Creating A Microphone from the speakers

The circuit design utilizes a regular speaker to be converted into a microphone
Although it seems strange to think that a loudspeaker can be used as a microphone because of their difference in functionality, it is indeed a fact that any speaker can be transformed to a microphone. To differentiate in a more technical detail, a loudspeaker is an electroacoustic transducer intended to radiate acoustic power into the air, with the acoustic waveform equivalent to the electrical input waveform. While a microphone is an acoustic-to-electric transducer that transforms sound into electrical signal.

In the circuit, the variation on the voice coil, made by the audio waves arriving at the speaker, creates a few electrical signals. This signals that are passing to the amplifier is being converted to audio signals coming out of the speaker. A range of voltage between 6V to 12V can be imposed on the circuit. During the first stage, the low impedance of the speaker corresponds to that of the transistor as it runs in common base mode. Direct coupling functions in the emitter follower as the second stage. Longer cables can be operated due to low impedance and less voltage gain.

Normal speakers when converted as a microphone may not provide excellent result, but hi-fi speakers when converted as microscope are better in quality than usual microphones. One most popular application of this circuit is the intercom wherein the operations are switched between end points.
Source:www.zen22142.zen.co.uk/Circuits/Misc/Sp-mic.htm