The science-fiction series Babylon 5 used this as a plot device in one episode - a group of hostages were located partly by identifying the background hum of machinery in the terrorists' video of demands.
It can be defeated by jamming the relevant frequency bands, i.e. by overlaying other mains recordings with random fluctuations. No deed to remove the original.
I'm not sure a notch filter would be enough because, even though the mains is supposed to be a perfect sine wave, some harmonics occur (sometimes with a much higher frequency). Perhaps by filtering all harmonics as well? Sound quality isn't exactly a must in these situations.
It requires a technically astute criminal, though, and most aren't.
It requires a technically astute criminal, though, and most aren't.
Exactly. Many don't even use gloves. I was wondering on how precisely you could locate the hum and if you could deliberately create a hum signature for a region.
