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.
Philip Harrison, from JP French Associates, another forensic audio laboratory that has been logging the hum for several years, says: "Even if [the hum] is picked up at a very low level that you cannot hear, we can extract this information."
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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.
Dr Harrison said: "We can extract [the hum] and compare it with the database - if it is a continuous recording, it will all match up nicely.
"If we've got some breaks in the recording, if it's been stopped and started, the profiles won't match or there will be a section missing. Or if it has come from two different recordings looking as if it is one, we'll have two different profiles within that one recording."