Hum — getting rid of the buzz

That low, steady hummmm under so many recordings has a single, boring villain: the electrical mains. Wall power doesn’t sit still — it alternates back and forth 50 times a second in most of the world, 60 times a second in the Americas. Any nearby cable, cheap power supply, or poorly grounded microphone can leak a little of that alternation into the audio as a pure, relentless tone.

Why it’s actually the easy one

Go back to the spectrogram. Hum is the simplest possible picture: a single razor-thin horizontal line, sitting at exactly 50 (or 60) cycles per second, present from start to finish. It doesn’t move. It doesn’t overlap much with the important parts of a voice. That makes it a sitting duck.

The tool for a single unwanted pitch is a notch filter — think of it as a very narrow pair of scissors that snips out one exact frequency and leaves everything on either side untouched. Tell it “remove 60 cycles per second” and it carves a thin notch there, killing the hum while the voice just above and below sails through.

The harmonics catch

There’s one wrinkle that trips up beginners. Mains hum is rarely just the base tone. The same electrical leakage usually brings along faint copies at exact multiples: 120, 180, 240… (for a 60-cycle hum), or 100, 150, 200… (for 50). These copies are called harmonics, and they’re why hum often sounds more like a “buzz” than a pure tone — your ear hears the whole stack.

So a hum remover doesn’t place one notch; it places a comb of them — one at the base frequency and one at each harmonic up the spectrum.

  loudness
     │  voice and music live in the gaps — untouched
     │   ▁▂▃▅▇█▇▅▃▂▁    ▁▂▃▅▇▇▅▃▂▁     ▁▂▃▅▅▃▂▁
     └──┬─────┬─────┬─────┬─────┬─────┬──────►  pitch
       60    120   180   240   300   360  Hz
        V     V     V     V     V          ← one narrow "notch" snipped at the
       hum  +harmonics (exact multiples)      hum and each of its harmonics

In cathar you say dehum --freq 60 --harmonics 5, and it snips 60, 120, 180, 240, and 300 cycles. If a hum still buzzes after you remove the base tone, you simply haven’t notched enough of its harmonics.

The 50 vs 60 gotcha: if --freq 60 doesn’t help, try --freq 50. A recording made in Europe, most of Asia, Africa, or Australia will hum at 50; the Americas and parts of Japan at 60. Guessing wrong does nothing, because the notch lands between the hum’s actual lines.

How the big tools do it

Once again the concept is universal — narrow notches at the fundamental and its harmonics — and the tools differ mostly in convenience:

• Audacity has a “Notch Filter” effect (you place them by hand) and the free “Hum Removal” and Nyquist plug-ins that automate the comb.
• Adobe Audition’s “DeHummer” gives you a tidy panel: pick 50 or 60, choose how many harmonics, done — exactly cathar’s two controls with a nicer face.
• iZotope RX’s “De-hum” adds a smart twist: real mains hum drifts a tiny bit as the power grid fluctuates, and the line isn’t perfectly stable. RX can track that drift and follow the hum, and can learn the exact harmonic fingerprint of a particular buzz.

When the hum won’t sit still — adaptive tracking

Fixed notches assume the hum line is nailed to exactly 50 or 60 Hz forever. In practice the mains frequency wanders a little, and cheap gear lets each harmonic breathe in amplitude too. A comb of static notches can leave a thin residue — a ghost buzz sitting between the notches you carved.

Cathar’s dehum --adaptive takes the tracking idea seriously. Real tape hum rarely sits on exact 50/60 Hz multiples — the lines land a few hertz off, and a 50 Hz European tape is not helped by a 60 Hz notch. Adaptive mode reads 50 vs 60 from the recording, finds each harmonic at the frequency it actually sits, and skips a line that isn’t there, so a file without hum is left alone. For each kept harmonic it demodulates the tone (shifts it to DC), low-passes to learn its slow amplitude wiggle (the tracker widens up the series, because transport wow scales with harmonic number), caps the envelope so a sung note parked on a mains line is not taken with it, and subtracts. Use it when --freq 60 gets you most of the way but a stubborn buzz remains, or when you don’t want to guess the region:

cathar dehum buzzy.wav --freq 0 --harmonics 6 --adaptive --out cleaner.wav

(--freq 0 means “detect it.” Adaptive mode will also switch 50 ↔ 60 on its own if you left the default --freq 60 on a 50 Hz tape.)

Steady, textbook hum? Plain dehum is faster and enough. Wandering, “alive” buzz, or a tape whose region you don’t know? Try --adaptive.

This is the rare corner of audio where the cheap and free tools are genuinely close to the expensive ones on steady hum, because the problem is so well-defined. Tracking drift is where the pro tools still earn their keep — and where cathar’s adaptive mode closes much of the gap.