Fifty hertz is worth knowing about if you record in the UK. It is the rate at which the mains supply reverses direction, and it leaks into anything with a cable attached: guitar amps, USB interfaces, ageing tape machines, even an indifferent laptop charger. The result is a low, steady drone beneath the music, joined by a family of harmonics that reach up into the midrange. The obvious cure — a notch filter — works, right up to the point where it takes the weight out of the bass. A clumsy notch also leaves an audible hole and a ringing resonance around its edges. Here is how to clean up the hum and leave the low end intact.
Confirm it is mains hum before you reach for a filter
Open a spectrum analyser — most DAWs have one, and there are good free options — and loop three or four seconds of a quiet passage. A gap between takes is ideal, because the hum sits there on its own, without a vocal or a snare to mask it. You are looking for a narrow, stable spike at 50 Hz with smaller companions at 100, 150, 200 and 250 Hz. Do this on the raw file, not on a processed bus: saturation and amp simulators turn a polite fundamental into a rich, buzzy tone with energy smeared a long way up the spectrum, which is far harder to subtract cleanly.
Hum or buzz?
The two behave differently. Hum is tidy: a strong fundamental and a descending ladder of harmonics. Buzz, usually from an earth loop or a switched-mode power supply, spreads energy much further up the spectrum and often sounds harsher and more broadband. A recording made through North American equipment will show its peak at 60 Hz instead. And a hum that wanders between roughly 49.7 and 50.2 Hz is still mains — the grid is not perfectly rigid, and neither is your analyser's estimate.
It also helps to know whether the hum runs constantly or only between phrases. Hum underneath a whole take responds well to static notches. Hum that appears in the gaps is better handled dynamically, with a gate, an expander or a spectral repair tool, because a static filter would thin the bass on every note for the sake of a few silent bars.
Fix it at the source, or at least try
An hour in the DAW can undo a minute's work in the room, but it is always cheaper to stop the hum getting in. Before reaching for plugins, ask what changed between the quiet take and the noisy one. Nine times out of ten it is a cable, a power supply, or proximity.
- Move the source, not just the mic. Single-coil pickups, and laptops with unearthed chargers, act as magnetic aerials. Rotating a guitar thirty degrees or moving an interface a metre away from a wall-wart can drop hum by 10 dB or more.
- Break the loop, not the earth pin. If a device hums only when connected to another, use a balanced cable, a DI box with ground lift, or a transformer isolator. Never defeat the safety earth on a mains plug.
- Separate power and signal. Run audio cables away from mains leads, and avoid long parallel runs — crossing at right angles is far less inductive.
- Suspect dimmers and cheap PSUs. Domestic lighting dimmers and unbranded USB chargers inject noise into the whole ring main. Unplug them and listen again.
If the hum survives all of that — a vintage amp with a tired filter capacitor, a tape machine with a failing head shield, a location recording you cannot redo — then it is time for the DAW.
Set the notch narrow, and count your hertz
The mistake most people make is reaching for the default bell. On a 50 Hz band, a Q of 1 is roughly 50 Hz wide, so the filter is already cutting hard at 25 Hz and 75 Hz — exactly where kick drums and bass guitars do their work. Narrow it down: a Q of 10 gives you a 5 Hz notch, and a Q of 20 gives you 2.5 Hz. That is usually enough to swallow a stable fundamental while leaving everything an octave either side untouched.
Use the smallest cut that does the job, too. If the hum sits 18 dB above the noise floor but your music sits 30 dB above it, a 12 dB reduction will bury the hum under the mix. A 30 dB cut is rarely necessary and always riskier.
How many harmonics do you actually need to notch?
At 50, 100 and 150 Hz the harmonics are usually exposed and worth treating. By 200 Hz and above they are often buried under the arrangement, and notching them costs you guitar body and vocal warmth for no audible gain. Sweep a boost around the suspect frequency to hear what lives there before deciding. If toggling the 250 Hz notch in and out makes no difference to the hum, delete it.
Alternatives that protect the low end better than a static notch
A notch is a blunt instrument in one specific way: it works all the time, whether or not there is hum to remove. Several gentler tools exist.
Dynamic EQ and multiband compression
Set a dynamic bell at 50 Hz with a sidechain that responds only to level in that band. The filter stays flat while the bass plays, then clamps down in the gaps, which is where hum is usually most audible anyway. The same trick works with a multiband compressor on the 40–60 Hz band, or a downward expander with a slow release.
Spectral repair and noise-profile subtraction
Tools such as iZotope RX's De-hum, or the spectral repair brush in a spectrogram editor, learn the hum's exact fingerprint from a silent gap and subtract it. They handle drifting frequency far better than a fixed notch, because the model tracks the peak rather than assuming it stays put. Use them lightly: heavy settings produce a watery, phasey artefact that is more distracting than the original drone.
Phase cancellation from a hum-only reference
If you can record ten seconds of the same rig with the instrument silent and every gain setting identical, you have a template of the noise. Align it to the noisy take, invert its polarity and blend it in underneath. On perfectly stable hum this removes the fundamental without touching the music at all. On a drifting supply it will beat against the original and make things worse, so null-test the result in mono before trusting it.
A notch filter is a scalpel. Set it wide and you have performed surgery on the song, not the hum.
Verify the fix before you commit
Match your levels before you compare. A filtered version is usually fractionally quieter, and quieter almost always sounds worse, which makes honest evaluation difficult. Then listen three ways: in mono, on headphones, and on something small and unkind, like a phone speaker. Low-end problems hide in full-range monitors and reveal themselves on a commute.
Finally, look at the spectrogram rather than trusting your ears alone. A very high-Q notch can leave a visible ringing tail after every transient, and a filter placed a few hertz off centre will leave a thin sliver of the fundamental behind. If the spike is still sitting there at 46 Hz or 54 Hz, you have notched the wrong place.
A short order of operations
- Find a quiet gap and confirm the frequency with a spectrum analyser.
- Fix cable, grounding and placement problems at the source if the take can be redone.
- Apply narrow, shallow notches at 50, 100 and 150 Hz — no wider or deeper than necessary.
- Replace static notches with dynamic EQ where the hum only shows in the gaps.
- Use spectral repair for drifting or broadband buzz, and use it sparingly.
- Level-match, A/B in mono, and check the spectrogram before printing.
Done properly, de-humming should be invisible. The listener should hear the take, not the treatment — and the bass should still move the room.
Photo: Anna Pou / Pexels
