This is very impressive I like this idea and would love to take its advantage I am also working on a same type of National Sons Day project you can see here.

Very interesting, thanks for sharing!

It would be interesting to see if the metallic sheet of the buzzer is transparent to infra red. If yes, then a very cheap DIY thermal camera sensor might be achieved by scratching the metallic deposited layer into a square grid, in order to form independent thermal pixels. Then, attach the scratched square grid to a PCB using solder balls, like the ones for the BGA ICs, and here we have it, a DIY thermal camera sensor.

This would be almost too good to be true.

metal is never transparent to infrared, just as it is to visible light. I think it may be fairly easy to etch away with some ferric chloride...

'Never' and 'always' are dangerous affirmations, they can be defeated by a single counterexample, like i.e. Germanium is a metal, yet is transparent to IR, so it is used to make Ge infra red lens.

The etching idea seems better than scratching, thought.

Anyway, a DIY IR thermal camera might be in reach using your observation. That is a project I will like to give it a try one day, I'll add it to the bucket list.

Germanium is a semiconductor with a band gap of 0.67 eV. They used to make transistors off this stuff.

A metal doesn't have to absorb visible light. It should then have a forbidden zone an eV or two above Fermi level. I'm not aware of any such metal though, and I wonder if such metal will absorb seriously anyway. 

I was always thought about semiconductors as being metals. Now I understand that was incorrect. My bad, sorry for the wrong affirmations. Thank you for pointing that out.