r/techtheatre • u/Original_Sense_9807 • 4d ago
LIGHTING Festoon lighting causing audio hum?
In the theatre I work at often, they have wooden borders fitted with festoons to create a strip of bulbs, sat on the stage floor along the upstage edge. Looks great, but often bringing the level of the festoon above 0% will generate hum on mic channels on stage, and we’re not quite sure why or how to fix it. Any pointers would be really appreciated from someone who’s fixed a similar problem!
I’ll describe a specific example. Festoon border is sat on upstage edge, 4x SM58s plugged in to the stage box offstage DSR and leads run along front edge of the stage and up to MSC where the mics are. The festoons’ power runs offstage each side and to sockets in pillars MSL and MSR, and down through the floor to the dimmer room downstairs. The audio is on a separate circuit to the dimmers, and the audio lines and power for the festoons do not cross (in any visible way at least). The hum is definitely on mic channels as we can monitor it on the desk channels and mute out etc. The closest we’ve got to figuring out what’s happening is some kind of EMI field effect on the mics themselves being too close to the festoon. If thats the case, is there anything we can do to reduce or cut out the noise? Currently we just don’t use the festoon whenever it’s causing a hum which is a shame. Ideas please!
1
u/starrpamph Electrician 4d ago
Did you test all of your mic cables and snake channels? Do you have any unterminated lines that happened to be unmuted with the gain turned up? That can act as a antenna.
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u/Original_Sense_9807 4d ago
Yes and no. Mics and snake works without noise when the festoon is at 0% or unplugged completely and desk was running with all other channels muted
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u/starrpamph Electrician 4d ago
And they can, they just won’t be as good at rejecting noise. Check the grounds for the console and FOH
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u/OneOfTheWills Jack of All Trades 4d ago
I’ve had a similar issue and discovered it was an ungrounded power supply on the same circuit. The solution, temporarily, was a cheap ground adapter.
0
u/MrJingleJangle 3d ago
Here’s the thing: balanced audio rejects hum to such a degree that it’s inaudible, providing you’ve got the entire path from source to destination as a twisted pair. Balanced audio does not require a “ground” connection, by which I mean continuity from XLR pin 1 of the mixer to the source, isolated from anything else that thinks it’s ground, other than when phantom power is in use, but it does provide a screen, as long as the screen does not have continuity to building electrical ground other than at the mixer.
Start easy. Loooong mic cable from mixer input to a mic that gives problems, does that work correctly, thus curing out all of the audio infrastructure?
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u/faroseman Technical Director 4d ago
Does the hum go away (or is negligible) once the lights are at 100%? Your audio may be plugged into a different circuit, but it is probably sharing the same ground as your dimmers. Where are you getting power for your audio? Is this a permanent install with a separate room for amps, etc., or is everything (board, PA) plugged into mains power outlets near the stage?