What is “Hum” Forensics?
Forensic audio analysis has been around long before the Watergate tapes and continues to make advances. But a monumental leap forward occurred about a decade ago right here in Colorado. You see, one of the foundational elements of any analysis is that of authentication. Edited and doctored recordings should always be considered when reviewing audio evidence. It’s more than just verifying the identity of the source. The source being a person or activity like a gunshot, car horn, or mechanical device. It’s also about verifying the date and time of the recording. Was it made just before the commission of the crime or a year earlier? Sometimes the words said are not as important as when they were uttered. It is the when that helps detectives put things in context. It turns out that within virtually every audio recording there is a silent witness.
There is something called the mains frequency. Inaudible to our ears it is a persistent hum generated by our electrical grid. Amplify the sound and you’ll hear a metallic buzz. Researcher Dr. Catalan Rigoras discovered that this hum fluctuates as the load on the power grid goes up and down. Think of it as a melody of notes arranged in an ever changing symphony. Since the load on the electrical grid is constantly changing, so too is the frequency of the hum. So forensic audio specialists and other researchers have embarked on am ambitious project to record this hum every second of every day and cataloging the signatures in a searchable database. It’s called Electric Network Frequency (ENF) analysis and they’ve been doing it for years. So if an audio recording is made anywhere near an electrical source it will record the hum as well. It can be an electrical outlet, a lamp, computer, anything. The distinctive hum signature acts like a time stamp or watermark. It is unique to that moment in time. The longer the recording the more easily it can be authenticated.
The hum is unique to the power grid so it can get a little more tricky in countries with multiple grids in service. But if a single grid is in use the value of the analysis increases dramatically. Say for instance that someone cuts and splices pieces of audio together…the hum signature won’t match. There will be gaps in the hum when compared to the source hum in the database. What happens when two different audio tracks are combined to make a fake original? There will be two hum frequencies in the background. So unless your recording is made in the desert or the middle of the forest the recording will likely contain the distinctive hum in the background. You might want to consider using this technology in your novel. Whether your character is talking on a cell phone or being recorded on a surveillance system the hum will be there. The key is to use some kind of doctored recording. Your protagonist can then do the ENF analysis to show that the recording has been tampered with (or is original). Either way its a really cool technology.
Posted on January 16, 2013, in Future Forensics, General, The Crime Scene and tagged Catalan Grigoras, Mains electricity, Metropolitan police, Metropolitan Police Service. Bookmark the permalink. 2 Comments.