[knodi123]

When measured, it apparently came out to about 262.5 Hz for Sonicare.

https://pubmed.ncbi.nlm.nih.gov/15575438/

And that makes sense. On the face of it, the idea that the bristles are vibrating in the MHz range is just absurd. "Ultrasonic" is by definition above the human range of hearing, which is about 20,000 Hz as you said. I own a sonicare. I've heard it. It's not inaudible. :-)

[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.