According to the BBC News, a police forensics lab has been recording the digital hum of London's electric grid for the past seven years, and this record will allow experts to pinpoint the exact time that any audio recording was made in the city.
Whenever any audio recording is made, it will always pick up background noise, which will include a hum produced by any electrical power lines or appliances in the area.
"The power is sent out over the national grid to factories, shops and of course our homes," said Dr. Alan Cooper, an audio forensics expert with the Metropolitan Police Forensic Audio Laboratory. "Normally this frequency, known as the mains frequency, is about 50 Hz."
This hum isn't constant, though. It changes, ever so slightly, based on the local demands on the power grid, and by recording these changes, the London police have constructed a digital audio 'timeline'.
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Now, using a method called Electric Network Frequency (ENF) analysis, police can examine any digital audio recording made in the city over the past seven years (and moving ahead into the future) and compare the background noise from it to this timeline. Matching up the background noise from each will provide a digital time-stamp, and allow them to tell unedited recordings from edited ones. This will prove to be incredibly valuable for authenticating audio evidence presented by both victims and police investigators.
"We can extract [the hum] and compare it with the database," said Philip Harrison, a director of the independent audio forensics laboratory, J. P. French Associates.
"If it is a continuous recording, it will all match up nicely. If we've got some breaks in the recording, if it's been stopped and started, the profiles won't match or there will be a section missing. Or if it has come from two different recordings looking as if it is one, we'll have two different profiles within that one recording."
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ENF analysis can be used for any power grid and any recording made within that power grid. For the U.K., where the entire country is supplied by one power grid, only one background hum record is needed, but for Canada and the United States, which are supplied by a total of five power grids, this technique becomes a bit more difficult. However, as long as there is a continuous record of the background hum from each grid, the time-stamp of an audio recording can still be found by comparing it to each of them.