Clever stuff (especially as hum is otherwise a great nuisance anywhere in audio)….

It is a technique known as Electric Network Frequency (ENF) analysis, and it is helping forensic scientists to separate genuine, unedited recordings from those that have been tampered with.

Dr Harrison said: “If we have we can extract [the hum] and compare it with the database, 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.”

BBC News – The hum that helps to fight crime