How ultrasound can spoil a microphone recording
An ultrasonic scrambler targets a microphone, not a person’s ears. It emits sound at frequencies above the range most people can hear. A microphone can still respond to some of that energy, even when the sound is inaudible to the person holding the recorder.
How an inaudible signal becomes interference
A microphone turns changes in air pressure into an electrical signal. In some devices, strong ultrasound pushes the microphone or its electronics beyond their clean operating range. Their response is then no longer perfectly proportional to the incoming sound: distortion can turn part of the high-frequency signal into lower-frequency artifacts, or overload the recording circuitry. The result may be noise or distortion that masks speech and makes the recording difficult to use.
This depends on the particular microphone and recording device. Some filter out ultrasound or resist overload better than others, so a scrambler is not a guaranteed shield against every recorder. Its effect also depends on how much ultrasonic energy reaches the microphone; distance, direction, and obstacles can matter.
The practical limit
The key vulnerability is a mismatch between human hearing and microphone behavior: a signal can be imperceptible to people nearby yet disruptive to certain recording equipment. But “inaudible” does not mean universally effective—the same device may spoil one microphone’s recording and leave another largely intact.
