See "Chirpy TX/RX": a protocol and implementation for transferring data out of an airgapped device over a piezo buzzer.

https://aka-gabor.xyz/lab/

This thing is awesome. Low power microcontroller and LCD watch movement coupled with the legendary battle tested Casio cases. Hardware is fully open source and hackable too. Even if you don't want any custom functionality, it's a temperature compensated quartz watch with ~10 seconds per year of accuracy.

I wear mine every day. I've been helping out with the firmware's maintenance too. We recently merged dozens of pull requests. People actually wrote games for this watch, it's insane. The movement software is undergoing a major refactoring in preparation for the new board's release so it should be even better in the future.

Really looking forward to getting the new Pro board and the custom LCD which has more features than the Casio LCD as well as none of the limitations. It's showing hints of becoming a fully custom watch. People have designed and manufactured custom titanium cases. Who knows what the future holds.

Here's some recent discussion of the non-pro version

https://news.ycombinator.com/item?id=41562846

I really enjoyed the previous iteration of this (https://blog.jgc.org/2022/10/pimping-my-casio-with-oddly-spe...) and I'm backing this one. These are fun upgrades to an iconic watch.

I've been wanting to buy a sensor watch for a while now. I'm actually really glad I waited to back this now!

I would love an SPL meter on my watch with weighting filters! Don't know much about piezo buzzers but I wonder if it could be used as a microphone for this purpose.