So there is this thing called chirp.io which allows you to transmit data via ultrasonic sound. No special hardware needed. Only a mic and a speaker. I wonder why this isnt used in smart watches instead of BLE as it requires even less power. Probably because sound is less penetrating than radio waves??!

Because sound, be it audible of not, causes damage to your ears if you are exposed for a long time. Also, some people can listen to high pitched frequencies.

Noise?

Also, is it even ultrasonic? I definitely used audible chirps from them in the past.

since there's many people who have no trouble hearing their voltage transofmers' noise, (me being one of them) this sounds like a... neat idea in theory, but a horrible one in practice.

also, any kind of sound transmission is very prone to noise (in the data quality sense), which is why phone modems never were too good

Could we do it with normal sound and disguise the data stream as music or hide it in music. That sounds fun.