weird buzzing sound

[Abraddesigns]

Hey CB
I'm designing lights for a show, and every time I take the instruments below 70% intensity the house monitors make this buzzing sound. It sounds like interference with the cables somewhere, at least to my semi-trained ear, so I went today and separated a rats nest of XLR cables, to tape the ones running the DMX signal apart from those carrying signal from the mics to the sound board. The problem still exists. The last show I did in this venue didn't have this problem. I ran Godspell for two weeks in this space with no problems of that nature. Then another local community theatre came into the space to put on a show and BOOM this problem starts. The only way I've been able to get the buzzing sound out is to set the EQ so there's no feed to the monitors on stage, which is bad...obviously...any suggestions? I know a bit about sound, but I've not had this problem. I figure it has to have something to do with the way cables are ran, but I'm not really sure what to do that I haven't already tried. Also, the theatre manager said this problem started when the other theatre came into the space, the show I'm designing now opens in two days, and this is driving me crazy.

 

[derekleffew]

(I wrote this a few years ago for a different purpose, but I'm not sure it ever got posted here.)

Noise in Audio Systems allegedly Caused by Lighting

The problem of annoying hums and buzzes, particularly of the 60Hz variety, is very common
in PA Systems and often immediately attributed to the Lighting System. While this is one
possibility, more often than not, I’ve found that it’s not really Lighting’s fault. We’re not here to assign
blame and point fingers, merely to fix the problem.
Somehow, errant RFI/EMF signals are getting into the audio system and being amplified. Audio’s
first line of defense against this is a good electrical earth ground on each and every piece of
equipment: mixer, amplifier, reverb unit, especially including the backline gear:
keyboards, guitar/bass amps, etc. Many times I’ve had a system sounding perfect until the
band arrives and plugs in!

Step One. To check that a good ground is present and all outlets/electrical devices are wired
properly, one cannot do without one of these:

(picture from Amprobe PY-3 110V AC Receptacle Tester)
An outlet tester, available at any hardware store, even in the electrical department at Wal-Mart
and similar, or (my personal favorite and likely the most frugal) Harbor Freight Tools for $2.99.
Follow the instructions on the device. If you don’t get two yellow (or sometimes two green, depending on the specific model) lights lit, you’ve got a
problem. Consult a licensed electrician. Use this device every single place there’s an Edison (parallel-blade, U-ground; NEMA 5-15)
outlet. An adjunct to Murphy’s Law says the one you forget to test WILL be the culprit.
Once you’re confident that all outlets are wired properly with all earth grounds intact, if
the problem persists, it’s time to move to Step Two.

This is a tough one, but I promise it will prove rewarding. Test each and every one of the cables used in the entire sound system,
especially the personal ones of the bass, guitar, and keyboard players. Remember, “One bad apple can spoil the
whole bunch of girls,” as Donny Osmond reminded us many moons ago.

This device,

(picture from Galaxy Audio Cable Tester | GuitarCenter )
a cable tester, available at Guitar Center and other fine establishments, costs around $60. I
actually prefer this one, the Whirlwind TESTER (catchy name, huh?),

(picture from TESTER - Catalog - Whirlwind )
But it costs more and does less, and doesn’t seem to be listed on Guitar Center’s website. If
you’re really strapped for cash, neither of the two testers above do anything a Digital Multi-
Meter (DMM), set on Continuity or more specifically, Resistance (ohms Ω) can’t do, but the
multi-meter requires a greater knowledge of electricity and can be dangerous in the wrong
hands. ELECTRICITY KILLS. WHEN IN DOUBT, CONSULT A QUALIFIED
ELECTRICIAN.
Again, once you have a tester, test every cable in the building. While watching the indicator
lights be sure to twist, wiggle and abuse the wires where they enter the connector’s strain-relief.
If you don’t, the performers and musicians certainly will.

Let’s stop here for now. Go back and complete Steps One and Two. Let me know when
you’re done, and we'll proceed with Steps Three through Nine-Hundred Sixty-Seven.

For further reading, I highly recommend a paper by Middle Atlantic, available for download in PDF form at Middle Atlantic Products - White Paper - Integrating Electronic Equipment and Power into Rack Enclosures .

Perform the above before re-running or re-routing any cabling.

