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It's very much possible. Even excluding the old acoustically coupled modems that eventually evolved into a direct connection to a phone line, there are also programs that will let you use a sound card as a modem (I've used some before for out-of-band communications when debugging an Ethernet driver, though I used direct audio cabling instead of actual acoustic signaling), and the general concept is becoming rather popular with IoT devices for pairing with a control app on a smart phone during setup (though this is closer to an RFID tag approach).

This approach has a number of pretty significant downsides though:

• It's very low bandwidth by modern standards. Even with ultrasonic frequencies, you're still looking at no more than a few hundred kilobits per second under good conditions. This makes it significantly less than useful except for passing very small amounts of data (such as with the IoT usage I mentioned above, where it usually just passes a 802.11 hardware address and some auth info so a Wi-Fi Direct connection can be established).


• Outside of very limited situations, it's really slow even aside from the signal frequency. The speed of sound in air is about 340 m/s (give or take a few dozen m/s based on temperature, pressure, humidity, and air quality), which is insanely slow compared to electrical signals or electromagnetic waves (which propagate at roughly the speed of light), which means that compared to Wi-Fi or Ethernet, signal latency is rather high. This doesn't matter much for very short-range communications, but once you get past a few meters, the latency starts to get noticable (imagine if the link between your computer and your router had a longer RTT than the entire rest of the network path to this website combined). Even the best conductors of sound can only get at best 35-40 times the speed of sound in air, which is still insanely slow.


• It's extremely sensitive to the environment. Ethernet is solid enough that it doesn't even need shielded if you have decent cabling. Wi-Fi can get bad at times, but is still at least able to reliably screen for specific frequency bands easily, and EMI is usually pretty easy to find and stop. Vibrations and sound are everywhere though. Again, this is more of an issue for longer-range communications, but it's still more of a problem than for Wi-Fi, partially because of the next point.


• Sonic transmitters with a high transmit power are rather dangerous, both to the environment, and to people. To reliably get any kind of range beyond about a few meters before the SNR gets so high that you have no recoverable signal, you need to be operating at high enough sound pressures to cause permanent hearing loss. That much energy can also pretty easily damage delicate objects.

It's been done. I used to have a 300-baud acoustic layer 1 device.

Anything that can transport information can be used as physical layer - sound waves as well as pigeons.

Some air-gap attacks use (ultra)sound to communicate across the air gap.

However, since the frequencies even for ultrasound are rather low (some kHz), the data rate would be low as well (some kbit/s). Also, the reach of sound waves limits the use to a single room.

Yes it’s possible. In fact, Amazon Dash buttons use ultrasound as a medium.

Certainly. Some options not discussed in other answers, but closer I think to the heart of your question:

• A shotgun mic receiver and a similar transmitter. Uses air as the
  medium.


• A piezo sensor receiver and a similar buzzer. Uses a rod of
  (say) wood, carbon fiber or beryllium rod as the medium.

In both cases, an electrical impulse provided to the transmitter will produce a mechanical impulse through the medium to be detected by the receiver, where it is converted back to an electrical impulse.

In both of these examples, we have not used any electricity to represent sound, but have actually used sound waves traveling through different media.

One of the first computers used soundwaves within a medium of mercury to store data so though it was not a networking medium it was still a method data retention and the same concept could be used for transmission and networking

I've done this before, not because it made sense to, but because I felt like it. Ham radio has all kinds of things for this, notably AX.25. Anything you send through a non-digital radio is audio being encoded into radio waves, and back by the receiver. Take the radio out of the equation, and you get what you're looking for.

Take a look at AX.25, as well as other digital modems, such as MT63 and PSK. fldigi is capable of many, although it's built for text, so you'd need to base64 any binary data.