Sound Signature

Electrical networks emit such a constant, locally recognizable hum that their sound can be used to help solve crimes.

[Image: Random sound file using Sound Studio].

A forensic database of electrical sounds is thus being developed by UK police, according to the BBC. "For the last seven years, at the Metropolitan Police forensic lab in south London," we read, "audio specialists have been continuously recording the sound of mains electricity. It is an all pervasive hum that we normally cannot hear. But boost it a little, and a metallic and not very pleasant buzz fills the air."

Any digital recording made anywhere near an electrical power source, be it plug socket, light or pylon, will pick up this noise and it will be embedded throughout the audio.

This buzz is an annoyance for sound engineers trying to make the highest quality recordings. But for forensic experts, it has turned out to be an invaluable tool in the fight against crime.

Even with—or, in fact, because of—slight fluctuations in the level of local electric power, such recordings can reveal sonic traces of where and when they were recorded; these barely audible details act as "a digital watermark," the BBC explains, secret audio artifacts that put "a date and time stamp on the recording."

You can thus acoustically prove that someone was in a certain part of, say, London at a certain time of day, and that a given audio recording is thus genuine (or faked), due to the exact signature of what electrical networks in that part of the city had been doing at the time.

It's like cosmic microwave background radiation, an immersive soundtrack—a sea of acoustic metadata—hidden in the built environment, detectable electronically, droning all around us at a volume usually below human hearing.