The time differences are used to triangulate the location where the sound occurred. It takes about 10 seconds for the triangulation to take place, after a computer algorithm has helped to dismiss things like echoes from nearby buildings.
"Sound echoes are the single biggest technical challenge for a technology that attempts to locate individual noises over many, many square miles," said Beldock.
Subscribing police departments usually end up with a much more detailed picture of gunshots across a neighborhood or city. With ShotSpotter, the San Francisco police has discovered that only about 10 percent of gunshots taking place were actually reported by the public through 9-1-1 calls, said Ali.
SST guarantees its system will detect and locate 80 percent of outdoor gunshot incidents within its coverage area — except those fired at point-blank range or from guns fitted with silencers — and says the figure is typically as high as 90 percent.
Police departments in the San Francisco Bay Area, including those in San Francisco, Oakland, Newark and East Palo Alto, seem satisfied with the system.
In the U.K., the West Midlands Police said a deployment there resulted in "technical difficulties" — mainly missed gunshots and false positives. SST responded that several of the missed shots were actually outside of the system's coverage area, and two involved guns the system has difficulty detecting — an air gun and improvised gun. The lower incidence of gunfire in the U.K. also means the rate of false positives is higher than in the U.S.
The use of ShotSpotter is now being expanded beyond gritty urban streets into a completely new environment: the savannas of South Africa. SST has started a trial of its system to detect gunshots from poachers, Beldock said.
The sound of a gunshot travels about twice as far in a savanna or even a jungle as it does in a town, so the density of sensors can be reduced. It's too early to gauge the success of that project, but the hope is that it will serve as a deterrent to poachers or help catch them.
Police in San Francisco also expect the system to act as a strong deterrent.
"We've been building a system since 2007," said Ali of the SFPD. "We've got to a tipping point where we have a large enough system in place. Our belief is we are going to see some serious changes in behavior as a result."
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