The first experiment will be to measure the intensity difference between the initial pulse and the resulting echo. I will try it in open-air first, no tube involved yet. We'll see how that goes. If that doesn't work, I'll need to scrap this concept and try something else.

The "something else" might be to try amplitude modulation rather than frequency modulation or time delays. My recorder is 16 bit resolution at 44.1 KHz. It seems like I may be wasting too much resolution on time (samples) instead of on amplitude. It also seems like there ought to be a way to use both to obtain plenty more than just 16 bits.