Boom catching light buzz ground problem

L

loudlael

Active member
Hello
I'm doing a project with a SD 442, 744, and Scheops cmit. We were shooting in a room with florescent lights "gaffed" along the top of the walls on about six places. There was only about two places that the boom op could put the mic to avoid a buzz that sounded like it was a grounding problem. I assume its electromagnetic current because of the lights not being proproly grounded? They were running off house power with three to two prong plug adapters. The noise was not audible with the naked ear just through the shotgun. I also was running wireless in the room but with no problem.

I've had this problem on one other shot where the only solution was to tape the wires to the walls, so as you can see I tried a lot of things even, wrapping the mic with tin foil.

Any help would be greatly appreciated. Thanks!
 
If the boom is catching a light buzz try switching to O'Douls.

Okay seriously... are you sure the mic isn't HEARING the buzz of the fluorescents?

If that is NOT the case... and it's actually interference instead of sound... then a 416 or NTG-3 should work in that environment.
 
It's not a grounding problem, fluorescent lights buzz and trow off a LOT of RF. You might have actually been picking up the acustic buzz of the lights but it also might be the RF. They use a high voltage in the tube to make it light up and it's really hard to avoid picking that up. There are I think, ones that are suppressed but as a rule fluorescent lights are bad news sound wise. If your picking up the RF it's probably in the mic and the cable is not going to make that much difference. I doubly suspect this because you said there were places the boom op could get a clean sound. The power to the fluorescents is just 120 (or 240 depending on your location) but the voltage in the fixture is quite high and it's switching back and forth at the power line frequency so you have a big transmitter.

The problem where taping the cables on the wall helped was probably not the same issue, though related. That one sounds like you had your cables running next to power cables. That is an audio 101 no-no. You can often "get away with it" but it's a bad idea that also will bite you in the tush. The cable problem becomes worse with high voltage and or a lot of amperage. So running your mic cable next to the 120 power feet to a fluorescent is probably not going to be a problem (they are low power users) but running it close to the tube/ fixture will be a problem because you have a high voltage.
 
Remember that fluorescent lamps OPERATE by sustaining an RF plasma ARC through the length of the tube. So BY DESIGN, they are a significant source of RF garbage. Whenever you get something as sensitive as a microphone that close to an active RF plasma, the only surprise is that the noise isn't a lot worse.
 
Thanks for the info about florescent lights guys. Just curious though how a 416 would be better? And so I take it there isn't any solution but to turn off the lights?

It seems besides plains trains and automobiles, lights are my next biggest challenge, or maybe its my new boom op every couple of days:violin:
 
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loudlael said:
Thanks for the info about florescent lights guys. Just curious though how a 416 would be better? And so I take it there isn't any solution but to turn off the lights?

It seems besides plains trains and automobiles, lights are my next biggest challenge, or maybe its my volunteer new boom op every couple of days:violin:
Click to expand...

If it's sound coming from the lights, a 416 or other mic wouldn't help. But if it's electrical interference, mics such as the 416 or Rode NTG-3 that work by modulating an RF signal in the capsule rather than a straight capacitor are more immune.
 
Since you said flos are gaffed (rigged)... I assume these may be flos that a friend is setting up? Like shop lights wired as individual flos? Kind of like a DIY project? (because regular flos such as Kino wouldn't be causing this problem)... if that's the case then you may also want to inform that guy that he should check to be sure the flos are using ELECTRONIC BALLASTS and not magnetic ballasts (shop lights or regular flos from Lowe's, etc. will probably use magnetic ballasts. You can change those out with high-frequency electronic ballasts for less then $20 per light.

Also... crazy as it sounds... try another XLR cable... if the one you're using has a break in the ground shield (especially at the mic end) then that could be making the problem worse.
 
And what are the odds of a shoot with a volunteer boom op using Kino Flo's? Well actually better than they should be but my bet is still on shop lights, and probably very cheap ones. Good thing nobody cares about sound but the sound guy, well till post anyway.

Man I'm getting cynical.

But more to the point. As a FYI it's not just Fluorescent lights. You can some nice hums and buzzes near power lines and with the right mic out at Chrissy Field (park in SF that used to part of a military base) there are some under ground cables? something (maybe part of the in earth transmitters the navy uses to send radio signals to submarines?) that get picked up like a geiger counter. I have also picked up with the same mic (Senn. Stereo) radar sweeps in Point Richmond (line of sight to a naval base at the time. Kind of cool if your not trying to record geese at the time and getting big vvwwhhhooomp vvwwhhhooomp vvwwhhhooomp on top of it.
 
So I talked to the "best Boy" today and all the flos are kinos. I couldn't seem to get a straight answer if the ballast were electric or mechanical, but they said they were newer. Or maybe I miss understood. I also did use different cable. the first day in the room I was about 30' away in another room, and the second day I was right by the entrance with a shorter cable. Maybe I'm missing something. The boom op for one of the days said that it could have been the house. So maybe Noise2 your onto it. I'll have to try another mic if we shoot there again.

thanks again for the info.
 
loudlael said:
So I talked to the "best Boy" today and all the flos are kinos. I couldn't seem to get a straight answer if the ballast were electric or mechanical, but they said they were newer.
Click to expand...

The the 'cheap' ballast would be using an inductor at the line frequency. The 'high frequency' ballast would be operating at a much higher frequency, and would 'smooth' out any flicker of the light production process of the florescent tube.

With the low frequency 'cheap' solution, and the switching of 'lots of current', there are a number of ways that the light could generate 'noise'. Some of it could be audible, because actual mechanical movement of the materials due to the 60 (or 50) Hz power frequency. In any situation where 'current is switched' there is the opportunity to generate RF 'noise' that becomes 'modulated' at the power frequency.

So, 'noise' can be picked up via actual 'sound', buzzing inductors, or via RF pickup...

I can still hear the 20K Hz flyback on CRT screens, as well as a number of other frequencies that are generated by 'old fashioned' serial communication lines, that are opperating under 15K and the circumstances for their generating audible 'noise' is 'good'...

Don't know if moving the 'ballast' of the light to 'away from the mic' would be possible.
 
I'd love to hear 10 seconds of audio having the problem... with everything you've said I'll be surprised if the problem is what you think it is.

Flo buzzing from 30' away? Not even if they WERE magnetic ballast would you have that problem... (no kino will have a mag ballast, so no worries there regardless).
 
Sorry I was 30' away the very excellent super amazing boom op was in the room. I'll post an audio clip. Where is a good site?
 
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