A pure 60 Hz tone points at a ground loop or an unbalanced connection; a strong 120 Hz and 180 Hz ladder points at a power supply; broadband buzz with the harmonics smeared points at a dimmer or a switch-mode supply.
Mains hum: the harmonic ladder tells you which stage is guilty
checked 2026-09-16responsible Room Tone Register (a named register, not a person)
The ladder
| Dominant rung | What it suggests | First thing to try |
|---|---|---|
| 60 Hz alone, clean | Ground loop, or an unbalanced run between two earthed devices | Lift the shield at one end only, or insert a transformer |
| 120 Hz strongest, even order | Full-wave rectification leaking into the audio rail | Move the supply away from the mic cable; test on battery |
| 60 + 120 + 180 evenly | Magnetic induction from a transformer or a mains cable | Re-route the cable; increase the distance |
| Harmonics smeared, broadband | Dimmer, fluorescent ballast, or switch-mode supply | Turn the suspect circuit off and listen |
Note the direction of the reasoning. People usually start by swapping cables. The ladder lets you start by naming the fault, which is faster and cheaper.
Why 50 Hz and 60 Hz change the filter
The fundamental differs, and so does the harmonic spacing. A high-pass at 80 Hz clears 50 Hz mains but sits dangerously close to a male voice fundamental; at 60 Hz the same filter has more room. This is why a filter preset copied from a European post house can thin an American dialogue track.
The distinction that matters
Hum and buzz are not the same fault. Hum is periodic and low-frequency — a ladder with clean rungs. Buzz is the same fundamental with a dense, irregular harmonic series, which almost always means a non-linear device is modulating the mains waveform. Fixing buzz with a ground lift does nothing, and fixing hum with a new power supply does nothing. Diagnose by looking at the shape of the ladder before you touch anything.
A magnetic hum field is not stopped by shielding foil. It is stopped by distance or by a mu-metal path. Foil works on electric fields, which is why wrapping a cable in foil sometimes appears to work and then stops working when the cable moves.
Near kin: buzz, hiss and common-mode interference
Hiss is broadband and has no fundamental; it belongs to self-noise or a gain stage, not to the mains. Buzz is the mains fundamental with a dense, irregular harmonic series, which means a non-linear device is modulating the waveform. Common-mode interference is the mechanism by which hum enters a balanced pair; it is not a separate symptom, and it is the reason a balanced connection helps at all.
| Term | What it is | How it differs from mains hum |
|---|---|---|
| Buzz | Mains fundamental with dense harmonics | Points at a non-linear load, not a loop |
| Hiss | Broadband noise with no fundamental | Belongs to self-noise or a gain stage |
| Common-mode | The coupling mechanism into a balanced pair | A path, not a symptom |
| Ground loop | A potential difference between two earths | One cause of hum, not the definition of it |
The common misread is calling every low-frequency noise 'hum' and reaching for a high-pass filter. A refrigerator's audible contribution sits mostly between 100 Hz and 500 Hz, where a mains-frequency filter does almost nothing, and the filter removes voice energy from the same band. Diagnose from the harmonic ladder before filtering anything.
Source log
- 01Sound System Interconnection (Rane Note 110)supports the identification of ground loops as the dominant cause of hum between interconnected devices
- 02Phantom power P48 and P12 (Schoeps)supports the balanced-connection and shielding practice used to reject induced hum
- 03Phantom Power: what it is and how it workssupports the behaviour of phantom-powered balanced inputs under common-mode interference
Each line points at one specific document. No line is a home page.
Leave this page Close this page if the noise is broadband hiss rather than a tone — that is a noise floor or self-noise question.