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Comment by superkuh | Hacker News Reader

Basically, ultrasound can fluidize the lipid membrane and cause leaks. MRI is used for mapping and guidance of the ultrasound mediated lipid shearing.

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Does this mean that ultrasound (e.g. when used for imaging) can cause bad effects as well?

Not really when it’s used in the ways they use for imaging. What this procedure uses is ‘focused’ ultrasound that is comprised of a helmet-array of transducers (it has serious Magneto vibes) that is very specifically calibrated to create areas of focused resonance within the brain, using the MRI imaging as guidance. Any of the individual transducers don’t have anywhere near enough power or penetration to be really dangerous by themselves.

This same technique is also being researched as an alternative for radiation therapy, since it doesn’t cause nearly as much collateral damage.

I know very little about medical ultrasound, but I do know that ultrasonic cleaners can seriously hurt you if they're powerful enough. On the other hand I recently had a treatment on my foot where they used ultrasound to stimulate blood flow. I just felt a slight warmth. So I imagine it's a matter of intensity, and where it's pointed.

does that apply to ultrasonic toothbrushes too?

Probably not- the faster sonicare toothbrushes vibrate at 512 Hz, while sonograms are between 2,000,000 to 18,000,000 Hz. There's probably a significant difference in power levels, too.

Wow, I had no idea that we even had the technology to produce sounds at that kind of frequency, let alone that it was routinely used!

The piezoelectric effect is used to do this[0]. This other wiki page[1] has some neat pictures of what the sound field looks like.

[0] https://en.wikipedia.org/wiki/Ultrasonic_transducer

[1] https://en.wikipedia.org/wiki/Ultrasonic_transducer#Transduc...

"Ultrasonic" is by definition above 20,000 Hz. According to wiki[0] an ultrasonic toothbrush typically operates on a frequency of 1.6 MHz.

[0]https://en.wikipedia.org/wiki/Ultrasonic_toothbrush

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. :-)

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.

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