Speaker as Microphone

The principle of an audio speaker is the exact inverse of an active microphone: membrane -> voice coil -> induced current. Therefore, a speaker can also be used as a microphone. The input leads of a speaker are used as outputs, run through an amplifier, and sent to a second speaker. When you talk into one speaker, the output comes out the other. Then, reverse the leads, and use the second speaker as a microphone and the first as the output.