A condenser microphone consists of a very light diaphragm and back plate to which is applied a polarizing voltage. The principle of operation is that sound waves impinging on the diaphragm cause the capacitance between it and the back plate to change in sympathy. This in turn induces an s.c. voltage on the back plate.
Because the capacitance is relatively small the electrical impedance is high and unmanageable so to overcome this condenser microphone in corporate an JFET to transform the impedance to a lower level suit-able for feeding an amplifier via screened lead.
The ECM operates in a similar manner except that it has a permanent charge voltage implanted in an electret material to provide the polarizing voltage. This can be effected in three ways, the most common being when the diaphragm is the electret material, in this case one side is metalized. This is known as the foil or diaphragm type. The electret material dose not make the best diaphragm and where higher performance is required the diaphragm is made of other material and the electret material applied to the back plate. This is known as the back type. A more recent variation is the so called front type.
Here the electret material is applied to the inside of the front cover of the microphone and the metalized diaphragm connected to the input of the FET. this works by vibration in capacitance between the front cover and the diaphragm caused bye the sound waves impinging on the diaphragm.
Like electret condenser microphone ECMs can be made to have directional or pressure gradient characteristics by allowing back feed to the diaphragm with suitable acoustic treatment.
A major advantage of the ECM is its immunity to vibration, resulting from the very low mass of the diaphragm. This is particularly useful in the field of telecommunications and hand held or body worm gear and where it is installed in equipment incorporating moving parts such as motor in dictation machines and cassette recorders.
The sensitivity is defined as the output voltage for a specified acoustic stimulus and load condition. It is expressed in dB/V/Pa. In the case of dynamic type it is expressed as the open circuit voltage appearing at the output terminals. In the case of electret type it is expressed with a specified resistance tends towards constant current characteristic.
The directivity frequency characteristics are obtained from a measuring circuit shown in fig. 1. A microphone is set up under the standard measuring conditions and it is turned around the reference point in a direction horizontal or perpendicular to the reference axis. In his state, the directivity frequency characteristics are expressed by the values of sensitivity levels measured according to the specified angle.