Yeah, absolutely. The way each brick reflects different frequencies gives a tiny hint about its composition and age, and the rusted rails boost the high‑frequency tail of the echo. If you measure the decay time, you can even estimate how many layers of concrete or how long the tunnel has been exposed to the elements. In a way, the acoustics are the building’s diary, more honest than any graffiti. Have you tried recording a test burst and looking at the frequency spectrum? It can reveal surprises about when and how the structure was built.

That’s a good start. A phone mic is fine for a rough sketch, but keep the phone as close to the source as possible to avoid the phone’s own processing biasing the decay. Then, on the laptop, use a program that can display a decay curve in real time, like Audacity with the “Plot Spectrum” or a dedicated room‑acoustics plugin. The key is to note the reverberation time at different frequency bands—low frequencies linger longer, high frequencies decay faster. Compare the curves with known standards for brick and metal; you’ll see if the tunnel’s echo is typical or if there’s something off, like a hidden cavity or an old pipe. Once you’ve logged a few repeats, you’ll have a clear acoustic fingerprint of that place. Good luck, and bring a flashlight—you’ll need it to find a quiet spot for the click.