IF CCTV cameras recorded sound as well as video footage, they could add a new dimension to fighting gun crime, say forensics experts. The news follows the acquittal of a man accused of a fatal shooting in Argentina, after analysis of a video soundtrack 鈥� rather than the pictures 鈥� proved he could not have pulled the trigger.
During a riot in Cutral-C贸, Patagonia, in April 1997 a bystander was shot dead. A journalist videotaping the event picked up 17 gunshots on the soundtrack, but the picture was so blurred no one could see who fired the fatal shot. Detectives charged a suspect, but a judge was not happy with the evidence. So he asked physicists at the CAB atomic energy research centre in Rio Negro to try to extract more evidence from the tape.
CAB鈥橲 Rodolfo Pregliasco and his team went back to Cutral-C贸 to reconstruct the crime. They lit firecrackers to simulate gunfire and, standing where the reporter had stood, recorded the sounds through a microphone, including the echoes produced as the sound bounced off nearby buildings.
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The principle behind their technique dates back to the echo analysis used to determine where President Kennedy鈥檚 assassin fired from in 1963. But that analysis was plagued with difficulties because a moving police motorbike had simply relayed the sound via an open radio channel back to HQ, where it was recorded.
The Argentinian team matched their firecracker echo patterns with the shot echoes on the tape, and found they could place the source position of 11 of the 17 shots. Eight of the shots could be attributed to a specific person seen In the video, while three others, including the fatal shot, came from men running near the police lines. While they couldn鈥檛 pinpoint the gunman, they could prove it wasn鈥檛 the main suspect 鈥� so he was acquitted late last year, they鈥檒l reveal in a forthcoming edition of the Journal of Forensic Science.
With more and more CCTV cameras recording our streets, experts say the Cutral-C贸 case adds weight to the argument that security cameras should also record sound.
鈥淭his sort of reconstruction method would certainly help investigations,鈥� says Philip Harrison, an audio forensic consultant with JP French Associates in York. And James Reames, a forensic technologist at JBR Technology near Washington DC, performs similar acoustic analyses in his insurance investigations. 鈥淭hese analyses are happening more and more,鈥� he says.