Mains electricity emits a constant hum, known as the ‘mains frequency’, which becomes embedded in digital recordings made anywhere near an electrical power source. This noise is being used to help police in the UK fight crime, as forensic scientists can authenticate audio evidence to find out whether it is genuine or has been edited.
BBC News reports that audio specialists at the Metropolitan Police forensic lab in south London have been continuously recording the sound of mains electricity for the last seven years. Minute fluctuations have been found to occur and the pattern of these changes in frequency is unique over time.
Comparing the unique pattern of the frequencies on an audio recording with a database that has been logging these changes for 24 hours a day, 365 days a year provides a digital watermark: a date and time stamp on the recording… It is a technique known as Electric Network Frequency (ENF) analysis, and it is helping forensic scientists to separate genuine, unedited recordings from those that have been tampered with.
The hum of the mains frequency can be extracted from a digital recording and compared with the database to see whether it matches as a continuous piece of audio or if it has breaks and different patterns, meaning it has been edited.