This is a good question, but people are pretty sensitive to pitch changes, I think we would detect the motor slowing down. It would be super easy to reveal as

This is a good question, but people are pretty sensitive to pitch changes, I think we would detect the motor slowing down. It would be super easy to reveal as well. A family member with the same toothbrush, your head finishes first. Motor slows down, pitch goes down. Compare the two. Replace the old head, now they're the same. Scummy practice revealed. Scandal. That said. I'm not totally sure on the mechanism that all electric toothbrushes use. It's much harder to detect subtle amplitude changes.