normally have their own power supply.