To be a little more pedantic, they send ultrasound frequencies and they “degrade” into regular audible sound waves when they collide with something (combination of harmonic effects and destructive / constructive interference). The reason they send ultrasound instead of using regular directional speakers is because it’s smaller wavelength is easier to aim so much more of the energy moves forwards, so you don’t hear much noise coming from the source itself.

That’s why they work for “crowd control”. A strong enough regular speaker would bother the operator too much.