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I'm writing a story in which someone is shot in a cave system. Another character is an adjoining tunnel some distance and at least a few tunnel intersections away. Would the sound of the shot reach her? Would it sound like a gun shot? or a rumble of echos?

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It turns out that gunshots can be used to map caves,

The decade-long search might have gone a little differently, though, if soldiers had access to a new system that can use sound waves from a gunshot to quickly map unknown caves and tunnels. The portable system, created by David Bowen of Acentech in Cambridge, Massachusetts, consists of two microphones placed at the entrance of a cave or tunnel, which are hooked up to a laptop loaded with software designed to decode acoustic signals.

A gun is fired four or five times, with about 5 seconds between each shot. Fifteen to 20 seconds later the map appears on the laptop’s screen, with simple graphs that display the area of the cave at different distances, and written explanations of the data, such as “30 feet ahead is a large opening”. A portable subwoofer can be used in place of a firearm as the source of the sound waves.

Source

so it is probable that the other person would hear the shot. It is likely that it would be a rumble and not a single noise since there are lots of surfaces to bounce off of in a cave.

Beyond that, it would be hard to say, since people usually don't shoot inside of caves. The noise level will be higher and that will result in damaged hearing.

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    I'd like to imagine that the last sentence of that quote originally read something like: *A portable subwoofer can be used in place of a firearm as the source of the sound waves, but where's the fun in that?" – tonysdg Sep 8 '17 at 13:00
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    Actually people shoot in "artificial caves" a whole lot where I live. Most long range rifle ranges locally (biggish city) are actually underground tunnels for security reasons. Few of them have any sound padding and as a general rule even high power ammunition (I've shot .500 S&W magnums from a revolver and .416 remmington magnums from a rifle) is quite survivable without ear muffs though it is unpleasant and for a short time (minute or two) I would hear worse. – DRF Sep 8 '17 at 15:19
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    I agree with @Andy here. While sonar-based systems might get a rough estimate of current cave and its dimensions, I wouldn't expect it to be any useful in anything more (like finding that you can squeese between two boulders to find small passage) than that due to highly irregular cave surface. It would probably work better in aquatic enviroments, though. – Revolver_Ocelot Sep 8 '17 at 16:23
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    All the cave mapping systems I've heard of recently are LIDAR or SLAM based, including DEPTHX, an autonomous underwater cave mapping robot (en.wikipedia.org/wiki/DEPTHX) – Andy Sep 8 '17 at 16:43
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    @Andy as I thought: a lot of sonars, firing repeatedly, mapping stuff in the line of sight, requiring several passes. Surely not a two mics and a few shots. – Revolver_Ocelot Sep 8 '17 at 16:54
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From my experience in caves, I'd say you could expect the gunshot to reverberate for just a second or two in passages that are barely more than walking size. In larger passages, maybe longer. Around a few bends it would probably sound muffled as well. And keep in mind that sound travels through small, impassable cracks and crannies in caves as well -- I've heard the muffled thuds of people crawling through passage hundreds of feet away with no direct path between us.

  • Guns are incredibly loud. Even around a few bends it would be defeaning. – whatsisname Oct 5 '18 at 0:21
  • Yes but caves are better at swallowing sound than you might think because the texture of all surfaces is typically irregular. In Kentucky caves at least, I would say that passages smaller than 20 ft in diameter don't typically have a substantial echo. – Andy Oct 5 '18 at 18:20
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So the answer is that the other person will almost definitely be able to hear the gunshot in the absence of background noise. Will it echo? That sounds like it would depend a lot on how much the walls in the cave system resemble the grooves in acoustic tiles from recording studios. Also tunnels that open into large caverns or even tunnels that gradually widen have a damping effect on echoes returning to the source.

The second person in this scenario actually doesn't need to hear an echo, they only need to hear the original sound, and especially if the tunnels are of consistent diameter, they will act to propagate the sound (and the shock) from the gunshot for very great distances. (Meaning the person a few intersections away might also get hearing damage from the pneumatic column between them transmitting nearly the same concussion to them as the person firing the gun)

YouTube is failing to provide good examples, but the next time you see a hollow handrail that's a few carlengths long, ask a friend to 'drum' one open end with the palm of their hand and have your head (not ear) near to the other open end. It makes a sound a bit like a dubstep bass drop and is actually much louder at the far end than for the person making the sound.

Visualising this scenario everyone is in corridor-sized tunnels, the previous answer shows huge cavernous rooms so if you mean could someone at the opening of Carlsbad cavern hear a gunshot by the elevators? Maybe not. That's a long hike between those two places.

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Would a gunshot echo in a cave...

It depends on the cave. To echo a sound wave needs a smooth surface to reflect off of.

Most caves aren't giant smooth caverns but are relatively convoluted or tight passages with random shapes to the walls, ceilings and floors which will randomly reflect the sound and quickly attenuate it.

The shock front of the 'bang' will travel a way, but it too will be attenuated.

Think of a muffler / silencer from an engine exhaust. No smooth surfaces and little noise made.

Within a cave it's hard to hear other people even a short distance away as they make their way through the cave, especially if there's tight passages.

  • A gunshot is several orders of magnitude louder than a large car engine with straight headers. If you discharge a firearm in a cave, unless the cave is made out of foam, anyone within any definition of "short distance" is going to know about it. – whatsisname Oct 5 '18 at 13:57
  • Caves, or waterless ones, are surprisingly quiet places that seem to absorb sound. Outside, a gunshot could be heard a very long distance away -- miles at night -- but in a complex cave it will be quickly attenuated simply because the sound will be reflected so many times. – GlennG Oct 5 '18 at 15:28
  • @GlennG can you link to some evidence about the acoustic properties of caves. Lots of reflections will attenuate sound, but the cave probably has a high acoustic impedance and acts like a wave guide. – StrongBad Oct 9 '18 at 18:32
  • Got me there! All I know is that as I crawl through a cave I can't hear other people even a few metres away. It's strangely quiet, anechoic even. – GlennG Oct 10 '18 at 14:02

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