There are secret messages 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.
But what if you need to get a few bytes from device A to device B (one of the hard problems in computer science!) and you don’t feel like making sure they’re both connected to the same network? Well, fortunately, another channel is available to us - sound, or for a more fun and less-audible experience, ultrasound.
What even is ultrasound
Sound that you can hear is, physically, the air vibrating, or being compressed and relaxed again and again, rapidly. Our ears (or our computers ears - microphones) can pick up this pressure variation. While a computer microphone picks this pressure variation up pretty much directly (and that is, in fact, what you see when you look at an audio waveform - it’s a measure of the pressure relative to ambient), the ear, and our perception, and most useful audio processing algorithms, work in the frequency domain. While you can take entire classes on this, the short version is: Every audio signal can be decomposed into frequency components - sine waves with a certain number of vibrations per second (the frequency, in Hz), which then lets you see how much energy the signal has at each frequency. If you’ve ever seen a bunch of bars go up and down on some kind of audio player - that’s exactly the frequency domain, usually with lower pitch / lower frequencies on the left and higher on the right, showing how much of the sound you are hearing is low-pitched, bassy rumbling, versus mid tones, versus highs.
That wording already implies something, which is that there is some reasonable low-mid-high range for these frequencies. For us humans, that is from about 10 to 20 Hz (sub-bass, more of a thing you feel in your stomach than you really hear) to about 20000 Hz, though as people grow older, that upper limit decreases quite a bit, and you become unable to hear the highest tones. Sound past that 20000 Hz barrier is what we call ultrasound - sound that is beyond (most) human ears ability to hear.
Ultrasound and you(r computer)
If you think of a thing that comes to mind when you hear “ultrasound”, it might be the thing that a doctor uses to look at what’s going on inside of you (most prominently, to check how a baby that is growing inside of you is doing). So you might be forgiven if you think of it as something that requires special devices with fun names like transducer or whatever. Not so! While our technical devices are, of course, designed to operate broadly in the same range as our ears, they generally can go a little bit beyond that. Yes, even your shitty laptop speakers and microphone are, technically, ultrasound-capable devices! 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.
So lets transmit some data
To recap:
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