Every room with electrical wiring has a 50 Hz or 60 Hz hum —
radiated by cables, transformers, and fluorescent lights.
Your phone's microphone picks it up even through silence.
This is how forensic labs date audio recordings.
every recording carries its location in the hum.
EU mains = 50 Hz. US/Japan = 60 Hz. ENF (Electric Network Frequency) analysis
compares a recording's hum against historical grid frequency logs — accurate to within minutes.
Police forensic labs date recordings this way. No timestamp required.
—
Hz · dominant mains component
waiting
50 Hz (EU)—
60 Hz (US)—
100 Hz (2nd EU)—
120 Hz (2nd US)—
0 – 200 Hz
—peak Hz
—dB rel.
—grid
▶ how does this work?
The microphone captures ambient audio. An FFT with a 32768-point window gives ~1.3 Hz resolution at 44.1 kHz,
enough to resolve 50 Hz from 60 Hz clearly. We integrate energy in narrow bins around each harmonic.
ENF (Electric Network Frequency) fluctuates slightly around nominal — 49.98–50.02 Hz in Europe —
because generators adjust to demand. These micro-variations are the "fingerprint" used in forensic dating.
Your phone captures this variation in real time.
Strong 100 Hz / 120 Hz harmonics suggest CFL or LED driver interference.
A very strong 50 / 60 Hz in a quiet room usually means a nearby transformer or UPS.