Microphones Part Two
The second part of my article relating to the ins and outs of audio microphones and their uses in music.
This mic is also called a capacitor mic. It uses a capacitor to capture sound. A capacitor is an electronic element that can store an electrical charge. basically its just two conductive plates parallel to each other. A capacitor stores electrical charge according to this formula:
Q = C x E
Q- charge in coulomb
C- capacitance in farad
The capacitance of a capacitor depends ( amongst other things ) on the distance between both plates.
Capacitors contain a relation ( certain ) charge and no current is running. When the front plate moves closer to the back plate the capacitance will change. The result is a current running from the back plate via the resistor to the power supply. According to Ohms law current through a resistor will result in voltage, this voltage is the audio signal. The voltage applied to the front plate is called the polarizing voltage. In a condenser mic the front plate is the diaphragm. It's thin, gold plate, plastic foil. The back plate is often made of gold plated ceramic. The diaphragm can be made very light giving the mic a very good high frequency response.
Condenser mics are sensitive to environmental factors ( heat and humidity ). They are more sensitive to shocks and are not so common for live work. Output voltage is very small so it must be adjusted with a pre-amp. This can be done by a tube built into the mic. The tube will give you that warm sound. Nowadays F.E.T's are used instead ( field effect transistor ). Both need a power supply to function.
Pre-amp voltage and the polarizing voltage are supplied by the phantom power. The voltage is transported over balanced cables on both cores.
Electret Condenser Mics
These work exactly the same as a normal condenser but this time one of the plates is made of an electret material. Electret material is capable of storing electrical charge permanently. Its charged with an electron beam so no polarizing voltage is needed. We still need phantom power for the pre-amp.
With the back electret condenser mic the technology to make them is cheap, so these mics are found in consumer equipment. In studios they are also used especially for close miking as they can be made very small.
These are made up of pulverized carbon placed in a closed disk called a 'button' coupled to a metal diaphragm. A voltage is applied across the carbon and sound striking the diaphragm crushes the carbon, making it more and less dense.
These are also known as pressure mics. They are good for grouping things together and capturing room reverberations ( room acoustics ).
These mics are also called 'pressure gradient mic'. They have two groups:
1. Unidirectional ( one direction )
2. Bi-directional ( two directions )
Bi-directional is usually called a figure of 8 polar pattern. When a omni-mic and a bi-mic are combined it is possible to make different unidirectional polar patterns.