The hum varies subtly, a matter of millihertz, due to power demand and supply, so that there is a consistent signature associated with any point in time since the recording has begun. Simultaneously, any other recording that’s made in the vicinity of a power line or appliance picks up the faint hum in the background. By matching the hum in the background of a telephone recording with the master record of all electrical humming over the last seven years, the police can ascertain the exact time when the recording was made. This Electric Network Frequency analysis can verify the provenance and continuity of a recording as well, and has held up as crucial evidence in UK court. BBC
