21 sats \ 4 replies \ @megaptera 21 Jun \ parent \ on: Nerd alert: ecash via sound? bitcoin
Would this be possible with a sound that is not possible to hear for humans? e.g. in a specific frequency? Then it could be used so pay contactless between two devices... :)
I think it's possible if the two devices (1) are close to one another (2) have the volume turned up (3) use a "regular" speaker + microphone (4) the background isn't very noisy
I doubt it is feasible via walkie talkies. I assume their range of transmissible sounds doesn't include much outside the range of human hearing, plus they transmit lots of static, interference, and environmental sounds which would probably ruin the effect. It's probably not even feasible in a setting like paying for coffee at a starbucks due to too many variables listed above. Humans are bad at following procedures precisely unless they are conscious of the requisite conditions and why they are needed, and I think #1 and #2 would trip normal people up.
E.g. I suspect people would try to pay with headphones plugged in and then not understand why it's not working. Even without headphones, some users would probably turn the volume down if the payment app turned it up without them understanding why. And you can't rely on "just telling them why" because people don't read what their apps tell them, they just complain when they don't work the way they wish things worked.
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It's possible. Phone transducers can send/receive up to around 20kHz.
Most adult humans can't hear beyond about 15kHz.
You'd need to keep in mind transmission distance which decreases with frequency which can be either a feature or a bug.
You can apply the same principles as Satoshi Jump using DTMF + The Goertzel Algorithm for tone detection. DM one of us if you'd like to learn more.
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My thinking on this question: I don't know the answer, but my gut says it's possible (though not too practical). While futzing with things like psk31, I've noticed that the sound level, even the audible sounds, the volume does not have to be very high at all if the devices are very close. For example, while setting up the 21 sat transfer audio file in the link up above, I used normal earbuds dangling over my laptops mic. I did that because the audio was too annoying my wife would have strangled me. I have rather poor hearing, but I could not audibly hear the sounds even though the data was being transferred earbud-->mic-->Fldigi to decode.
As far as outside the range of normal hearing thresholds, that's probably something for a ham radio person to answer. User bitpunkfm suggested waver (ggwave) over on nostr. I'd never heard of it, but it might be doing what you're describing with "ultrasonic" messages. See youtube vid, the first couple of tx you can hear, the ultrasonic message is at about the 33 second mark.
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