Microphone Sensitivity

Author: N.H. Crowhurst

A microphone does not amplify sound. The intensity of the waves is very small, whether the diaphragm is arranged to pick up particle movements, as in the ribbon type, or pressure fluctuations. The regular telephone microphone, which is of the carbon type, is effective only for two reasons:

1. It is provided with a mouthpiece to collect all the sound from the mouth of the person speaking into it.

2. It really works by "modulating" an electric current, not generating one. The current comes from a battery at the telephone exchange, and the vibrations of the diaphragm modulate this rather large current instead of generating small ones from the vibrations themselves.

At the receiving end, too, advantage is taken of an earphone to get all the received sound energy right into the listener's ear. (The output of a telephone receiver is not great enough to drive a loudspeaker.)

The carbon microphone modulates an electric current

A small loudspeaker can be used to make a microphone that is quite sensitive by ordinary standards. Sound waves impinging on the speaker cone move the voice coil and generate small currents. This type of microphone does not give as good quality as a properly designed one, but it is often used in intercommunication sets of the kind used in offices. If you try connecting two small loudspeakers in different rooms, you will only be able to hear by having someone speak very close to the "microphone" and putting your ear very close to the other loudspeaker.

Basic intercom using moving coil speakers

Practical intercom using an amplifier

Practical intercommunication sets use an amplifier, between the two speakers, which makes communication much easier. Even then the quality is not good, but has the well-known shrillness associated with such "squawk boxes." The reason for their use is that they are much more sensitive than any other form of microphone, and so make the system less expensive, by requiring less amplification and less attention to special wiring. (If a loudspeaker were connected directly to a moving-coil microphone, it would be virtually impossible to hear anything, however close the speaker was to the microphone and the listener to the loudspeaker.) Thus, any practical form of microphone needs electrical (or, more properly, electronic) amplification, to get enough current to be of any use in driving a loudspeaker.