The hum from mains electricity is proving to be a valuable ally for British forensic scientists in fighting crime.
A report by the British Broadcasting Co. said the hum helps investigators determine if a recording to be used as evidence is authentic or merely cleverly edited.
Since 2005, audio specialists at the Metropolitan Police forensic lab in south London have been recording the hum of mains electricity - about 50 Hertz (cycles per second).
"Even if (the hum) is picked up at a very low level that you cannot hear, we can extract this information," added Philip Harrison of JP French Associates, another forensic audio laboratory logging the hum for several years.
Such a noise will be embedded throughout any digital recording made near an electrical power source, such as a plug, light or pylon.
"The power is sent out over the national grid to factories, shops and of course our homes. Normally this frequency, known as the mains frequency, is about 50Hz," said Dr Alan Cooper, a senior digital forensic practitioner at the Met Police.
Ten years ago, Romanian audio specialist Dr. Catalan Grigoras, now director of the National Center for Media Forensics at the University of Colorado, Denver, discovered the pattern of random changes in frequency is unique over time.
If the unique pattern of the frequencies on an audio recording is compared with a database logging such frequency changes 24/7, a digital watermark of sorts can be made on the recording.
The technique - Electric Network Frequency (ENF) analysis - is helping forensic scientists separate genuine unedited recordings from tampered ones.
Harrison said they can extract the hum and compare it with the database.
"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," he said.
But this may be more complicated in other countries where more than one grid supplies power to the country.
Still, all investigators need to do is have the hum continuously logged on each power system and for a recording to be compared against each of them.
"This has really been a key tool in the box to help us with this kind of work," Harrison said.
Cooper was recently called as a witness on ENF in court, which was trying a case against a gang accused of selling weapons.
At the time, undercover police had recorded an arms deal, but the defense claimed the police had merely strung together separate recordings.
"We carried out various forms of analysis, including the mains hum frequency analysis and we found some good quality signals, and that the alleged date and times of the recordings matched with the extracted data from the recordings themselves," Cooper said.
The analysis showed the recordings had not been tampered with, and led the court to find Hume Bent, Carlos Moncrieffe and Christopher McKenzie guilty and order them jailed for 33 years.
While the Met Police were the first to automate the system, Cooper said the technique is now starting to be used by others around the world.
But he admitted there are always new challenges ahead.
"Digital forensics is constantly in flux, and the technology is changing every day. 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," he said.
— TJD, GMA News