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BAS Data Sheet No 4
 

Aviation Microphones

There are 3 basic types of microphone used in aviation: Carbon, Dynamic and Electret

Carbon Microphones

The carbon microphone was originally invented by Graham Alexander Bell over 100 years ago and set the standard for all aviation transceivers of US origin up to the present day. A carbon microphone is basically a small container containing very fine carbon granules with one side of the container made flexible and connected to a diaphragm that responds to speech. The voice sound waves cause the diaphragm to respond to the voice pattern which in turn causes the carbon granules to be compressed in sympathy.   A DC voltage of between 5 and 10 volts is generated in the radio and applied across the carbon granules.   In the static situation, the granules have a specific resistance and pass a steady DC current.   When the granules are compressed by the diaphragm in sympathy with the voice pattern, their electrical resistance varies resulting in the current flow becoming proportional to the impressed speech audio.  This current flow is seen as an AC audio voltage superimposed on the steady DC voltage level, and when passed to the radio, the AC output is extracted as the microphone input.  Typically, this AC audio voltage level from a carbon microphone is of the order of 0.5 to 1 volt, and this level has become the standard that all US are designed transceivers to accept.

Dynamic Microphones

As the years went by, technological advances developed smaller and more efficient ways of converting the audio speech waves to an electrical signal. The first of these was the dynamic, or moving coil microphone in which the diaphragm is mechanically connected to a coil which moves in a strong magnetic field.  The design is very similar to a loudspeaker but much smaller.   Indeed, miniature speakers are occasionally used as dynamic microphones.  The resultant voltage output from this moving coil is of a very low order, around 10 millivolts, some 100 times less than the output of the carbon microphone.  While the dynamic microphone does not require any energising voltage, when used with US designed transceivers the low output requires amplification to the standard 0.5 to 1volt level to drive the radio input circuitry.  This amplifier is usually built into the microphone and is powered by a DC energising voltage to produce the required audio output level.  This version is usually referred to as an amplified dynamic microphone although confusingly the word 'amplified' is not always used. European designed transceivers, in addition to the standard or carbon mic input, usually have a second microphone input to which an unamplified dynamic microphone can be connected directly.   This second low level mic input is for directly connecting a dynamic microphone, it does not need the transceiver to provide any DC energising voltage.

Electret Microphones

A more recent development is the electret microphone.  In an electret microphone the voice diaphragm is connected to two plates which sandwich a small piece of piezo crystal.  As the pressure on the crystal varies, it produces a very small audio voltage which is picked up by the plates.  This voltage requires considerable amplification to become useable and a small amplifier is directly connected to the piezo crystal.  The amplifier requires a DC voltage to operate and in an aviation transceiver this is usually provided by the DC energising voltage present at the microphone input. The electret microphone, because of its small size and excellent frequency response, has become the commercial microphone of choice. However, in most electret applications, the output voltage level is designed to be much lower than that needed to fully drive aviation transceivers.   In aviation applications, the electret mic is designed to have considerably more amplification to provide the 0.5 to 1.0 volts needed to drive an aviation transceiver.  For this reason, most commercial electret mics not designed for the aviation market have insufficient output to fully modulate an aviation transceiver.