A recent art project used ultrasonic waves to jam cellphone microphones. The idea was to prevent eavesdropping from recorder applications or a compromised, or otherwise always-on mic.

A side effect of this, and perhaps these devices as well, as depending on the volume, these effectively create RF interference in a way in the circuit, as in the electromagnetic signal beyond the mic is changed, and could lead to unintended side effects such as the inability to speak to 911/999 emergency services.