Halve the chain at the first device that still hums when everything upstream of it is disconnected; then work down the surviving half. The ladder's strongest harmonic tells you which half to suspect first.
Find the source of a hum by bisecting the chain
checked 2026-09-16responsible Room Tone Register (a named register, not a person)
Before you touch anything
- Look at the spectrum. Note the strongest harmonic. 60 Hz clean means a loop; 120 Hz dominant means a supply; smeared harmonics mean a non-linear load. Write the rung down; you will use it in step four.
- Note whether the hum is present with all source devices off but the chain powered. If it is, the fault is downstream of the sources and you can skip half the chain immediately.
- Note whether the hum changes when you touch the equipment chassis. If it does, you have found a floating chassis and the fault is a missing or duplicated earth, not a cable.
The bisect order
| Step | Action | If the hum stops | If it continues |
|---|---|---|---|
| 1 | Disconnect everything from the recorder or interface; leave it powered | Fault is upstream | Fault is the recorder, its supply, or the room |
| 2 | Reconnect one device at a time, from the closest to the recorder | The last device added is implicated | Continue until all are reconnected |
| 3 | On the implicated link, lift the shield at the destination end only | You had a ground loop | Suspect magnetic induction |
| 4 | Move the cable 300 mm away from mains runs and transformers | You had induction | The cable itself or the connector is at fault |
| 5 | Substitute the cable | Fault was the cable | Fault is in a device |
| 6 | Power the suspect device from a different circuit | The circuit is at fault | The device is at fault |
The order matters. Lifting a shield is step three, not step one, because a lifted shield on a device that is genuinely faulty hides the fault and creates a new one. And moving the cable is a measurement, not a guess: 300 mm is chosen because mains magnetic fields fall off fast enough over that distance to show up as a change.
When to stop looking
Stop when the residual hum is below the room's noise floor. A hum you cannot hear under the quietest passage is not a problem to solve, and chasing it costs days. Write the residual down with the room's floor beside it; that pair is the evidence that the job is done.
A transformer-based isolator will break a loop and will not fix a faulty supply. If the ladder said 120 Hz dominant, an isolator is the wrong purchase and will disappoint.
Source log
- 01Sound System Interconnection, PDF (Rane Note 110)supports the interconnection and earthing topology used in the bisect order
- 02Plane Wave Expansion in a Duct, Penn State Acousticssupports the duct and enclosure coupling that explains why re-routing a cable changes a hum
- 03Superposition of Waves, Penn State Acousticssupports the summation of a hum with programme material at the point of measurement
Each line points at one specific document. No line is a home page.
Leave this page Close this page once the residual hum is under the room's floor. Below that point it is no longer a problem you can hear.