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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 was used. But dozens of factors - from the angle and direction of the 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 occurred.
Now researchers from BAE Systems, in Austin, TX, 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 recently presented its findings at the annual meeting of the Acoustical Society of America in San Diego.
"The field of forensic gunshot analysis is relatively new," said Steven Beck, principal scientist for BAE Systems. Analysts receive the 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."
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. Researchers were surprised at how important the angle of the gun was to the sound signature, or waveform, caught by each microphone. A shot fired into the air produces a wildly different waveform than one fired in the direction of a recorder, even when both shots come from the same gun. Similarly, 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. Analysts could take advantage of this extra sound in a good recording to identify the direction of the weapon, said Beck.
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