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Author Topic: Random Intermittent Noise in Sound System  (Read 3113 times)

John K. Harris

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Random Intermittent Noise in Sound System
« on: December 29, 2017, 11:08:07 pm »

Hey All! This is my first post on here because I have a interesting question that I am searching an answer for. I work as the resident AV Tech at The Salvation Army Kroc Center in Hawaii. On the side I have my own sound/lighting business that I do.

My issue is with the PA system at the Kroc Center. Sometimes there is a unique intermittent noise that sounds like something is rewinding/giggling chipmunky type of sound coming out from the mains. I have not heard of this type of sound before. The noise occurs randomly and is not duplicatable. It has happened with vendor techs that we have here and mainly on external client's shows when we rent out our Worship and Performing Arts Center. To my knowledge it has not occurred on Sunday Services for church. I have been working at the Kroc Center for two years working almost every Sunday (just missed one Sunday for vacation) and I have never heard the noise.

Sometimes the noise has happened when the vendor techs only have a couple channels on and the rest of the channels have no signal. It has happened to at least a few of the vendor techs. I would like to say that it normally happens when I am not A1 but that would imply human error. We have brought in the company that installed the audio system to troubleshoot but the noise doesn't ever happen when they are here, because it is not a duplicatable sound.

I have included a schematic of our system.
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Kevin Graf

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Re: Random Intermittent Noise in Sound System
« Reply #1 on: December 30, 2017, 09:00:18 am »

Find an amateur Ham radio operator, he will know another Ham that has the skill and equipment to search for a problem like this.
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Keith Broughton

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Re: Random Intermittent Noise in Sound System
« Reply #2 on: December 30, 2017, 09:30:24 am »

This is one of the tougher trouble shooting problems you can have.
Start by keeping a record of time of date, time and duration of the noise to see if there is some kind of pattern. From this, you may be able to determine a "likely" time to trouble shoot.
usually, the idea is to eliminate components from the audio signal chain to get down to what device(s) are getting the noise.
I would start with disconnecting the inputs to the amps and work back, connecting one device, or cable run, at a time.
Also, check around the area for some antennas that might indicate a radio transmitter.


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Mike Caldwell

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Re: Random Intermittent Noise in Sound System
« Reply #3 on: December 30, 2017, 09:36:47 am »

Find an amateur Ham radio operator, he will know another Ham that has the skill and equipment to search for a problem like this.

Maybe....maybe not, while maybe great in radio operation many will have no clue about the kind of system and equipment in John's system.


As for the noise you described, that is kind of how I would describe cell phone interference.
If that is the case check for open mic lines, bad mic cables, mic cables or wall plates with the shell tied to pin 1. Lot's of place for RF ingress in a system like you have, assuming it is an RF ingress problem.
What are your wireless mic frequencies.
« Last Edit: December 30, 2017, 09:40:17 am by Mike Caldwell »
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Scott Helmke

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Re: Random Intermittent Noise in Sound System
« Reply #4 on: December 30, 2017, 10:00:44 am »

Divide and conquer. Looks like there are several places to start looking - output of video switcher, output of main mixer, output of speaker processor. Likely you could Y-cable various outputs into some kind of recording device, like a cheap MP3 recorder that can run for a day or two. If you get a report of noise, you can go back and check the recording. Don't hear the noise, move the recorder further downstream.

If at all possible train the users to listen more critically. Not just "noise in the speakers" but "noise in the left side", etc.
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Ed Walters

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Re: Random Intermittent Noise in Sound System
« Reply #5 on: December 30, 2017, 10:53:39 am »


I have included a schematic of our system.

That wireless in the simple  system...what band? Is it off/muted when not in use?  Often, so-called simple systems are always live.  What you describe sounds like LTE. T-Mobile started activating 600 MHz in Hawaii last summer..... 

If that (or other) wireless are in the 600 bands, start by physically disconnecting that wireless and see if the problem stops. Disconnect it instead of turning it off so other helpful people don't turn it back on for you...

Ed Walters
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Simon Lewis

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Re: Random Intermittent Noise in Sound System
« Reply #6 on: December 30, 2017, 11:31:39 am »

Also, check for things that might be affecting mains power or generating RF noise, such as arcing thermostat contacts...
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Kevin Maxwell

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Re: Random Intermittent Noise in Sound System
« Reply #7 on: December 30, 2017, 11:36:17 am »

Hey All! This is my first post on here because I have a interesting question that I am searching an answer for. I work as the resident AV Tech at The Salvation Army Kroc Center in Hawaii. On the side I have my own sound/lighting business that I do.

My issue is with the PA system at the Kroc Center. Sometimes there is a unique intermittent noise that sounds like something is rewinding/giggling chipmunky type of sound coming out from the mains. I have not heard of this type of sound before. The noise occurs randomly and is not duplicatable. It has happened with vendor techs that we have here and mainly on external client's shows when we rent out our Worship and Performing Arts Center. To my knowledge it has not occurred on Sunday Services for church. I have been working at the Kroc Center for two years working almost every Sunday (just missed one Sunday for vacation) and I have never heard the noise.

Sometimes the noise has happened when the vendor techs only have a couple channels on and the rest of the channels have no signal. It has happened to at least a few of the vendor techs. I would like to say that it normally happens when I am not A1 but that would imply human error. We have brought in the company that installed the audio system to troubleshoot but the noise doesn't ever happen when they are here, because it is not a duplicatable sound.

I have included a schematic of our system.

https://www.youtube.com/watch?v=3OII3oeWruA
Listen at about 15 seconds in. Does it sound like that?
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Chris Eddison

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Re: Random Intermittent Noise in Sound System
« Reply #8 on: December 30, 2017, 11:59:05 am »

Do you have access to a simple recording device that has a long run time and a line input? Something like a Zoom handheld recorder. If so then start inserting it at various points throughout the signal chain and just leave it running. Note the time you hit record and then get the operators to record the time the noise occurs. With a decent size card and the lowest recording quality it'll do you should only be hitting record once a day. If it's not happened that day, delete the file and start again tomorrow.

This will do 2 things - it'll give you a recording of the issue if it's there, which might be useful for analysis. More importantly though, if the sound occurs and the recorder doesn't pick it up then you know it's getting in to a system component sometime after that point.
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Keith Broughton

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Re: Random Intermittent Noise in Sound System
« Reply #9 on: December 30, 2017, 12:27:37 pm »

Do you have access to a simple recording device that has a long run time and a line input? Something like a Zoom handheld recorder. If so then start inserting it at various points throughout the signal chain and just leave it running. Note the time you hit record and then get the operators to record the time the noise occurs. With a decent size card and the lowest recording quality it'll do you should only be hitting record once a day. If it's not happened that day, delete the file and start again tomorrow.

This will do 2 things - it'll give you a recording of the issue if it's there, which might be useful for analysis. More importantly though, if the sound occurs and the recorder doesn't pick it up then you know it's getting in to a system component sometime after that point.
Good concept!
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