Electrical Network Frequency analysis, ENF analysis, matches background hum against power grid logs. I talked to one of the researchers who works on it, and also set them a challenge. Thanks to @answerinprogress, @hannahwitton and @SteveMould!

MORE FROM TOM: https://www.tomscott.com/
(you can find contact details and social links there too)

WEEKLY NEWSLETTER with good stuff from the rest of the internet: https://www.tomscott.com/newsletter/
❓ LATERAL, free weekly podcast: https://lateralcast.com/    / lateralcast  
➕ TOM SCOTT PLUS:    / tomscottplus  
THE TECHNICAL DIFFICULTIES:    / techdif