Earlier quoted context omitted.
> sensors are calibrated to ignore all sounds except for those that most closely match the “impulsive” sound of an explosion > The blast of a gun is different from other explosive sounds > “It’s a very, very sharp wave,” Beldock said. “No other sound works that way.” > Other impulsive sounds — including fireworks, backfires and helicopters — can also trigger the remote sensors They're calibrated just to hear explosio…
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)-…
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.