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For what it's worth, neither bullets nor helicopters are propelled by explosions. While the distinction is somewhat technical, gunpowder doesn't explode--it jus

For what it's worth, neither bullets nor helicopters are propelled by explosions. While the distinction is somewhat technical, gunpowder doesn't explode--it just burns really fast. This is less important for small arms than for, say, artillery, but being able to control burn rates is critical for the design of most firearms/cannons and things like solid-fuel rocket motors (which function as one big propellant grain)--in many of these cases, detonation would likely prove catastrophic. All that to say that helicopter blades can create similar pressure waves as gunshots. Supersonic bullets passing overhead make very distinctive sounds, but the noise of the gunshot itself is much less unique. I never really got comfortable living in places with a large number of cars that backfired a lot, because I couldn't easily tell it apart from gunfire.

throwaway0a5e 6y ago
>gunpowder doesn't explode Define "explode". Any chemical reaction that produces more outputs than inputs by volume can explode if you trap that pressure. The flame front on gunpowder isn't fast enough to make a pressure wave that sounds like a bang if there's nothing to trap the combustion byproducts (like the space behind a bullet as it travels down the barrel). You can harmlessly set off gunpowder (old school black powder and equivalents) as a party trick. Don't try that with Semtex. I'm not an acoustics expert but I think it's going to be fundamentally very hard if not impossible to build a shotspotter type system that both works in an urban environment (where sound bounces off all sorts of things) and doesn't get a false positive from things like motorcycles backfiring.