Using a single cable such as speaker wire or doorbell cable, this circuit can be remotely positioned, for
example, at the bottom of a garden or garage, and used to detect all sound in that area. The cable can
be buried in a hosepipe or duct and is concealed out of sight. The mic is an ordinary dynamic mic insert
and should be housed in a waterproof enclosure with the rest of the circuit.
The mic output is amplified by the two transistors, the output is fed down the cable via the 220u capacitor.
Here, it has a dual purpose of preventing the DC supply from upsetting the bias of the circuit, and also
allowing the smaller ac audio output to pass down the line. At the power supply, the audio is recovered
by the 10k preset and 220u capacitor. It is used to feed a small audio amplifier (such as the 2watt design)
shown earlier on this site.
Source: zen's circuit directory