Audio (acoustic waves in air) at 65KHz have a wavelength that small. This is ultrasound. When you beat two ultrasound waves together you can get beat frequencies for example in the audible range, inside the head. And yet you cannot hear either ultrasound wave separately, by itself, as it is higher than the ear can detect. May seem like magic but really it is just simple acoustic physics.

Audio (acoustic waves in air) at 65KHz have a wavelength that small. This is ultrasound. When you beat two ultrasound waves together you can get beat frequencies for example in the audible range, inside the head.