 

[museav]

I have had noise in audio and video systems that was caused by the lighting and dimming, especially when the grounds or neutrals were not properly addressed. e.g. shared neutrals. But since the problem you are addressing was not there until the last show then unless that group modified or damaged something related to the power, dimmming or audio, the problem seems unlikely to be related to any of the installed systems. Is there any way to contact that group to assess whether they may have rewired or modified anything?

It would be useful to know what portions of the audio and lighting systems are permanently installed versus what you are bringing in or are temporary. You mentioned a "rat's nest" of cables and DMX, is that referencing the installed systems or cabling run just for your show? Is there some way to minimize the added or temporary components in order to try to assess if the problem is with the installed systems or not?

Related to that, do you represent the facility, a user group or both? If you are a user then is this something the theatre venue should be addressing? I know that many venues would be rather unhappy with a user's LD messing around with the installed audio systems, especially if they did so without any coordination with those responsible for the venue.

A ground loop or 60Hz noise issues should be fairly tonal and equalizing out the related frequencies should be possible. The result may not sound good and is far from a long term solution, but it should not require removing all of the signal.

 

[Abraddesigns]

Under "normal" circumstances I wouldn't touch anything that i wasn't directly involved with. And by that I mean, this venue didn't have someone running the sound for the last show I did there, so I took for granted it would be the same for this show (which I will say feedback is notorious, especially when people are switching off on mics) The 'rats nest" in the booth was pre instlalled...in fact I haven't ad to run any cables other than patching, and the cables were installed for that, I just had to switch cables to get lights set up on channels and dimmers. The only cable I messed with when taping was the DMX cable running from the board. I also asked the director if she wanted me to try and fix it. This particular venue is kind of out in the middle of nowhere, and no one there seems to understand tech at all. My steady job, non freelance, is sound/lighting tech for the local college. but the equipment I work with there has spoiled me it seems. I have a Digico D1 live sound board, and a nice ETC board in the light booth that I work with at my normal job. The system at this venue is kind of spread out. The sound board is back stage, and the light board is at the back of the house. I guess it's just weird to me that I ran a show there last month and didn't have this problem. Another tech crew came in, and rearranged quite a few things which is when this problem arose. I'm not trying to get electrocuted...I just want to be able to run the show without that buzzing sound. I'm not sure if it's because not many people in this venue think/know much about tech that they don't seem to want to try and fix it, but it's driving me insane, and if I were to go see a performance that would take me out of the world of the play completely, it's distracting. I've talked to the theatre manager about it, and he didn't know what to do about it either, while at the same time after I brought it up to him he told me he didn't know, then just went to his office and didn't come out for over an hour. Is it something I should keep working on, or should I just make a note for someone in the future? the show opens tomorrow, and I'm afraid if we can't get it out of the system today this buzzing sound will be there for the run of the show.

 

[FMEng]

Can you ask the person that made the last alterations to come back? They should have a clue where to start because they would know what they touched.

There are a bunch of possible causes, too many to go through here. But, the two most common are a ground loop in an audio circuit, or a un-balanced line in the chain between console and power amp. A long, un-balanced audio line is nothing but an antenna, and your dimmers make great transmitters.

My best guess is that someone changed a balanced cable with an un-balanced one. The easiest way for that to happen is to substitute a TRS (tip-ring-sleeve) for a TS (tip-sleeve) patch cord. What exactly do you turn down to stop the buzzing?

 

[Abraddesigns]

I figured it out today...I asked the theatre manager if, since there were no sound techs involved with this production if he minded if I traced cables. Almost instantly I found the problem. Two of the cables were cables someone had "made" and poorly at that I will add. They tried to make an XLR to 1/4", and in the middle of two cables was a golf ball size section of electric tape. I disconnected the mics and it took care of the buzzing. Since the mics were condensers(sp?) and they have the them all over the stage the director and myself found mic placements that picked everything up, and now I'm free to run my lights at whatever intensity I want. Thank you all for the help, it's always amazing to me the amount of knowledge contained in this one site.

 

[Abraddesigns]

What exactly do you turn down to stop the buzzing?

I turned down the monitor feed to the stage, and the overall volume volume for each input. Now that I figured out where it was coming from, it made sense that it went away when I turned down those things. And it wasn't that it went away completely, you could just hear the music over it, as well as most of the people on stage...I started today by going down the line and taking out cables until I could turn the system on, and it wouldn't buzz. Then I traced the cable from the board and found the problem real fast.