As the NYPD prepares to expand its ShotSpotter gunfire-detection system, officials are trying to determine why so many recorded gunshots don't spark emergency 911 calls.

Close to 75 percent of shots detected by the acoustic sensors spread around the Bronx and Brooklyn don't have corresponding 911 calls, an NYPD spokesman said Wednesday, adding that other cities using ShotSpotter have reported similar findings.

But other cities aren't as densely populated as New York City. NYPD officials believe there may be a number of reasons for the lack of 911 calls, ranging from fear of criminals to the dynamics of how sound travels in urban areas.

This week NYPD Commissioner William Bratton said the system will expand to other parts of the Bronx, Brooklyn and Manhattan. The NYPD is expected to come out with a detailed report on ShotSpotter activations near the end of the year.

When Mayor Bill de Blasio unveiled the technology in March, he said it would help reduce crime by increasing the likelihood of catching shooters and recovering weapons. But so far ShotSpotter's effect on crime are relatively modest, according to NYPD data.

The technology has detected 1,342 confirmed gunshot episodes since March and April, when the city put ShotSpotter in parts of the Bronx and Brooklyn. Of those recorded gunshots, 911 operators received just 366 calls reporting gunfire, said NYPD spokesman Stephen Davis. "Either they don't hear it -- which I find hard to believe in a densely populated area that they don't hear it -- [or] they don't recognize it as gunshot," Davis said.

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The 1,342 confirmed gunshot activations led to the recovery of evidence or witnesses who saw the shooting, 273 times, or 20 percent, Davis said. Investigators recovered physical ballistic evidence or weapons in 249 of those cases, Davis said.

Shootings have increased weekly in the city over the previous year despite the presence of ShotSpotter, anywhere from about two percent to 11.5 percent before declining 1.7 percent, according to the latest NYPD crime statistics.

Twenty-one arrests in the city have come as a result of ShotSpotter, officials said.

The so-called "snitch factor" reflects a fear among the public that if they call to report shots fired they may be discovered by criminals, said Liz Einbinder, a spokeswoman for SST Inc., the Newark, California company that makes ShotSpotter.

Sound also travels so its location may be difficult for someone who hears gunfire to pinpoint its location, Einbinder said, adding that others who may have heard gunshots don't bother to call -- a gunshot "fatigue" effect.