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The hum that helps to fight crime
By Rebecca Morelle, Science reporter, BBC News
A hum that comes from mains electricity has allowed forensic scientists to establish whether recordings are genuine. A rape victim has come forward to the police. She says she has confronted her attacker and has secretly recorded him admitting his guilt. A suspected terrorist has been taped planning a deadly attack, and the police want to use this evidence in court. Or someone has been captured on CCTV threatening an assault. Increasingly, recordings like these are playing a role in criminal investigations. But how can the police be sure that the audio evidence is genuine, that it has not been tampered with or cleverly edited?
Forensic scientists have come up with the answer: they can authenticate these recordings with the help of a hum.
For the last seven years, at the Metropolitan Police forensic lab in south London, audio specialists have been continuously recording the sound of mains electricity. It is an all pervasive hum that we normally cannot hear. But boost it a little, and a metallic and not very pleasant buzz fills the air. "The power is sent out over the national grid to factories, shops and of course our homes. Normally this frequency, known as the mains frequency, is about 50Hz," explains Dr. Alan Cooper, a senior digital forensic practitioner at the Met Police.
Any digital recording made anywhere near an electrical power source, be it plug socket, light or pylon, will pick up this noise and it will be embedded throughout the audio. This buzz is an annoyance for sound engineers trying to make the highest quality recordings. But for forensic experts, it has turned out to be an invaluable tool in the fight against crime. While the frequency of the electricity supplied by the national grid is about 50Hz, if you look at it over time, you can see minute fluctuations.
"It's because the supply and demand is unpredictable," says Dr. Cooper. If millions of people suddenly switch on their kettle after watching their favourite soap, the demand for electricity may outstrip the supply, and the generators will pump out more electricity, and the frequency will go up. "But when supply is greater than demand, the generators will slow down and the frequency will go down," explains Dr. Cooper. "The grid operators will try and compensate for this, but you can sometimes see some very significant fluctuations."
A decade ago, a Romanian audio specialist Dr. Catalan Grigoras, now director of the National Center for Media Forensics at the University of Colorado, Denver, made a discovery: that the pattern of these random changes in frequency is unique over time. By itself, this might be an interesting electrical curiosity. But when you take into account that most digital recordings are also embedded with this hum, it becomes a game changer.
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