The Incriminating Hum of Audio Recordings

By Anupum Pant

If you go to the Wikipedia’s page for a seemingly banal topic “Frequencies,” at the bottom of the page you’ll find a section that says “Line current.” You may want to read through it, but you won’t have to because I’ll tell you what it says, in short…

This section is particularly interesting because it mentions that when sounds are recorded, a very faint hum that you can’t hear unless you magnify the sound, gets recorded with it. This hum comes from the electricity which powers anything, say a plug socket, which was close to the microphone when the sound was getting recorded.

In the American electricity grids, a 50 Hz AC current produces a different hum in the recording than in 60 Hz European grids. So, sounds recorded in America, and sounds recorded in Europe, or other places which use the 60 Hz frequencies, can be distinguished fairly easily by forensic experts. This gives a great power to forensic experts. And a headache to sound engineers who never want a hum being embedded in their audio.

Now, what the WIkipedia page doesn’t tell you is that this hum carries a greater amount of information than just telling the recordings from continents apart. In fact, there’s apparently enough information to almost know the date on which this recording was done.

There exist laboratories around the world, to assist forensic experts with this task of determining the recording date, which have continuously been recording just the hum of electricity for several years now. They study the tiny fluctuations (in the order of thousandth of a hertz) in these frequency and keep an archive of dates and tags. It has been found that patterns in these hum fluctuations have been varying continuously and in a very systematic fashion over the years.

The recordings sent to the forensic experts can use processing to isolate the hum and match it with the hum of a date from the archives. And then a date on which, or at least a rough date, on which the recording was done.

via [BBC]

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