Police use 24/7 power grid recordings to spot doctored audio

Forging audio recordings is a lot harder than it used to be, thanks to a new method of authenticating recordings based on the buzz of the electrical power grid at the time they were recorded.

The oscillations of alternating current (AC) produce a distinct frequency – 50Hz in the UK, 60Hz in North America – that varies slightly with the demand placed on the power grid. At times of high demand, the frequency will be lower; when demand is low, it will be higher.

Audio specialists at the Metropolitan Police forensic lab in south London have been recording the hum of this frequency continuously for the last seven years – 24 hours a day, 365 days a year – and they say they can now use that data to verify the authenticity of audio recordings.

The method works because sound recordings invariably pick up a hint of the AC power hum from nearby electrical sockets, lights, and other power sources.

It's a problem that's well known to sound engineers, who work painstakingly to remove the buzz from their recordings. Often it's to no avail; amplify the recording enough, and the buzz is still there.

In the kinds of amateur recordings that are often entered as evidence in court, however, typically no effort has been made to edit the power grid hum from the audio, which allows investigators to use the subtle variations in the frequency of the noise to determine whether the recording is genuine.

If a witness claims a recording was made in the morning on a certain date, for example, then the telltale buzz picked up by the recording device had better match the buzz logged by the police forensic lab at that time.

Similarly, if a portion of a recording has been edited out, the resulting stutter in the electrical hum will be detectable, even when the edits are imperceptible to the human ear.

Sources say this method of audio forensics is now in use by law enforcement agencies in various countries throughout the world, though the Metropolitan Police were reportedly the first to automate the system.

According to researchers, the process is aided in the UK by the fact that the entire country is served by a single power grid, which means AC hum data recorded in London can be used to check recordings made anywhere else.

Other countries, such as the US, are served by multiple power grids, which means researchers must monitor the buzz at multiple locations to build a useful reference library.

Although the technique has reportedly already been instrumental in determining the outcomes of several trials, the technology continues to evolve as new types of recording devices are introduced.

"Digital forensics is constantly in flux, and the technology is changing every day," says the Met Police's Dr. Alan Cooper. "Every time a new format comes out, we need to be able to extract the data from those recordings and find different techniques to find out more about them."