The secret hum
Stories from Physics for 11-14 14-16
In the UK, mains electricity oscillates with a frequency of 50 Hz. This results in the emission of a signal, inaudible to people, but detectable on electronic audio recording devices. The frequency varies by the order of 0.001 Hz in response to changes in demand on the National Grid. These variations create a unique fingerprint that may become embedded in a recording. The signal acts as a timestamp which security services can use to determine the exact time at which an audio recording was made.
Electrical engineers have created models of how the grid frequency changes during big sporting events, such as the World Cup, or after an episode of a popular soap opera, in order to be able to predict times of peak demand.