I wonder if you are hearing a "beat" tone (a third tone which is heard when two tones of different frequency are played. Playing tones at 500 and 600 Hz will also cause a tone at 100 Hz to be heard). This is how 3D positional audio systems work -- they beam two ultrasonic tones, and the difference tone appears to emanate from where the ultrasonic beams intersect.

Sending 50kHz vibrations into the earn should interact with a 49kHz tone to cause the listener to hear a 1kHz tone.

(This is all from an acoustics course I took over a decade ago, so take it with a grain of salt!)