Here's my technique: Give a brief, relatively high-pitch bark or "yip" directly into an opening.
Brief: If you give a long yell, you're still making noise when the first part of the echo/reverb returns, so you're going to miss some of it unless you cut it short.Naturally, my friends think I'm nuts the first time I do this when we're out together, even though they already know that I can't walk past a drain grate without looking down it longingly. But once I've explained the technique, they usually try it as well.
High-pitch: This carries more information (i.e., there are more waves per second) and is less prone to degradation as it bounces around.
Conversely, you can also listen for sounds emitted by the tunnel itself. For example, you may hear running water and can gauge the diameter of the tunnel and the distance to the source of the water/sound from that. Similarly, when you are inside a tunnel, you can tell how large it is up ahead for several thousand feet from the sound of traffic going over manholes (which is one of the most interesting sounds you can hear underground because all the required elements are so massive).
I should note that I'm probably more attuned to sound qualities like reverb and "tone" than most people because I am a musician and deal a lot with electronic processing of sounds. Still, most people could pick this up with practice.
- Human echolocation - An article at Wikipedia.
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