Vibration. Do you know them? Do you understand what they're doing?
I'm not really a physicist, but I've taught quite a lot in the field of vibration. Why? Well, vibrations have a very specific effect.
In my case it is about vibrations from a record that is set in motion with an enormous frequency. Ultrasound you'll get. Sound waves (vibrations) that go so fast, we can't hear them.

It was once invented that if you put those vibrations on the body now, they can also move your body. Logically, when you bump into a tree, it also shakes. When you bump into that tree over and over again, it shakes again and again. But take a good look at that tree. The place you bump into. That's where the tree is thick. A short stiff movement is your effect. But at the top of that tree, where the branches are thinner, you see it swinging with a much larger movement. Logical, too. A thin branch is more flexible and a thick branch is stiff so the trunk is the most rigid part of a tree.

(rather not go out trying)

But now I'm talking about wood, so one and the same fabric. Now imagine I put such a trembling plate on your shoulder. Then there's underneath that trembling record:
1. Skin
2. Fat (maybe little, but still something)
3. Muscle and tendons (yes, hopefully some more of that)
4. Bone
Yeah, blood vessels, nerves, the whole rambam.
But let those parts vibrate independently of each other (no, don't want to disassemble your body now). Make yourself a performance. Compare that skin to a piece of leather, the fat with a nice jell-pudding, the muscle with a nice steak and your bone with a piece of wood. Does it all react the same to that vibration?
So no!

What happens when that bone is adjacent to a muscle where that muscle is shaking through my vibration and the bone is almost standing still?
Friction! So that's exactly what's happening here.
And so you can let heat occur deeper into your body (when that vibration reaches that depth) and also the associated effects. So, within physiotherapy, this is the treatment with ultrasound and use this principle.

We can also convert the same reflection of waves into images. You could choose to darken fabric that allows little vibration to pass through dark and fabric that lets through a lot of vibration, just light (or vice versa). Do you now place such a triplate on a part of the body that can pass quite a lot of vibrations: The belly, what could you see?

Many ladies among you have already lived through this life and the image is amazing. Luckily, I was also allowed to watch the ultrasound that was once made when my wife was pregnant.

A thrill!

Trembling, it's great to hear, to feel and oh, when you want to see it:

https://www.youtube.com/watch?v=lRFysSAxWxI

https://americanhistory.si.edu/science/chladni.htm

https://www.youtube.com/watch?v=tFAcYruShow

#education

Physics, vibration and visible sound.