a shot in the dark: the acoustics of gunfire

JenlucpiquantWho shot first, Han Solo or Greedo? If you're a diehard Star Wars fan, you know the answer is Han Solo. It's a popular bit of geek trivia, referring to a classic scene in what is now known as Star Wars Episode IV: A New Hope. Fans have engaged in heated online debates, signed petitions, and broken off budding friendships over this pressing issue. And now, thanks to advances in the acoustics of gunshot forensics described at last week's Acoustical Society of America conference, those in the "Han Shot First" camp — and Jen-Luc Piquant reminds you that there is no other acceptable answer — can bring in the mighty fists of science to make an even stronger case.

You remember the scene. Han meets bounty hunter Greedo at the Mos Eisley Cantina. Greedo is there to collect on behalf of Jabba the Hut, who wants payment for a load of smuggled cargo that Han dumped to avoid getting caught. The two — well, one man, one strange-looking alien — sit across from each other. Greedo has his blaster pointed at Han, who, as they chat, positions his own blaster beneath the table. In the original theatrical release, Greedo says, "Ive been looking forward to this for a long time," and Han replies, "Yes, I'll bet you have." He then blows Greedo away before the bounty hunter manages to take his shot. (Pic below via Mental Floss.) Except fans were outraged when director George Lucas modified the scene for the film's re-release in 1997. In the modified scene, Greedo fires first, missing Han Solo despite the point blank range (Greedo is a pretty horrible shot for a bounty hunter), and Han only fires and kills Greedo in retaliation. Apparently Lucas didn't want there to be any moral ambiguity whatsoever surrounding Solo's ultimately heroic character — but that ambiguity has always been a big part of the character's charm. (Think the redemption of Spike in Buffy the Vampire Slayer for a similar character arc.) You don't mess with the Star Wars canon, people — even if your name is George Lucas.

But what if you were a cantina patron whose view of the table where Han and Greedo was sitting was blocked? When the authorities asked for your account of the shooting, how much could you really tell them about what happened, when you only heard the blaster shot? You might be able to tell them quite a bit, actually assuming you'd boned up on the latest acoustics research on gunshot forensics.

Okay, granted, nobody's done a bona fide study on sci-fi blasters, but acousticians Steven Beck (BAE Systems in Austin, Texas) and the FBI's own Hirotaka Nakasone have collaborated on a number of studies of gunshot recordings of various types of firearms, wit microphones placed in many different configurations and angles relative to the gun being tested. They've combined those findings with visual evidence (photographs of gunshots) to develop more accurate waveform models for forensic audio analysis of actual gunshot events.

A waveform is a kind of acoustic signature of a given sound. So waveforms can contain a lot of useful information for someone who's trained to interpret them, once there's a reliable catalog of waveforms associated with specific sounds, of course — in this case, different kinds of gunshots at different ranges and angles. For instance, a trained analyst might be able to determine whether a sound is a gunshot or not, or how many guns were used, or — most relevant for Star Wars fans — who fired first. It turns out that a gunshot "acoustic event" (i.e., "sound") has more than one component. First there is the muzzle blast (the loud bang that propels the bullet forward), which sounds like a short pop in laboratory conditions, but is louder with more reverberation in a more real-world setting. Factors that can affect the resulting waveform from this part of the gunshot include the make of firearm, model, length of the barrel, and the type of ammunition used.

The muzzle blast is followed by a ballistic shock wave, i.e., the crack as the bullet zips through the air, traveling faster than the speed of sound. It's basically a supersonic boom, as the shock wave forms a cone behind the speeding bullet. You don't always hear it either, if you happen to be standing at more than a 90 degree angle to the line of fire. How big the shock wave is, and how long it lasts, depends on such factors as the bullet's speed and its size, while the usefulness of a recording of the ballistic shock wave depends on where the microphone is located, relative to the line of tire. Per Beck and Nakasone's lay language writeup for the conference:

"Currently analysts use simple blast waveform models and cross correlation techniques to study acoustic gunshot waveforms and try to answer basic forensic questions." However, "There are many variables that can affect recorded gunshot signals and forensic audio analysts need to be aware of their influence on the resulting waveforms."

That's why the two acousticians performed so many different kinds of studies of recordings made in both the lab and the field, trying to take into account the many, many factors and conditions that could change a waveform in significant ways, and make it less accurate (and hence less useful) for forensic purposes.

The resulting waveforms matched closely with their theoretical models, at least for the high-quality recordings they made themselves in the lab, with top-notch equipment. Beck and Nakasone acknowledge that for real-world purposes, the kinds of gunshot recordings an acoustic analyst encounters are likely to be done with handheld devices like cell phones, with a lot of background noise. "Waveforms recorded under more realistic conditions can vary significantly — even to the point of being unrecognizable and forensically unusable," they admitted in their lay language paper.

The culprit is reverberation, or the echoes that distort the waveform, not to mention from the recording equipment itself. Most gunshots occur in urban settings, so the sound will ricochet off pavement, buildings, even nearby parked cars. Beck and Nakasone focused their efforts on recordings of gunshots in the field, and then compared those with those made in the lab, but there are also "acoustic event locating systems" used by military and law enforcement officials.

Such a system was implemented in parts of Washington, DC, back in 2006. It was even featured on an episode of Bones, in which the female serial killer known as the Gravedigger is assassinated by a long-range sniper outside the courthouse for an appeals hearing. The mighty crime-fighters at the Jeffersonian use the sensor system to pinpoint the likely source of the gunshot via acoustic triangulation.

There's more than one kind of acoustic sensor used in these kinds of systems. Per How Stuff Works (a.k.a., "The Enormous Time Suck for the Eternally Curious"), some sensors detect the sonic boom a speeding bullet produces as it moves faster than the speed of sound, while others detect the optical effects produced by a muzzle blast. The DC system uses the former variety, spacing a dozen or so sensors per square mile. Those sensors can each detect the sound of gunfire within a two-mile radius. Since the speed of sound is well-established, all you need to do is measure the difference between the time it takes for the sound of the gunshot to reach three different sensors to figure out the exact location using standard GPS. That's acoustic triangulation.

Then this information gets relayed to the nearest 911 call center, and the police are on it, people. At least that's the main idea. It's pricey — DC spent millions of dollars on the system — but it can cut the response time for a "shots fired" 911 call in half, according to DC police officials. If only we had the film equivalent of such a system, or highly trained forensic audio analysts, to settle once and for all what every Geek and Gamer Gurl knows: that Han Shot First. (FYI, video below is a parody of Katy Perry's "California Gurls.")


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