Sound system noise from lighting system

Question

I'm doing a lot of work in a relatively old theatre this year. The space is part of a church that probably hasn't put any money into maintenance and upgrades for the theatre (except for light bulbs, which we sometimes have to beg for anyway) in over a decade.

Part of the space's oldness is antiquated wiring. I'm told by veterans of the space that electrical noise will come through the power system and get into the sound system. I've heard it, and sometimes it's impossible to tell where it's coming from. Supposedly, a hair dryer plugged in at the other end of the building can cause interference in the theatre.

The biggest problem, though, seems to be the lighting system. When I turn the sound system on before a show (following dimmer check, but before any lights are brought up), there's no noticeable buzz in the speakers. As soon as I hit the pre-show light cue, though, a buzz shows up.

When the lights cross-fade, the buzz modulates. I haven't been able to figure out what to do about the problem—not that I have much sound experience, but the guy who's been doing it for years just chalks it up to the space.

His normal setup includes a large umbilical cable running from the mixing board in the booth to the speakers and amps on- and backstage. That cable runs along a wall about ten feet up; on the other side of the wall are the two Strand CD80 dimmer racks that house the theatre's 48 dimmers. There are also usually power and signal cables for the amps and speakers running across the floor near the racks.

I've managed to mitigate most of the buzz in the house by turning down the speakers and boosting the amplification (so the overall output is reduced, but the signal is stronger compared to the noise). But there's still a noticeable buzz during silences, and it comes through quite loudly over the monitors.

Are there any mixing techniques that can further reduce or eliminate (ha!) the audible component of the interference, or do I need to recommend an overhaul of the equipment inventory and standard setup procedures?

Answer 1

There are two possible causes (that I can think about) that would introduce lighting-related noise. Determining the cause is difficult - once you know why, you can determine the steps to take to eliminate it.

The first possible cause is RF (usually from dimmers) being picked up by some of the wiring. If you suspect this is the cause, you can determine where it's coming from by a process of elimination. I'd start at the amps and work my way back. Is the noise there if the inputs to the amps are unplugged? Are there crossovers, etc?

If you can determine the source of the noise, the next thing to eliminate would be noise introduced via the power. This won't work for amps, or anything that draws a lot of current, but if the noise is coming from the board, crossovers, EQs or similar low-current electronics, try running them off a good quality UPS - something that generates the power off the batteries would be ideal.

You might get lucky, and narrow it down to a single source, but my guess is that its systematic - you can do lots of things to reduce it slightly, but short of rewiring the building you're going to have to live with it.

Answer 2

I think trying to mix around this will be an exercise in frustration - your best bet is to try to fix the problem at the source. The dimmers (and/or associated cabling) are the most obvious likely source of the buzz. Even if the snake you're running has balanced lines, I'd want to move it away from them. And the other cables as well - if there are unbalanced line level cables going through there they can pick up buzz.

Ideally, I'd want all the audio cabling on the other side of the theater from the lighting stuff. But if that's not possible, take a look at where some of the audio cabling is lying in relation to the dimmer packs & lighting cables. If you can move them apart even by a foot or two, or arrange things so they are not running in parallel, you may be able to reduce the buzz significantly. You may have to experiment a bit to see which lines are causing the buzz - it may end up being more than one. With some luck, you may be able to solve the issue for now without rewiring. But I'd still lobby to reroute all the audio cables.

Answer 3

As you mentioned, boosting the signal is essential. That's really sound engineering rule one, the signal should always be as high as possible without distorting in all parts of the chain.

What's worked for me to minimize hum and buzz, in order of usefulness:

1. Move signal cables away from power cables and power equipment (especially fluorescent lamps).
2. Every signal that can go through a balanced cable should go through a balanced cable.
3. Try to see if you can find which signal cables, if any, are the noisiest, and stop using them.
4. Make sure the cable shielding is properly connected to ground.
5. If you have a device that introduce buzz/hum and it uses a "wall wart", make sure that's a good quality power adapter. Many cheaper adapters that output for example 9VDC will not filter away the 50/60Hz signal properly. I had this problem with a Boss compressor, but noticed that it didn't appear when I used the original Boss power adapter.
6. Flip the power cables for all sound equipment around 180 degrees, one device at a time, so the "left" pin goes in the "right" hole, so to speak. Listen if it makes a difference. (This doesn't work in all countries).
7. See if you can find exactly which devices that introduce most noise. Try putting power conditioners / hum filters on these devices.

Answer 4

The fix to this is to isolate the audio system's AC from the lighting system's AC. This can be done professionally and with some expense with a large isolation transformer, but on a budget or on the fly, try to get the audio and lighting systems on separate circuits and if possible, on separate legs. Also, I've been in situations where the lighting controller was located too close to the mixing console and it caused interference.