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As the name implies, the condenser microphone or capacitor microphone uses a capacitance that varies in line with the incoming signal to generate the varying output voltage.
The name condenser microphone still persists. The microphone was invented in the days when capacitors were still called condensers.
Capacitor microphone operation
As the name implies the condenser or capacitor microphone relies on changes in capacitance for its operation.
The actual condenser microphone element consists of a thin membrane in close proximity to a solid metal plate. The membrane acts as the diaphragm and is electrically conductive. Older microphones used a thin metal foil but more modern types may use a plastic coated with gold or aluminium. One common type is gold-sputtered mylar.
This construction creates a capacitor which may be in the region of 10 to 50pF. For the condenser microphone to operate it requires a DC voltage to be applied. This can be supplied by the battery shown, but for high end microphones it may also be supplied along the coaxial line to the microphone – this is known as phantom power. The most common voltage this is 48volts.
This voltage not only provides the voltage needed for the microphone to operate electrically but also pulls the diaphragm taut.
When sound waves hit the microphone, the diaphragm moves backwards and forwards. This changes the level of capacitance and as a result small voltage changes are seen across a high load resistor connected across the microphone element. .
As the impedance of the condenser microphone is very high, a buffer amplifier is needed. This has the effect of converting the signal so that it has a much lower impedance. This amplifier is also powered either from the internal battery or from the phantom power line.
Condenser microphone features
The very low mass an inertia of the diaphragm results in the microphone having a flat and extended frequency response. In fact condenser microphones offer the widest frequency response an best transient response of any microphone allowing them to faithfully pick up the attack of a drum or the “pick” of an acoustic guitar. Also, condenser microphones usually offer much higher sensitivity and lower noise than dynamic microphones.
However the basic microphone element requires a low noise preamplifier to ensure that the microphone element is not loaded. It also requires power for the capacitor and preamplifier. This is normally provided as phantom power from the mixer, or from a small battery within the microphone.
As a result of the high sensitivity, these microphones can be overloaded by very loud sounds, so care needs to be taken in selecting them for applications where they will not be overloaded.
The condenser microphone is not as robust as the dynamic microphone as its internal construction is relatively delicate. Whilst this results in a low inertia system that gives a good response, it also means it is less robust.
The other main issue to be aware of when using a condenser microphone is to avoid humid environments. High levels of humidity have been known to cause internal flashover between the diaphragm and the back plate of the microphone element.
Condenser microphone summary
The condenser microphone has many advantages and a summary of their key features is given below.
|Condenser Microphone Key Features|
|Typical output impedance||Typically around 200Ω or less.|
|Transducer impedance||Impedance of the transducer itself is very high – many MΩ.|
|Typical frequency response||Can be as good as 20 Hz to 20kHz or better.|
|Typical applications||High quality sound recording, some sound support systems where additional sensitivity is needed.|
|Durability||Care needs to be taken when using and handling these microphones. They can be damaged more easily that dynamic microphones.|