Microphones
David Kinden - 10th August 08
Microphones covert acoustic sound pressure waves to an electric voltage. They are composed of three parts. The diaphragm is a membrane which vibrates due to the incoming sound wave. The transducer coverts the mechanical vibration to an electrical signal. The case holds everything together, provides insulation from handling noise and effects the directional response of the mic.
There are two types of microphone commonly used in modern recording. The moving coil/dynamic microphone, and the capaciter/condenser microphone. The terminology varies depending on where abouts in the world you live.
Dynamic Microphones
The dynamic mic has a round diaphragm connected to coils of wire. Sound waves move the diaphragm and hence the coils. The coils are suspended inside a stationary magnet. The moving coil in this magnetic field generates a voltage that is proportional to the sound pressure. The high frequency response of dynamic mics is limited by the mass/inertia of the moving parts, the response typically starts falling at around 10 kHz. They often exhibit a resonant peak around a few kHz, this lifts vocals and increases intelligibility.
Capaciter Microphones
Capaciter microphones use a diaphragm which forms one half of a capaciter. The other half of the capaciter known as the back plate is held stationary in the microphone casing. As the diaphragm moves, the distance between the capaciter plates changes. These changes result in an output voltage proportional to the incident sound pressure. The far lighter diaphragm on capaciter mics mean they are able to move faster, giving them a better high frequency response than dynamic mics.
This makes them ideal for capturing instruments such a cymbals and acoustic guitars. A voltage is required across the capaciter plates, this is sometimes supplied by a battery, more usually from a 48 volt 'phantom power' source. Most modern mixing desks and audio interfaces provide a phantom power source. Dynamic mics do not require phantom power but will not be harmed if the are used while it is switched on. Be sure to turn down your monitor volume off before switching phantom power on. Otherwise the voltage spike will produce a nasty click through monitors and headphones that is damaging for speakers and ears.
Sensitivity
Sensitivity is the size of the electrical voltage generated by the mic compared to the size of the acoustic signal it is picking up. Therefore a mic with high sensitivity will need less gain from the mixing desk, thus achieving a better signal to noise ratio. Capaciter mics are generally more sensitive than dynamics. This means they are useful for recording medium to low volume instruments, but may distort if used on very loud things such as a kick drum or bass amp.
Directional Response - Polar Patterns
The sensitivity of a microphone can vary depending on which angle the sound wave is arriving at the diaphragm from. The sensitivity can be plotted on a graph to produce the directivity, or ‘polar pattern’ of the microphone.
When a source is directly in front of the mic it is said to be on-axis. When the source is moved away from the front it is off-axis. At low frequencies most microphones give an omnidirectional polar pattern, they are equally sensitive in all directions. Cardioid microphones have a big drop in sensitivity off-axis at high frequencies, and are least sensitive from the rear.
This explains some vocalists prefer to angle their mic up or down slightly. This alters the frequency response of the mic slightly. The polar response is a result of the casing design of the mic. Dynamic and capaciter mics can both exhibit the different polar patterns outlined below.
The polar response is plotted in two dimensions for clarity, but this shape is actually a three dimensional orbit around the diaphragm.
The cardioid polar pattern is useful in a number of ways.
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Separation between instruments is improved. A cardioid mic on the snare drum could be angled directly off axis from the hi-hats resulting in less high frequency spill.
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In live setups the on-stage monitor faces directly up at the vocalist mic. A cardioid response limits the amount of sound from the monitor entering the mic. This means the volume of the monitor can be higher before feedback incurs.
