According to Wikipedia, a loudspeaker is an electroacoustical transducer that converts an electrical signal to sound, while a A microphone, sometimes colloquially called a mic or mike, is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. The loudspeaker involves electromechanical processes where the amplified audio signal must move a cone or other mechanical device to produce sound like the original sound wave.
This following loudspeaker microphone circuit diagram takes an ordinary loudspeaker and allows it to be used in reverse, as a microphone. The circuits allows you to use a cheap loudspeaker as a microphone.Sound waves reaching the speaker cone cause fluctuations in the voice coil. Circuit diagram specification: speaker impedance may be 4 ohm to 64 ohm. The 8.2 ohm resistor value may be changed to match the actual speakers own impedance.