5 ms·

https://www.washingtonpost.com/investigations/shotspotter-detection-system-documents-39000-shooting-incidents-in-the-district/2013/11/02/055f8e9c-2ab1-11e3-8ade

by Cactus2018 6y ago

https://www.washingtonpost.com/investigations/shotspotter-detection-system-documents-39000-shooting-incidents-in-the-district/2013/11/02/055f8e9c-2ab1-11e3-8ade-a1f23cda135e_story.html https://www.washingtonpost.com/investigations/shotspotter-de... .... 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. ....

kulahan 6y ago
Yep, this seems to match my point. Thanks! :)
saghm 6y ago
> 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?
- tarmon 6y ago
  Well technically helicopters are propelled by explosions. :)
  - refurb 6y ago
    Most helicopters are turbine driven. Still an explosion? Kind of one long continuous explosion from start to stop.
- throwawayway9 6y ago
  I forget where I had read it, but the spotters were also picking up and recording conversations. Inadvertently, of course.
  - pininja 6y ago
    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-system-recording-private-1793846543 https://splinternews.com/is-nyc-s-new-gunshot-detection-syst...
- na85 6y ago
  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?

kulahan 6y ago

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

saghm 6y ago

> 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?

tarmon 6y ago
Well technically helicopters are propelled by explosions. :)
- refurb 6y ago
  Most helicopters are turbine driven. Still an explosion? Kind of one long continuous explosion from start to stop.
throwawayway9 6y ago
I forget where I had read it, but the spotters were also picking up and recording conversations. Inadvertently, of course.
- pininja 6y ago
  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-system-recording-private-1793846543 https://splinternews.com/is-nyc-s-new-gunshot-detection-syst...
na85 6y ago
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?