Electric 'hum' used as investigation tool

London's Metropolitan Police say the recorded "hum" of background noise produced by the electricity grid has helped them solve crimes. A database of the noise, which varies in pitch with the amount of demand on the distribution network, can help verify whether audio or video recordings gathered as part of a police investigation are genuine, police officials said. The database, created by Britain's National Grid to analyze ongoing electricity demands, can be compared with evidence from an investigation to determine the exact date and time someone was speaking in a recording, the BBC reported Thursday. Police said they have been able to use the database, comparing it to background noise to authenticate covert recordings made by victims of a crime in which the perpetrator had apparently admitted his guilt, senior digital forensic practitioner Alan Cooper said. "If we can extract the background hum from the recording, we can exactly date and time when it was made and also establish whether there has been anything added or taken away from the recording," Cooper said. "The fluctuation occurs because the frequency drifts in sympathy with the supply and demand," he told the BBC. "It builds up a unique fingerprint. From a statistical standpoint, it is very, very strong evidence."