A suspected terrorist has been taped planning a deadly attack, and the police want to use this evidence in court. Or someone has been captured on CCTV threatening an assault.
Increasingly, recordings like these are playing a role in criminal investigations.
But how can the police be sure that the audio evidence is genuine, that it has not been tampered with or cleverly edited?
Forensic scientists have come up with the answer: they can authenticate these recordings with the help of a hum.
Electric find
For the last seven years, at the Metropolitan Police forensic lab in south London, 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.
For the last seven years, at the Metropolitan Police forensic lab in south London, 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.
"The power is sent out over the national grid to factories, shops and of course our homes. Normally this frequency, known as the mains frequency, is about 50Hz," explains Dr Alan Cooper, a senior digital forensic practitioner at the Met Police.
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.
by Rebecca Morelle, BBC | Read more:
Illustration: Unknown
