I wonder if high-intensity ultrasound can damage human hearing even though we can't hear it. With a wide-angle dispersion, it's possible that the waves will directly impinge on an ear without our being aware of it.
They do cause damage.I remember a traffic light which always made my ears, or my head, feel uncomfortable whenever I walked in front of it. (This was decades ago.) I believe it was an ultrasonic pitch operating as a vehicle detection system.
For the new study, neurobiologist Markus Drexl and colleagues at the Ludwig Maximilian University in Munich, Germany, asked 21 volunteers with normal hearing to sit inside soundproof booths and then played a 30-Hz sound for 90 seconds. The deep, vibrating noise, Drexl says, is about what you might hear “if you open your car windows while you’re driving fast down a highway.” Then, they used probes to record the natural activity of the ear after the noise ended, taking advantage of a phenomenon dubbed spontaneous otoacoustic emissions (SOAEs) in which the healthy human ear itself emits faint whistling sounds. “Usually they’re too faint to be heard, but with a microphone that’s more sensitive than the human ear, we can detect them,” Drexl says. Researchers know that SOAEs change when a person’s hearing changes and disappear in conjunction with hearing loss.