I remember reading about a device that does something like an acoustic heterodyne to overload the analog front ends of eavesdropping microphones. I did a quick search here didn't find anything related to this. Can anyone point me in a direction to learn more about this?
Eavesdropping microphones can be jammed by overloading them with ultrasonic sound waves. The microphone and the front end analog circuitry, when overloaded, can generate noise in the audible frequency band, reducing the sensitivity of the microphone to sound in the normal audible frequency range.
This article can get you started, but the term "microphone jammer" will generate many other results. The jammer may irritate and/or harm animals that can hear ultrasound, and there's a possibility of adverse effects in humans -- even though they can't directly perceive the energy as an audible sound. Experiment with care.