Ultrasound high frequency sound waves are inaudible to humans. The most common use for devices that utilise them are found in hospitals for defining the gender of a baby, for scaring away insects or for tracking human movement through sensors.
But the problem lies that the application of ultrasound by humans with bad motives is the one that is most feared. Assistive Listening Devices or ALD are most prone to interference. But besides them micro electrical mechanical sensing chips that are found in car airbags or in smartphones can be hacked by a command that the human ear will not hear.
And aside from the fact that technology can be affected by them there are side effects on humans too, like nausea, fatigue, high blood sugar levels etc. Thus the application of ultra sounds can harm both machines and humans. This domain of sounds needs to be studied more. Especially since it can be a crucial aspect of our lives.