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Interesting article on using 'hum' to find edit points in audio recordings. Not sure if this is useful evidence in all contexts but a great way to see how sound has an 'subliminal' presence and effect :)
http://www.bbc.co.uk/news/science-environment-20629671
Is this also happening with professional gear instead of iPhones or digital dictation devices?
This technique has nothing to do with "removing" hum from recordings - it's all about using the very slight - but trackable - variations in mains frequency (50Hz or 60Hz depending on where you are) - to authenticate recordings.
Localised recordings of mains AC frequency can be spectrally tracked so that the variations in the AC frequency can be compared with those found in other recordings such as those captured by the police during investigations. If discontinuities are found in the recordings then it can be a strong indication that editing has occurred within the clandestine recordings.
It does require that a fully tracked recording of localised mains frequency is available for reference.
This is a tried and tested forensic audio technique.
Professional audio devices have many ways of removing mains hum, but I think these forensic detectives will almost always find its presence, as it is transferred through the air electromagnetically between devices. Audio professionals attenuate noise so it can't be heard by a listener. But it is still there at very low levels. Even if you try to remove it with a band filter, it will remain with other very low noise.
That was a very interested read, thanks for that. I would say that this does still apply to pro audio equipment, as those hums can be present in the acoustic room tone of a recording and aren't not solely heard through signal interference. Some microphones are so open and sensitive that they can often pick up those sort of sounds when you really don't want them to. And with 24 bit recording some of that noise level can be buried deep down in the lower parts of the dynamic range. For example, Earthworks QTC50s are so sensitive to low frequencies and have such an open omnidirectional sound that you can hear all sort of buzzes and hums that you would normally miss. So much so that I have had to remove all electrical equipment from the room I was recording in, and even changed the lighting I was using.
