mains hum geolocator

ac power grids hum at exactly 50 hz or 60 hz depending on country. this electromagnetic hum leaks into microphones from nearby power supplies, cables, and fluorescent lights — and it tells forensic audio analysts where and when a recording was made. this technique is called electric network frequency (enf) analysis, used by investigators worldwide to authenticate recordings and detect edits.

50 hz — most of europe, asia, africa, australia, south america (west). 60 hz — north america, central america, japan (east), brazil (east).

✦ works best with a power adapter nearby · turn up mic gain · laptop power supply or a lamp often leaks ENF

• enf authentication is real forensic science. uk courts have accepted enf analysis as evidence. a recording made in germany will show a 50 hz signature in its noise floor, permanently embedding the grid into the file — even if exported as mp3.
• the grid frequency drifts continuously. grid frequency varies ±0.1 hz around the nominal 50/60 hz in response to load changes. national grid operators publish archives of this drift. by matching the drift fingerprint in a recording to the archive, investigators can pinpoint the exact time it was made — or detect post-recording edits.
• modern laptops suppress this. switching power supplies at 100–400 kHz don't leak at 50/60 hz directly. what leaks instead is the ground-loop coupling from the ac-side. old transformers and fluorescent lights are the most reliable sources for demos.
• cross-country grid ties. europe's synchronous grid connects 35+ countries — they all share the same 50 hz signal (and same drift). north america's eastern and western interconnections do the same at 60 hz.