What I've Learned

I was watching a movie when the power went out. In the silence, I heard gun shots.

I opened a window and listened, staring out at the night. Cold air bathed me as I strained to hear. There it was again, a sharp crack that reverberated through the winter air.

After ten years in the Army, including combat in the first Iraq war, I know what gunfire sounds like. Whatever was going on, it couldn't be good.

Did the phone still work? If it did, a call to 911 was in order to report shots fired. The window was half down when several more shots rang out. I pulled the window back up, listening.

There it was. The distinctive crack of a rifle. And then a moment later, another. What the heck?

Turns out, what I was hearing was trees. The ice storm of 1998 had begun, and branches--and in many cases, whole trees--were snapping like kindling. It would be nine days before the power came back on.

That my ears could have mistaken ice-storm damage for gunfire is not really surprising. (I say this to console myself.) The sound of a gun shot is made up of two parts: the muzzle blast and the crack when the projectile breaks the sound barrier as it flies through the air. On a cold night, a snapping branch can create a fairly good imitation of this.

Let's back up six years – from 1998 to 1992 – and go from the frozen Northeast to sunny California.

In the early 1990s, East Palo Alto (20 miles south of San Francisco) was the murder capital of the United States. There were more deaths there per capita – most of them drug-trade related – than anywhere else in the country.

In near-by East Menlo Park – another high murder area – John C. Lahr, a seismologist, wondered if the techniques used to pinpoint the epicenter of earthquakes could be used to pinpoint the location of gunshots.

He set up some microphones in the area and modified a seismological software program. When shots were picked up by the mics, Lahr used the program to determine their location. To test for accuracy, his wife monitored a police scanner to see if the sounds were indeed gunshots and where they had occurred. Lahr found that he was able to map gunshots to within 30 or so meters.

The setup wasn't perfect. The software had trouble distinguishing gunfire from fireworks and cars backfiring, but when Lahr demonstrated his system to police, there was interest in the idea.

Since then, using seismology techniques to locate gunfire has come a long way, and a some major cities--as well as some smaller ones--have listening posts that notify the police of shots fired. There are a number of systems – ShotSpotter, Bomerang, and SENTRI being but three – that are used by various agencies.

Because there are two sounds involved – the muzzle blast and the sharp, high-pitched sonic boom of the projectile when it exceeds 761 mph – systems are now better able to distinguish gun shots from other shot-like noises. And good, old-fashioned triangulation, with the help of GPS satellites, can map the locations to within feet.

Police can respond knowing before they arrive if one or more than one gun is involved in an incident. There have been cases where officers showed up at the scene of a shooting before any 911 calls had even been made. Victims were treated sooner, and shooters got caught.

To some, this all smacks of Big Brother and an erosion of Second Amendment rights. Be that as it may, the science involved fascinates and inspires my untrustworthy ears.