Suggestion: Please, would you be more specific since there are many over 400Hz devices, e.g.
Electronics-transmitters, receivers, radars, transponders, computers, printers, VCRS, etc.
Power-Induction ovens, microwaves, converters, motors, transformers, cables, generators, medical devices, etc.
Controls-Modulators, demodulators, multiplexers, etc.
Instrumentation-sensors, telemetry, motion sensors, proximity sensor, level sensors, etc.
Etc.
for pictures.
It could potentially be used on or within all of the above posted equipment except circuitry consisting of chips, ASIC, and components not using it, e.g. capacitors, semiconductors, etc.