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Transmitting data via ultrasound without any special equipment
flying all around you all the time, being transmitted via, most of the time, electromagnetic waves going from antenna to antenna. ELOs “Secret Messages” is a song about posting conspiracy theories via WiFi.
And while they do usually pretty heavily start to cut frequencies once you get to about 18000 Hz or thereabouts for technical reasons, you can probably still get a signal through, and it’ll be almost or completely inaudible to most people. The resulting audio looks like this, with the waveform below, and a spectrogram- essentially, a lot of those spectrum analyzers laid on the side and taped to each other, with the colour getting more intense the higher the bar - on top: Not entirely: This type of signal is actually used by calling sofware to help figure whether there are other devices close to you that are already in a given meeting, in addition to other stuff like bluetooth beacons.
Or read this on Hacker News
