In more detail
Because the variation follows a pattern that is the same across a grid at any moment, comparing a recording's hum to reference logs can help date it or reveal cuts and insertions. Europe's mains is 50 Hz and North America's 60 Hz. The frequency drifts by a few hundredths of a Hertz (Hz) as supply and demand shift, and microphones or mains-powered recorders pick up the hum at the base frequency and its harmonics (100, 150 Hz on a 50 Hz grid). Analysts filter the audio around one of these lines and track the peak over time in a Spectrogram.
Why it matters
ENF is one of the few signals inside a recording that ties it to an outside, logged record. A break in the ENF trace can mark an edit, and a trace that matches the grid log at a different date than claimed contradicts the story. It only works if the hum was captured at all: battery recorders far from mains wiring may have none, and heavy compression or noise reduction can wipe it out.
Example
Frequency resolution depends on window length: a 16-second analysis window gives bins 1/16 = 0.0625 Hz apart, so the peak is then interpolated to follow smaller changes. Harmonics help: a 0.02 Hz dip at 50 Hz appears as a 0.04 Hz dip at 100 Hz and 0.06 Hz at 150 Hz, which is easier to measure. On a 60 Hz grid, look at 60, 120 and 180 Hz instead. The Audio Forensics runs this kind of hum analysis.