Hum Remover

How the Hum Remover works

Mains hum is the low, steady drone that leaks into recordings from the electricity supply. It comes from ground loops between a computer and an audio interface, unshielded guitar pickups, cheap phone chargers, dimmer switches, fluorescent lights and long unbalanced cables running beside power leads. Because it is locked to the frequency of the power grid, it is one of the few kinds of noise whose exact pitch is known in advance, and that makes it unusually easy to remove cleanly.

50 Hz or 60 Hz

The grid runs at 60 Hz in North America, most of South America, Taiwan, South Korea, the Philippines, Saudi Arabia and western Japan, and at 50 Hz across Europe, Africa, most of Asia, Australia, New Zealand and eastern Japan. The Detect setting measures how much energy the first 20 seconds of the file have at 50, 100 and 150 Hz and compares it with 60, 120 and 180 Hz, then picks the stronger family. Choose the frequency by hand if you know where the recording was made, or if the file is mostly music with strong bass notes that could confuse the detector.

Why harmonics matter

Pure hum at the fundamental is a deep tone, barely audible on laptop speakers. What makes hum annoying is its harmonics: multiples of the fundamental at 100, 150, 200 Hz and up for a 50 Hz grid, or 120, 180, 240 Hz and up at 60 Hz. Rectifiers in power supplies and the magnetic saturation of transformers create strong even and odd harmonics, which turn a hum into a buzz. Ground loops are often heaviest at the second and third harmonic. The default of six harmonics covers up to 300 Hz or 360 Hz, which deals with most hums; raise it to 10 or 12 for a harsh buzz from a lighting dimmer or a guitar amp.

How the hum remover's notch filters work

A notch filter cuts a very narrow band around one frequency and leaves everything else alone. Its sharpness is described by Q: the centre frequency divided by the width of the cut. At Q 30, the notch at 50 Hz is under 2 Hz wide, so a man's voice whose fundamental moves through 85 to 180 Hz passes almost untouched. Each harmonic's notch is given a Q that rises in proportion, so every notch keeps the same width in hertz rather than getting wider as the frequency rises. That keeps the total amount of audio removed tiny even with twelve notches.

Lower Q makes wider notches that catch hum that drifts slightly off frequency, which happens with generators, old tape transfers and some battery inverters. Higher Q is more surgical and is the right choice for music, where a wide notch at 100 or 200 Hz can take the body out of a bass line or a kick drum.

Tips

• Find the hum first. Run the file through the Spectrum Analyzer. Hum shows as a comb of sharp peaks at regular spacing, which tells you both the mains frequency and how many harmonics stand out.
• Remove hum before broadband noise. Clear the hum here, then use the Background Noise Reducer for any hiss that remains. A noise print taken from hummy audio wastes effort on frequencies a notch handles better.
• Fix it at the source next time. Ground-loop hum usually disappears when everything is powered from the same socket, a laptop runs on battery, or a balanced cable or ground-loop isolator replaces an unbalanced one.
• Watch the bass on music. Bass notes near A1 (55 Hz) and B1 (61.7 Hz) sit close to the fundamental. If a bass line loses a note, raise Q or reduce harmonics.

When it will not help

Buzz with a rough, changing tone from a damaged cable, crackle from a bad connector, and interference from mobile phones (the "dit-dit-dit" GSM buzz) are not locked to the mains frequency, so notches will not catch them. Hum from tape transfers where the playback speed was off will also have moved away from 50 or 60 Hz; lower Q to widen the notches.

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