A standard electret microphone can detect sub-audio "sounds" when some simple equalization circuitry is added.

Electret microphones are capable of high-quality ... to properly drive a balanced audio connection. The circuit is designed to fit inside a common BM700 or BM800 microphone body, and the PCB ...

and figured out a way to explain how the circuit works. While not the most important component, the most obvious component inside a Clapper is a microphone. [Rafale] is using a small electret ...