Interesting technique if you want to put a date stamp on a recording. They use what is called 'electrical network frequency' matching the fluctuation in the noise in the recording with the known fluctuation in the grid.
robertheaton.com
How to date a recording using background electrical noise | Robert Heaton
Three men were accused of selling firearms to South London gangs. At their 2012 trial in Croydon Crown Court, the prosecution played the jury a recording, taken undercover, of the trio allegedly arranging a sale. But the men’s lawyers claimed that the recording was a fake, and that the police...
robertheaton.com
