[jml7c5]

The Sonicare is marketed as a "sonic" toothbrush (a marketing tactic that I never really understood), so it's not too surprising that it's in the audible range.

I suspect that you, like myself, assumed parent was writing "ultrasonic" by mistake and didn't look too carefully at the wiki link they posted. But to my surprise, true ultrasonic toothbrushes are real! Take a look at the link; apparently they typically operate at 1.6 MHz.