Two mics into the same recorder. The boom is clean, the lav on the subject has a steady low hum through the entire take, and it gets slightly worse when he turns his head. Same lav worked fine in a different room last month. I can notch it out but I would rather understand what causes it so I stop doing it.
Confirm it is actually hum before you chase the room. Look at the spectrum: mains hum is a hard line at 50 or 60 with harmonics stacked above it. If it is broad and low instead, you have a different problem — clothing rub, a failing connector, or the lav power supply itself.
A cracked cable near the connector also gives a noise that changes with movement. Wiggle test before you blame the room.
If you are stuck with the take as recorded, a proper hum removal tool handles this far better than a plain notch because it tracks the harmonic series. Still fix the source next time.
@salted_hash_h · 9mo ago · 3 replies
Almost certainly induced hum rather than a ground loop, and the head-turning detail is what tells you. A lav cable running near mains, a dimmer, a fluorescent ballast or a laptop charger acts as an antenna, and body position changes the coupling. Fixes in order: find and kill the offending device, because dimmers are by far the worst offender, reroute the cable away from mains and cross it at right angles if you must cross, and if your recorder is plugged into the wall, run it on batteries instead. Notching 50 or 60Hz also mangles the harmonics, so five minutes at the source is worth it.
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@salted_hash_h · 9mo ago
Do not, everyone learns this by ruining one interview. If you ever cannot avoid a dimmer, turning it fully to maximum often takes it out of the chopping range entirely.
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@reamp_rosa · 9mo ago
There was a dimmer switch on the wall directly behind him. Feeling stupid.
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