Remove Electrical Hum from Audio Recordings

Understanding electrical hum: 50Hz vs 60Hz

Electrical hum in audio recordings comes from the AC mains power supply. In North America, Japan (eastern), and a few other regions, mains power runs at 60Hz. In Europe, the UK, Australia, Asia, and most of the world, it runs at 50Hz. When this electromagnetic energy couples into audio equipment, it appears as a tonal hum at the mains frequency — 60Hz in a US recording, 50Hz in a UK recording.

The hum doesn't stop at the fundamental frequency. Harmonics appear at integer multiples: a 60Hz hum produces additional tones at 120Hz, 180Hz, 240Hz, 300Hz, and beyond. A 50Hz hum produces harmonics at 100Hz, 150Hz, 200Hz, 250Hz. These harmonics are often louder than the fundamental, especially the second harmonic (120Hz or 100Hz). Effective hum removal must address the fundamental and all audible harmonics — removing just the base frequency leaves the higher harmonics clearly audible.

Common causes of electrical hum in recordings

• ●Ground loops — the most common cause. When audio equipment is connected to different power outlets on different electrical circuits, a voltage difference between grounds creates a current loop that induces hum. The classic scenario: a laptop on one outlet and an audio interface on another.
• ●Unbalanced cables — cables with only two conductors (TS/RCA) pick up electromagnetic interference from nearby power cables, monitors, and electrical equipment. Balanced cables (TRS/XLR) with three conductors cancel this interference by design.
• ●Electromagnetic interference (EMI) — recording near transformers, power supplies, fluorescent lights, dimmer switches, or monitors introduces hum. Laptop charger transformers are a frequent source — recording while charging with the charger near the microphone or audio interface.
• ●Poor shielding in cheap equipment — budget microphones and audio interfaces with inadequate electromagnetic shielding pick up ambient electrical noise from the recording environment.
• ●USB power noise — some USB audio interfaces pick up electrical noise from the computer's USB bus, which carries both data and power on the same cable.

Identifying hum with spectral analysis

Before removing hum, confirm its frequency. In Audacity, select a section of audio with audible hum and go to Analyze > Plot Spectrum. You'll see sharp spikes at the hum frequency and its harmonics. If the spikes are at 60, 120, 180, 240Hz — that's 60Hz mains hum. If at 50, 100, 150, 200Hz — that's 50Hz mains hum. Knowing the exact frequency matters because notch filters need precise targeting.

Sometimes hum isn't exactly at 50 or 60Hz. Power supply frequencies can drift slightly, and some interference sources produce hum at other frequencies entirely. USB noise might appear at different frequencies depending on the device. Always verify with spectral analysis rather than assuming the frequency.

Removal approaches: notch filtering vs AI

Notch filtering places very narrow EQ cuts at the hum frequency and each harmonic. A notch at 60Hz, 120Hz, 180Hz, 240Hz, and 300Hz removes five harmonics of 60Hz hum with minimal impact on the voice. In Audacity, use Effect > Notch Filter at each frequency, or use the built-in hum remover in iZotope RX. Notch filtering is precise and effective for pure tonal hum — it's the best approach when hum is the only problem.

AI noise removal handles hum as part of its general noise processing — it doesn't need you to identify the frequency or apply individual notch filters. Upload the recording, and the AI removes hum alongside any other background noise. This is faster and handles cases where hum coexists with broadband noise, room echo, and other problems. For recordings with multiple noise issues, AI is more efficient than applying separate fixes for each problem.

Fixing hum at the source

• ●Plug all audio equipment into the same power strip — this eliminates ground loops by ensuring all devices share a single ground reference.
• ●Use balanced cables (XLR or TRS) between microphones, interfaces, and monitors. Balanced connections reject electromagnetic interference that unbalanced cables pick up.
• ●Move the laptop charger away from the microphone and audio interface. Even 3-4 feet of distance significantly reduces electromagnetic coupling.
• ●Use a DI box (direct injection box) with ground lift when connecting instruments or unbalanced sources to a balanced input. The ground lift breaks the ground loop without compromising audio quality.
• ●Record on battery power when possible — disconnecting the laptop charger during recording eliminates the most common source of USB-coupled electrical noise.
• ●Turn off or move away from fluorescent lights and dimmer switches — these produce significant electromagnetic interference at harmonics of the mains frequency.