Quoted from SF Weekly:
"In a nutshell, the spotters are sophisticated microphones that can discern between gunfire and other percussive noises (firecrackers, Fred Sanford’s car backfiring) and relay the exact location of the shooting to police."
Oakland and Redwood City as well as a few other cities around the US already have it in use. Wonder what this will reveal about SF, in terms of unreported shootings of course, but also about how the police respond to said shootings.
Here's the article.
And here's a follow up article.