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.

Forensic scientists…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…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 in the order of a few thousandths of a hertz…

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.

Comparing the unique pattern of the frequencies on an audio recording with a database that has been logging these changes for 24 hours a day, 365 days a year provides a digital watermark: a date and time stamp on the recording…

Hmmm?