Subsonic Microphone

Commercially-produced subsonic microphones are too expensive for the cost-conscious user. However, as long as a high-precision measuring device is not what is required, you can build such a microphone yourself using a few components from the spares box.The job of a microphone is to convert a sound into an electrical voltage as faithfully as possible. The conversion happens in two stages: first from sound into mechanical form in the microphone capsule, and then from mechanical into electrical form in the transducer itself. The chief characteristics of a microphone are its directional response and its frequency response. The directional response is only of peripheral interest to us here: it is the frequency response that concerns us more. How can a microphone, which according to its specifications has a frequency response starting at say 50 Hz, be made to respond to infrasound at frequencies below a few Hertz? To answer this question, we first need to go into a little theory.