ENF (Electrical Network Frequency)
Definition
The instantaneous frequency of the mains electricity supply (nominally 50 or 60 Hz), which drifts continuously and uniquely with time and is captured as background hum in mains-proximate audio recordings.
- Nominal value
- 50 Hz (Europe, most of Asia and Africa) or 60 Hz (Americas, part of Asia)
- Behaviour
- Drifts continuously around the nominal value with grid load
- Capture mechanism
- Induced hum from mains-powered devices near the recorder
- Forensic use
- Timestamping and continuity checking of audio recordings
Common questions
Why does ENF drift rather than stay fixed at 50 or 60 Hz?+
Grid operators let frequency float slightly around the nominal value as generation and demand shift through the day, then correct the cumulative drift over subsequent hours. That fine, non-repeating fluctuation pattern is what makes each moment in time distinguishable in a reference database.
Does every audio recording carry a usable ENF signal?+
No. It requires the recording device or its environment to be near mains power, for example through an AC adapter hum, fluorescent lighting, or an induction pickup, so recordings made purely on battery power in a mains-free environment often carry no detectable ENF component.
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