An average person on the street may find it hard to tell the difference between an unaltered recording and one that's been edited or tampered with in some way, but forensic experts are often tasked with just that responsibility. Thankfully there's one technique that's nearly foolproof so long as the aural evidence is there. For the past several years, authorities in London have been recording a continuous log of the "mains" frequency — a slight hum (normally inaudible to human ears) that accompanies the distribution of electricity. The mains frequency provides a unique, constantly shifting sonic fingerprint that forensic specialists can match up against nearly any audio sample.
So long as a provided clip was recorded near a power source, they're usually able to amp up this "hum" and compare it to the exhaustive database. If a sound file is genuine, they'll be able to identify the exact time it was recorded based on fluctuations in the mains frequency. Edited clips, on the other hand, either won't have a valid match at all or will contain multiple timestamps if several recordings have been deceptively pieced together. Sadly the technique, known as Electric Network Frequency analysis, isn't of much use if a source file was captured somewhere in the wilderness or other areas isolated from electricity. But it's just another example of technology lending a powerful hand to investigators.
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