From BBC News, an interesting piece on how forensic scientists are using a digital hum to authenticate audio recordings:
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.
While the frequency of the electricity supplied by the national grid is about 50Hz, if you look at it over time, you can see minute fluctuations.
The process is known as Electric Network Frequency analysis. How this research came to be:
A decade ago, a Romanian audio specialist Dr Catalan Grigoras, now director of the National Center for Media Forensics at the University of Colorado, Denver, made a discovery: that the pattern of these random changes in frequency is unique over time.
By itself, this might be an interesting electrical curiosity. But when you take into account that most digital recordings are also embedded with this hum, it becomes a game changer.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.