Why hum on your recordings can be good

Now, to ease me into discussing more technical things on this blog...

I read an article on the BBC website today that talks about a good use for the hum you get on your recordings, which I found interesting (although from a sound recording perspective, I still don't like that bloody hum).

Mains hum is a frequency that audio equipment picks up for a variety of reasons. In most of the world (the blue-coloured countries on the map below), the hum is emitted at 50Hz.

 

However, a quote from Dr. Alan Cooper in the BBC article explains "because the supply and demand [of electricity] is unpredictable", each day there are different fluctuations in the amount of energy distributed over the National Grid which cause small fluctuations in the tone of the hum. Because there's one National Grid to supply electricity to the entire nation, the hum throughout Britain is exactly the same frequency at the same time, wherever you are.

When recordings are played back as evidence in court hearings, people might claim that the recording has been edited to change the meaning of what's been said. Thanks to the hum, a technique called Electric Network Frequency (ENF) analysis can now be used to determine whether or not recordings have been altered because, as the fluctuations in frequency are unique to each day, the hum extracted from the recordings can be compared to the recordings of the hum that the Metropolitan Police have been capturing for the last seven years. If the hum in both recordings matches, the recording hasn't been tampered with. If it doesn't match, it's evidence of manipulation and can't be used as evidence in court.