How to Fix Audio Hum

Open the file in Audacity (convert lossy sources to [WAV](/convert/mp3-to-wav) first), select a section of pure 'silence' that contains only the hum, and run Analyze > Plot Spectrum (FFT size 8192). A sharp spike at exactly 50 Hz means European/Asian mains; 60 Hz means North American mains. Additional spikes at 100/150/200 Hz (or 120/180/240 Hz) are the harmonics — these tell you about the source character. Pure fundamental with weak harmonics is induction pickup. Strong odd harmonics (150, 250 Hz) point to half-wave-rectified switching supplies. Strong even harmonics (120, 240 Hz) suggest full-wave rectifier noise from a transformer or wall-wart. Note the exact frequency to 0.1 Hz precision; the notch filter you apply later must match.

With the recording chain set up, monitor the hum live and remove components one at a time. Unplug the microphone cable from the interface — does the hum stop? If yes, the mic, the cable, or the cable run is the source. If no, the hum is downstream. Disconnect the laptop charger and run on battery — hum that disappears with the charger removed is a switching-power-supply ground loop, the most common case. Move the laptop and interface to a different mains outlet — hum that disappears means a building wiring issue (different circuits at different ground potentials). This 60-second triage points to one of three categories: ground loop, induced, or USB-bus, each with a distinct fix.

A ground loop forms when two pieces of equipment connected by an audio cable are also both grounded to mains earth via different paths — the loop becomes an antenna for the 50/60 Hz field around mains wiring. Symptoms: hum appears when the audio cable is plugged in and disappears when unplugged; lifting either device's earth (do not actually do this — it is unsafe) silences the hum. Fixes that are safe: plug all audio gear into a single power strip on a single circuit so both ends share the same ground reference. Use balanced XLR or TRS cables between every link in the chain — balanced lines reject common-mode hum by 60-80 dB. For unavoidable unbalanced links (consumer RCA, guitar amp aux), insert a Hum-X (Ebtech), an audio isolation transformer (Jensen, Lundahl), or a DI box with a ground lift switch. Never bypass mains earth via a cheater plug — risk of electric shock.

USB audio interfaces draw power from the host computer. If the host has a noisy switching supply (every laptop), or shares a USB bus with other power-hungry devices (external SSDs, phones charging), the 5V rail picks up ripple at 50/60 Hz that couples into the analogue front end. Symptoms: hum scales with computer load, varies between battery and mains-charging operation, or changes pitch when CPU activity changes. Fixes: use a powered USB hub with a high-quality linear power supply (iFi iPower, Audioquest Jitterbug). Move the interface to a USB port on a different controller — Macs separate USB-A and USB-C ports onto independent buses. For chronic cases, switch to a Thunderbolt or self-powered audio interface (RME UCX II, Apogee Symphony). Verify with the disconnect test: unplug all other USB peripherals and re-record a noise sample; if hum drops, USB power is the path.

Unshielded or poorly shielded cables next to mains conductors pick up the 50/60 Hz magnetic field directly into the signal. Symptoms: hum appears or worsens when the cable is moved near a power strip, transformer, monitor, or wall-wart. Fixes: route audio cables at least 30 cm away from any mains cable. Where audio and power must cross, cross them at 90 degrees rather than running them parallel. Use star-quad cables (Mogami W2534, Canare L-4E6S) for high-induction environments — the four-conductor twist provides 30 dB more rejection than standard two-conductor mic cable. CRT monitors, fluorescent ballasts, dimmer switches, and unshielded toroidal transformers radiate strongly; replace fluorescents with LED on a non-dimmable switch and move dimmable lighting to a separate circuit. Single-coil guitar pickups are unshielded by design — humbucking pickups, RFI-shielded cavities, or distance from the source cure them.

When hum is already in the recording and the source cannot be re-captured, notch filters at the fundamental and harmonics remove it. iZotope RX De-hum is the gold standard — it identifies the fundamental and tracks every harmonic up to 20 kHz automatically; one click does the job. Free path in Audacity: Effect > Notch Filter at 60 Hz, Q = 10; repeat at 120, 180, 240, 300 Hz. Q = 10 keeps each notch ~6 Hz wide, narrow enough to avoid colouring music. For deeper hum, raise Q to 30 (narrower) and increase repetitions. Adobe Audition: Effects > Noise Reduction / Restoration > DeHummer with the corresponding profile. Compromise: notching at 60 Hz dips the lowest octave of bass and kick drum, so apply the minimum cuts that brings hum below the noise floor. Save before/after takes — the 'cure' can be worse than the disease for music with significant low-end content.

Six rules eliminate hum at source. (1) Single power strip: every piece of audio gear plugs into the same outlet, on the same circuit, sharing a single earth reference. (2) Balanced cables everywhere — XLR and TRS between interface, monitors, and outboard. (3) Distance: keep audio cables 30 cm from power cables; cross at 90 degrees. (4) Shielded mains cables for the laptop charger; replace bargain-bin wall-warts with linear supplies for any DC-powered audio gear. (5) Acoustic and electrical isolation from CRTs, fluorescents, dimmers, and routers. (6) Earth-loop break with a Hum-X or DI box at any unbalanced link you cannot eliminate. With these in place, the recorded noise floor of a quiet room mic is typically -60 dBFS or below, well within ACX/broadcast spec — see [what-is-audio-noise-floor](/guide/what-is-audio-noise-floor).