By Charles E. Blue
The characteristic crack and bang of a gunshot blast may contain vital clues about a crime: who shot first, or even what type of firearm did the deed. But dozens of factors, from the angle and direction of the gun’s muzzle to the quality of the microphone, can change the way a gunshot sounds in a recording, making it difficult even for expert analysts to tell exactly what went down at the scene of the crime.
Now researchers from BAE Systems in Austin, Texas, and the FBI Forensic Audio and Video Analysis Unit have taken a major step toward improving scientists’ understanding of how the sound of gunshots corresponds to the manner in which the bullets were fired. The team will present its findings at the Acoustical Society of America’s annual meeting, which runs from Oct. 31-Nov. 4 in San Diego, Calif.
“The field of forensic gunshot analysis is relatively new,” said Steven Beck, principal scientist for BAE Systems and one of the presenters at the upcoming meeting. “[Analysts] get these recordings” – often from cell phones or other devices with limited recording capabilities – “and they have to try to figure out what’s going on. If you don’t understand [the impact of] all these variations, you can make the wrong conclusions,” Beck said.
Guns produce two distinct sounds: the “bang” caused by the rapid expansion of the gasses that push the bullet out of the barrel, and the “crack” caused by shockwaves made by a supersonic bullet. In a controlled laboratory environment, the researchers placed microphones at a range of angles and distances from each blast, to catch the sound pattern of a single round from multiple points of view.
One of the team’s biggest surprises was how important the angle of the gun is to the sound signature, or “waveform,” caught by each microphone. A shot fired into the air produces a wildly different waveform from one fired in the direction of a recorder, even when both shots came from the same gun. Likewise, a heavy rifle blast fired at a great distance from the microphone can produce a sound signature that is almost indistinguishable from one produced by a smaller, lighter gun fired at close range.
An extra “bang!” caught by recorders sitting at a 90-degree angle to the barrel provided another surprise for the physicists. The team traced the sound to gasses leaking and expanding out of the side of certain types of firearms. In a good recording, said Beck, analysts could take advantage of this extra sound to identify the direction of the weapon.
The presentation, “An introduction to forensic gunshot acoustics,” by Steven D. Beck et al. will be at 10:15 a.m. on Thursday, Nov. 3 at the Town and Country Hotel and Convention Center in San Diego, Calif.