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

The simple schematic is in Figure 4; the concept also worked well in LTspice, using their default “NJF” JFET, and formed part of Figure 2’s simulation. Figure 4 A circuit that simulates an electret ...

The schematic design and preliminary budget for the Reimagine Miller project were approved by the Marshalltown Community School District Board of Education during the regular Monday meeting ...