For those who don't know, if there weren't nonlinearities in the ear you wouldn't hear difference tones (https://en.wikipedia.org/wiki/Difference_tone). Those nonlinearities are interactions within the audible range though, not interactions between audible frequencies and ultrasound.

(Difference tones are a convenient way to tune a violin: when two strings are a fifth apart (as they should be), the difference tone is at the octave, and it's easier to get an octave right than a fifth.)