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Author Topic: 'Hanging' Mics and EMI - A first....!  (Read 2061 times)

John Ward

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'Hanging' Mics and EMI - A first....!
« on: February 24, 2007, 12:07:07 pm »

I was going to post this as a reply to the 'hanging mics' but thought it might deserve a new topic for discussion, so 'hang' with me through the history and description.

This past week I installed a recording system in a tax appraisal office. I had designed and installed a similar (but considerably larger and more complex system) in another district's office a few years ago and expanded it last year. The heart of the systems components consist of Shure mics, a digital processor, an audio/USB converter and propritary software. The first system also included some portable carts that contained Presonus mic pres/ comps that were 'free standing' but could be tied into the main room(s) system. The first install incorporated a Crown USM 810.

For this install due to budgetary constraints Shure M200 mics (5), AA Digitool MX and a Tapco USB/Audio interface was used. The long and the short of it was that I installed and tested the system last week, all was good (except for some latency issues with the proprietary software - it worked well with several other recording softwares - but that needs to be addressed by the programmers). The Digitool handled all routing and processing (filters, gates, etc.) and having never used the Tapco unit before, I was pleased with the quality of the components and circuitry for this application. The audio was crystal clear. Spent about an hour or so going over everything with the office IT guy, and everything was hunky-dory - - till Wednesday.

I get a call saying that the system had a horrendous buzz, and it could not be isolated by the IT guy. (What can you expect from an IT guy, anyhow?). Of course my first question was had he tried to get into the processor (he chose the password to lock it, and I knew he was going to download the Digitool's software - the front panel is sort of awkward for first time users). You can imagine what I may have been thinking at this point.

Cut to the chase - get on sight and indeed there is so much interference that audio is almost totally inaudible. Go thru the whole thing, quickly figure that it is originating in four of the mics and not in the Digitool or Tapco unit. Knowing that Shure recently  had a design change in the M200 series, I start with the mic's preamps as a possible source - and they appear not to be the problem. At this point I'm a little bewildered, as first, seemingly nothing electrical had changed in the building since the install and everything to this point could be eliminated as a source of the interference except the mics themselves. I called Shure's tech support and they were very helpful, offering technical info on the mics and opinions as to what may be the source of the issue. We discussed all the variables (there was a ton of florescents, data lines, etc. in the plenum but I was extremely careful to stay completely away from that in routing cable). This was not my first rodeo, or so I thought...
The conclusion was that there was indeed a possiblilty that the mics themselves could be bad. It was explained to me that some recent changes in the design could be the problem, and that there were a number of different component 'designs' floating around within that model. At any rate the Shure tech was very helpful and short of being on sight with me did all he could to help with the issue.

I then went ahead and pulled the mics from their positions within the ceiling tiles, letting them just hang to the floor, and proceeded to test them individually again to eliminate the possibility that the interference was due to proximity of the mics to anything electrical or data. Sure enough, the four problem mics still had the buzz to the point that the audio signal was inaudible.

I went ahead and pulled the mics completely, and not really believing four mics could be bad, tested them by going directly into the Tapco converter's preamps and into the laptop. The signal was perfect! My question then was with everything else eliminated - and I mean every other component / line in the system was eliminated as the source, why would these things work totally by themselves and not while anywhere near the ceiling? Again, keep in mind everything worked well the day of the install and everything was extremely clean. I even made sure the a/c, video projector/system in the room got turned on (it was very cold the day of the install), etc. to eliminate those possible potential problems.

O.K. - Now I have one working mic in the ceiling, four that work quite well when not in the ceiling. So I begin the process of reinstalling mics one at a time, constantly monitoring the signal as I go along. I'm within a literally a half inch of the original point of installation with the first mic - still very clean. I get it in it's original spot, and whammo - the buzz is outragoeus! Then the lightbulb in my pin head turns on....The mic is placed next to the metal ceiling tile grid - actually the gooseneck is in slight contact with it. Therein lies the source of the noise. The cure was a slight change in position and a wrap of the gooseneck with a little electrical tape. Same with the other three mics. The one mic that never experienced the noise issue was installed with the supplied rubber boot, but from an aesthetic standpint it didn't look so good, and I had never had cause to use them before, so I forego them in the remainder of the installation.  Oh - the 'booted' mic was not near the grid work.

In conclusion - seeing as how everything ws fine day of original install, there must have been some electrical component in the building that was not turned on or whatever at that time. Obviously, the painted metal ceiling tile grid was the culprit, and the Shure's metal gooseneck liked the grid a whole lot. They worked very well together. I could lay the mic cable itself on the grid - in fact, just for grins even wrapped the cable around some data lines, layed it over a florescent fixture, and wrapped it around some electric tails leading to a fixture and there was absolutely no interference, no noise - nada. Just the gooseneck was the problem.

My question is - has anyone ever experienced this particular problem before - with Shure or any other brand, and was I just being a total dufus? I've probably installed a few hundred of these mics, many in the same fashion (next to grid - either for positioning purposes or aesthetics as they are less obtrusive next to the grid) could have changed so radically in a few day's period that would have changed the grounding scheme of the grid?
The only thing I learned that could be different from the week before was that a remodeling crew had been in to do some work in a restroom adjacent to the hallway and next to the room I was working in. I think they may have installed some automatic soap dispensors......

Feel free to chime in and let  me know if you think there was something very basic I overloked. I'm not afraid to be called an idiot if deserved. We all live and learn, and I certainly learned on this one. Cost me a half a day and $10.00 in gas, not to mention a somewhat hacked off client! Fortunatly he is very happy at this point and understands the issues.
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Don Boone

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Re: 'Hanging' Mics and EMI - A first....!
« Reply #1 on: February 24, 2007, 01:22:14 pm »

That's a new one for me. I put a lot of mics in ceilings but they are usually Crown PZM-11LLs that output line level so maybe that kept the problem from popping up on my installs.

The remodeling makes me suspicious. When ever something suddenly stops working or is really noisy I always ask if any work has been done recently. You might want to carefully meter the ceiling grid for AC. A remodeler may have stripped a wire that lets a Hot or Neutral put noise or even AC on the grid

Don
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Jason Lavoie

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Re: 'Hanging' Mics and EMI - A first....!
« Reply #2 on: February 25, 2007, 10:24:59 am »

I've had similar things happen to me.
I do a lot of hockey arenas, and in a few cases I've had digital noise coming into my system simply because the metal box for my mic jack was touching the metal box for the scoreclock (which is powered from a different panel waaay on the other end of the building of course)

as for the ceiling grid, I don't think they are required to be grounded so it is quite possible that anything from a malfunctioning balast to those soap dispensers could cause problems.
and even if the grid is grounded, the level of current required to send hum into your system is way below that required to trip a breaker. I agree with the other poster that checking from the grid to ground with a meter would be a good idea (the IT guy should have a meter, so maybe he can do that for his own curiosity)

also, if the grid wasn't grounded, except by accident somewhere, or by touching grounded light boxes then it is possible that something like temperature could have made a connection better or worse.

Jason
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