Power grid fluctuations hidden in audio recordings proved a powerful tool for police forensics

February 14, 2018, Agency for Science, Technology and Research (A*STAR), Singapore
Power grid fluctuations hidden in audio recordings proved a powerful tool for police forensics
The I2R A*STAR team together with their collaborators from SPF. Credit: A*STAR Institute for Infocomm Research

Audio and video recordings are important sources of evidence in criminal investigations, especially as more electronic devices are in use now than ever before. However, for recordings to be admissible, investigators often need to determine the time they were made, which can be difficult. Now, a team led by Vrizlynn Thing at the A*STAR Institute for Infocomm Research (I2R), in collaboration with the Singapore Police Force, has developed an impressive new system that reliably estimates the time of recordings by identifying small fluctuations in the frequency of the electrical power grid.

The 'electrical network frequency' (ENF) of power grids is centered around 50 or 60 Hertz, and is picked up in audio recordings as a background hum. The ENF shifts up and down randomly, which provides each recording with a unique fingerprint that can be compared to the long-term records captured continuously and maintained at forensic labs.

"The are consistent across different places within the same ," explains team member Lilei Zheng. "As a consequence, recordings captured in different places at the same time will have ENF fingerprints showing the same fluctuations."

By visually inspecting the ENF, human investigators can reliably match recorded fluctuations to a time in the long-term records, but this is a laborious task best done by a computer. In response, the I2R team developed a similarity criterion called bitwise similarity (bSim) that mimics the way humans judge the similarity of two signals.

The team tested bSim by using it to identify the timing of 187 audio recordings made around Singapore using various mobile phones.

They found that bSim greatly outperformed previous similarity metrics, which were thrown into doubt by small deviations even when the general shapes of the signals were clearly similar. "bSim enables us to focus our attention on the overlapped parts instead of being drawn away by the deviated parts," says Zheng.

"The science behind ENF pattern matching has been proven to be reliable, like fingerprints and DNA," says Thing. "It has been used in courts in various jurisdictions and the cases cut across many different crimes. We hope to extend our work from to videos, which not only contain audio but may also enable us to 'see' the ENF through variations in lighting."

"This innovative solution towards audio authenticity verification developed by I2R has already proven itself in actual use, and we are excited about the potential it holds," says a representative of Singapore Police Force.

Explore further: Rare audio of indigenous languages saved by invention 100 years later

More information: Lilei Zheng et al. Time-of-recording estimation for audio recordings, Digital Investigation (2017). DOI: 10.1016/j.diin.2017.06.001

Related Stories

Bird recognition

January 3, 2018

Birds play an important role in a wide variety of ecosystems as both predator and prey, in controlling insect populations, pollinating and seed dispersal for many plants, and in releasing nutrients on to land and sea in the ...

Diverse causes behind frequency fluctuations in power grids

January 10, 2018

The use of renewables like the sun and wind can cause fluctuations in power grids. But what impact do these fluctuations have on security of supply? To answer this question, scientists from Juelich and Goettingen worked together ...

Can patients record doctor's visits? What does the law say?

July 10, 2017

Traffic stops, office conversations, and even doctor's visits—more and more people today are choosing to record life's encounters. If you are doctor, there is a good chance that at least one of your last 10 patients recorded ...

Recommended for you

What can snakes teach us about engineering friction?

May 21, 2018

If you want to know how to make a sneaker with better traction, just ask a snake. That's the theory driving the research of Hisham Abdel-Aal, Ph.D., an associate teaching professor from Drexel University's College of Engineering ...

Flexible, highly efficient multimodal energy harvesting

May 21, 2018

A 10-fold increase in the ability to harvest mechanical and thermal energy over standard piezoelectric composites may be possible using a piezoelectric ceramic foam supported by a flexible polymer support, according to Penn ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.