Comment by Cactus2018 | Hacker News Reader

Comment by Cactus2018 | Hacker News Reader

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https://www.washingtonpost.com/investigations/shotspotter-de...

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Wrapped in a weatherproof container roughly the size of a watermelon, each ShotSpotter sensor combines microphones, hardware, software and a clock linked to the Global Positioning System, which uses satellites and radio navigation to pinpoint precise times and locations.

In the cacophonous urban environment, sensors are calibrated to ignore all sounds except for those that most closely match the “impulsive” sound of an explosion, said James Beldock, a senior vice president for ShotSpotter.

“It’s a very, very sharp wave,” Beldock said. “No other sound works that way.”

The blast of a gun is different from other explosive sounds because it is directional, meaning that the noise changes its frequency as the bullet moves through space. A person may hear a gunshot a half-mile away if the gun is fired toward him. But a person 200 yards away may hear nothing if the gun is fired away from him.

Once sensors register a potential gunshot, they transmit the data to the ShotSpotter computer network for analysis. The computer server compares the time that each sensor logged the sound to calculate the likely location of its source, a process of triangulation and multilateration.

“That sound will reach a sensor 100 yards away at a different time than it reaches a sensor 200 yards away,” Beldock explained.

The more sensors that capture the noise, the more accurate the location. A sound detected by 10 sensors can be located to within two feet, he said.

The computer system also classifies the likely source of the sound based on its sharpness, frequency and consistency across sensors. This is critical, because other impulsive sounds — including fireworks, backfires and helicopters — can also trigger the remote sensors.

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> 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 explosions, and in fact _no other explosive sound works that way_, but then they're triggered by...helicopters? Which are not propelled by explosions?

Well technically helicopters are propelled by explosions. :)

Most helicopters are turbine driven. Still an explosion? Kind of one long continuous explosion from start to stop.

I forget where I had read it, but the spotters were also picking up and recording conversations. Inadvertently, of course.

These sensors are usually on rooftops, and while I believe they could pick up conversations if we’re next to it or yelled loudly (as in, physically capable of capturing frequencies of human voices), I don’t believe they have a use for recording lots of audio to some permanent storage. Their main purpose seems to be posting time stamped events of a particular sound occurrence with a couple seconds of sound surrounding the event. [0]

> Only two seconds before a gunshot and four seconds after a gunshot are recorded, the company claims.

If they were recording full conversations on purpose without permission that would expose a lot of liability, and unless every customer is in on it (I doubt these distributed public entities are coordinated) I think there would be some documented story by now.

[0] https://splinternews.com/is-nyc-s-new-gunshot-detection-syst...

It's no doubt referring to the "whap whap whap" sound of the tail rotor's wake causing interference patterns with the main rotor's wake.

Next time you're near a helicopter in flight, notice that the "whap whap whap" chopping sound is more prominent at certain angles. Because it's constructive interference the pressure wave can be quite large and sharp... just like a gunshot, I guess?

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.

>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.

Yep, this seems to match my point. Thanks! :)

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