The ones in microphones ... electret detectable out to 10 inches. This was with the original wax recipe, but there are now much better materials available, like polyethylene. [Jay] heated a a ...

The entirety of the build is minimal, consisting of a GY-MAX4466 electret ... a capacitor for power line noise suppression. [Stephen] uses various enclosures from Thingiverse for the microphone ...