For this version, I plan to use a 555 as a fixed frequency squarewave generator, probably around 500 Hz. The 555 drives a small loudspeaker at the end of a tube. A little further in, a microphone records the sound. The tube is held vertically and other end of the tube dips into the water. The sound reflects off the surface of the water and goes back toward the microphone. Some software is used to analyze the varying time delay between the speaker's output and the returned echo. The series of sampled time delays is used to generate an audio file.

500 Hz provides about 2 mS for the reflection to occur before the next waveform arrives. That is about a 2 foot round-trip. To accommodate larger water waves, I might need to lower the frequency (maybe to 100 Hz or so). If the water wave varies over a 1 foot range, that corresponds to about 1 ms of time, which at my recorder's max sample-rate (44,100 per sec) is about 44 samples. But that's only 5 or 6 bits resolution. So I need to increase the effective resolution somehow, possibly by mechanically amplifying the wave height. A factor of 2 or 4 is probably the max, bringing the resolution to 7 or 8 bits. Still not so good!