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What Causes the Noise in the Sound System?

The noise could be coming from a fire alarm relay wire inside the same conduit as the microphones. The noise could also be coming from several other sources of interference.

What Causes the Noise in the Sound System?


Greetings Douglas,

I am the supervisor of a music venue, and I've been trying to hunt down noise in our system. I discovered that the wires running to the input of a relay controlled by the fire alarm panel (the relay shuts off the audio system when the alarm sounds) were ran through the same conduit as my 34 channels of microphone cabling from the stage. This is against best practice in audio. Do you know of any reason why this would be done? Do you see any issue with relocating the input wires?

Thank you, J

You are correct in thinking that including any wire in the same conduit as microphone wire is against sound system best practices. I've worked in sound reinforcement, sound production, TV, radio, and in fire alarm systems. They are all different from each other, especially fire alarm systems.
  • Fire alarm system installers and even fire alarm technicians generally are very good with fire alarm systems, but not so good with sound reinforcement systems.
  • Sound reinforcement technicians generally are very good with sound reinforcement systems, but not so good with fire alarm systems.

But before attempting to relocate any wires, however, you need to think about troubleshooting to find out where the noise is coming from.

Is the noise coming from:
  • The fire alarm relay wire?
  • Ground loop noise injected into the microphone cables?
  • Ground current carried on the conduit and inductively induced into the microphone cables?
  • Other sources of noise?
  • Some combination of two or three sources of interference?


I have seen all of these possibilities, and trying to fix the noise problem is very frustrating until you know the source of the noise.

Fire Alarm Relay Wire

A correctly installed fire alarm shutdown relay will be powered-on during normal circumstances. This is so that if there's a problem with the wiring, you will know about the problem and get it fixed right away. The fire alarm system should have been installed according to National Fire Protection Association (NFPA) rules, so at all times, there should be current running through the relay input wire.

To test if it is the source of the noise, you have to disconnect both wires for the relay from the fire alarm panel so the fire alarm system isn't injecting any signal into the microphone cables. Make sure there is no power from the fire alarm control panel going through the fire alarm wire inside the conduit.

I know you are perfectly capable of disconnecting a wire, but when working on anything connected to the fire alarm system, just to protect yourself from liability, you need to get a fire alarm service company in there to help you. When the service company is there, you may as well have them move the wire anyway. Even if moving the wire doesn't solve the noise problems, moving the wire out of the conduit now may prevent problems later.

Yes, the noise could easily be coming from the fire alarm relay wire, but be prepared to discover that the noise is coming from somewhere else.

Another thing to watch out for is that if the relay is properly installed and powered-on, disconnecting it will shut down the sound system. You might have to TEMPORARILY bypass the relay to keep the sound system working while testing the relay wire.

Troubleshooting

To really troubleshoot, you have to start with a silent system. That means unplugging every input and output from the mixer, and listening to the mixer with headphones only. That includes all processing equipment like sound effects, equalizers, speaker amplifiers, even recorders.

Then, while listening to the headphones, plug one cable back into the mixer - one plug at a time.

If you hear noise, don't assume anything. Leave the plug with the noise plugged in and unplug everything else again. If the noise is still there, follow the wires and check to see where the noise could be coming from.

Troubleshooting isn't easy, and troubleshooting often leads to where you don't expect. Just keep at it.

Ground Loop Noise Injected into the Microphone Cables

With any noise, you might be able to hear the noise in one place, but because all the inputs and outputs are electrically related to each other the noise can come from another place.

Most sound equipment isn't very well protected from ground loop interference. Even with balanced inputs that have a plus and minus wire, AC current riding on the shield for the wires can be amplified.

Unbalanced inputs, like for auxiliary inputs are much worse. These have a single center conductor for the signal, and a shield for the signal return. If there's any noise current riding on the shield, that becomes amplified noise.

Watch out for any contact between shields and ground, especially on the stage. Anything hard wired should be suspect. Metal outlet plates, even though they're needed, could be your source of noise.

Ground Current Carried on the Conduit

I don't know how long the conduit is, but if it's less than 10 feet (3 meters) the conduit itself probably isn't an issue.

Conduit is usually hidden. Conduit, though, is an electrical conductor. I have measured the ground on two outlets in an auditorium to be 0.7 volts RMS different from each other. This makes for electrical current in the conduit which can be magnetically coupled between the conduit and the microphone cables.

Other Sources of Noise

Watch out for other sources of noise. Even though they seem innocuous, the noise could be injected into the system from unbalanced inputs or outputs on the processing equipment.

Open Mind

Just keep an open mind when looking for the noise, it's probably not coming from where you expect, and it might be coming from several sources at once. If it is coming from multiple sources, each one, by itself might not be noticed. When added together, though, the total from the multiple sources might be the issue.

Douglas Krantz
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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