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Mains hum and the grid frequency (ENF)

Indoor recordings hum with the electricity supply: 50 Hz (nominal, Europe) and harmonics, with magnetostriction strongest at 100 Hz. The grid's actual frequency wanders by tens of millihertz as load and generation balance — the electric network frequency, ENF — and a long recording carries that wander as a continuous, involuntary log of the grid. Version 0.4 adds a reader for it, developed on the Haarlem-loft case study (9 h of hum tracked to ±50 mHz).

The cached per-minute spectra are far too coarse for this (5.9 Hz bins); ambiscape.enf makes its own pass over the raw W channel.

Tracking

from ambiscape import enf

tr = enf.enf_track(sess, step_s=300, win_s=60, harmonics=(1, 2))
enf.enf_summary(tr)
# {'mean_hz': 49.9958, 'sd_mhz': 21.0, 'max_dev_mhz': 88.0,
#  'coverage': 1.0, 'harmonic_agreement_mhz': 0.9, ...}

hum_peak measures one window (zero-padded FFT + parabolic interpolation → millihertz precision on a 60 s window); enf_track walks a whole session, skipping the sliver reads that recorders' overlapping 2 GB splits produce at take boundaries.

Reading the summary

  • harmonic_agreement_mhz is the authenticity check: the 50 Hz and 100 Hz hum are independent acoustic lines driven by the same electrical frequency, so millihertz agreement confirms you are looking at the grid and not at a machine. In Haarlem a second stable line at ~49.8 Hz — a rotor just under synchronous speed — masqueraded as mains in coarse spectra; its wander was 17× the grid's and it tracked nothing.
  • A Continental-Europe trace should sit near 50.000 Hz with SD ~20 mHz, rarely leaving ±50 mHz (the ENTSO-E normal band). Systematic offsets mean either a mechanical line or a recorder sample-clock error.
  • Forensics: matched against published grid-frequency archives, an ENF trace timestamps a recording to the second, independently of the recorder clock — a cross-check for schedule.clock_offset.

As a corpus descriptor

enf_summary's median_rise_db and coverage say how electrified a room sounds: a machine-room measured +45 dB of 50 Hz line, a quiet hotel room essentially none. Compare across sessions before interpreting any low-frequency tonal finding — the hum is in nearly every indoor recording.