Forging recorded conversations just got harder, as power grids can now be used by the police to detect fake and doctored audio.

As reported by the BBC, forensic experts within the police force and criminal investigation authority use recordings, often used in cases of rape, fraud and assault when a suspect is secretly taped, to prove someone’s innocence or guilt.

However, making sure the evidence is genuine is the tricky part.

In order to combat this issue, specialists at the Metropolitan Police forensic lab in south London have been recording the hum of power grid frequencies, the alternating current (AC) which varies depending on pressure placed on a grid at particular times, for seven years.

When demand is high, frequencies — around 50Hz in the UK — will be low, and when power demand is low, the frequency rises.

Why? The team says that by tracking changes in the hum of these frequencies — for the last seven years without stopping — they can use this data to authenticate recordings submitted as evidence, and are able to tell when a transcript has been tampered with; a technique known as Electric Network Frequency (ENF) analysis.

Whenever you record audio, the footage will always pick up the humming of an AC power unit, whether from lights, sockets or other power sources. Therefore, when a homemade recording winds up in court, investigators can compare the variations in noise on the tape with those catalogued in the 24/7, 365 days a year database.

If someone claims a recording is made on the morning of December 21, for example, the authorities can check this against their data to determine whether the buzzing from AC power units match up. In addition, if some clever editing has resulted in a portion being cut out or shifted, then that same hum — not necessarily detectable to human ears — will change and stutter.

“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.”

The ENF technique has been used in several cases with success, and according to the Met Police, will now be used widely by the force. However, this means more work for audio specialists — as every time a new format or grid modification comes out, they must be able to extract the data they need as well as keep the catalogue continuously flowing.