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Author Topic: Everything is hum. Or is it?  (Read 7314 times)

Mike Sokol

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Everything is hum. Or is it?
« on: February 26, 2014, 10:10:32 am »

I was reminded over the weekend that sound volunteers in churches often have trouble describing interfering noises in their sound systems. The first customer said they were hearing a lot of hum from their professional CD player. Taking a listen on headphones, I heard sort of a washy/gritty hiss that only occurred when in <play> mode. Certainly not a hum, but not something I could remember hearing before. A quick look at the CD player was all it took me to figure it out. They had used a Y cable to sum the stereo outputs of the CD player into a mono feed for the channel. That's a bad idea to begin with of course, but to make matters worse they had summed the Left audio channel and the digital S/PDIF output together. Of course I was now hearing the 44.1 clock mixing with the analog audio and doing horrible things. Their house engineer didn't think it was a big deal since 44.1 kHz is above hearing range. But I assured him that a mic pre did not like being fed with an S/PDIF signal. I changed out the Y cable for a Whirlwind podDI and all was well.

The second hum "issue" was from a church in Georgia where they complained about hum on the pastor's RF mic when it was turned off. Again, a quick listen on headphones showed me the channel sounded completely clean while the RF mic was turned on. But turning off the mic produced a bunch of popping and RF noises. Looking on the receiver I could see a lot of RF signal level even with the mic turned off which produced the clicks and RF scattering noise. Obviously it was hearing RF interference from an outside source, which was compounded by the fact they had the receivers squelch up at max threshold. I showed them how to find a new clear frequency and set the squelch, and advised them to watch for future interference since nobody there had any idea of other churches in the area with RF mics.

Of course, neither of these situations were ground-loop hum at all. I had packed my road kit with various XLR pin-1 lifts and Whirlwind ISO boxes just to be ready to break some ground loop hums. But you just can never really trust descriptions from volunteer board operators. And to make things crazier, musicians tend to call everything "feedback" even when it really IS hum, or slap-back echo, or buzz, or hiss. In my live-sound class at SU I make my students do exercises where they produce these sounds with their mouths as part of their quiz. So if you're in Winchester and hear humming noises in B-flat, it might be my students practicing their hums.   
« Last Edit: February 26, 2014, 10:34:50 am by Mike Sokol »
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John Roberts {JR}

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Re: Everything is hum. Or is it?
« Reply #1 on: February 26, 2014, 10:25:11 am »

I thought everything was called a "ground loop".  :-)

JR
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Mike Sokol

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Re: Everything is hum. Or is it?
« Reply #2 on: February 26, 2014, 10:39:28 am »

I thought everything was called a "ground loop".  :-)

JR

Perhaps.... but not all Ground-Loops hum, some of them crash. Here's one you really want to avoid.

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Stephen Swaffer

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Re: Everything is hum. Or is it?
« Reply #3 on: February 26, 2014, 06:46:37 pm »

durOver a decade of repairing industrial electronics taught me that often the most difficult part of troubleshooting is determining the true problem.  The lack of accurate observation and description is certainly not limited to church sound volunteers!  I lost count of the times I was called back to a machine a day, or a week or a month later for the "same" problem only to find the only similarity in the problems was that the machine was not doing what it should-but the symptoms were as different as those of a common cold and a broken leg.
 
It doesn't help that volunteers understanding is often skin deep.  Last year after a play practice at church someone came to me in a panic-the sound system has a major issue-had to shut it down to stop the racket!  They were certain we had a major issue.  I knew the pastor used a headset mic and since he was either acting or directing it was always hot-he muted it as desired.  When I left the board after practice it was still hot in case he needed to make an announcement-but then he shut the transmitter off instead of muting it.  I can't get them to understand the difference between "off" and "mute", but I also knew that that was the only input still hot and capable of creating an issue so finding the problem was simple.

In troubleshooting, I always ask myself first, "What is really the symptom here?", then "What has changed?"  Getting the right answer to both questions usually takes multiple questions to those "running the show"!
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Ivan Beaver

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Re: Everything is hum. Or is it?
« Reply #4 on: February 26, 2014, 08:02:52 pm »

I thought everything was called a "ground loop".  :-)

JR
I thought it was always the fault of "dirty power"-----------------
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Jonathan Johnson

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Re: Everything is hum. Or is it?
« Reply #5 on: February 27, 2014, 12:45:09 am »

Last year after a play practice at church someone came to me in a panic-the sound system has a major issue-had to shut it down to stop the racket!  ... I also knew that that was the only input still hot and capable of creating an issue so finding the problem was simple.

I have a maxim: "the bigger the problem appears, the easier the solution will be." Everything is down? Nothing works? The possible causes are relatively few and usually simple to fix.

It's the "slightly annoying" issues that people think they can live with -- those are the issues that are often the most difficult to troubleshoot. Especially intermittent problems that don't seem to occur when I'm there to observe.

Which brings up another maxim of mine: "the only thing more frustrating than something that doesn't work when it should, is something that does work when it shouldn't."
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Brad Weber

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Re: Everything is hum. Or is it?
« Reply #6 on: February 27, 2014, 11:14:10 am »

I thought it was always the fault of "dirty power"-----------------
When dealing with many mechanical noise issues I will often have them turn off or cut off power to the AHU or other offending equipment and when that stops the noise I then sometimes claim that it is an obviously a matter of their having 'noisy power'.  It's interesting to see how people respond.
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Steve M Smith

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Re: Everything is hum. Or is it?
« Reply #7 on: February 27, 2014, 11:34:25 am »

This noise discussion reminded me of a story my father told me.  He used to be a central heating system designer (not a plumber or installer).

A few weeks after a system was fitted in a woman's house, she called to complain about an occasional noise the system would make.

My father sent someone out several times to check the system but always, no fault was found.

Eventually, after another call, my father went out and sat in her kitchen drinking tea having given the instruction "as soon as it make the noise, tell me".

After about half an hour, she said "that's it".

My father's response was "madam, that's your fridge!"


Steve.
« Last Edit: February 27, 2014, 12:16:50 pm by Steve M Smith »
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John Roberts {JR}

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Re: Everything is hum. Or is it?
« Reply #8 on: February 27, 2014, 11:50:31 am »

This general ignorance about technology can cause us problems trying to negotiate consumer oriented service support.  A few years ago I had an intermittent DSL problem due to a faulty telephone line that was not so bad that it stopped the POTS from working, but would step on the DSL. I diagnosed this by putting an oscilloscope on the phone drop, so I knew the problem was not on my end.

I had to literally lie to the consumer support decision tree, to not get vectored to some phone bank operator in Bangalore telling me to reboot my modem.  >:(

After I lied to get to talk to a line service rep in my country, they threatened me with having to pay a penalty fee, if they sent out a line man and didn't find anything wrong.

After way too much effort, I got them to send out the truck and he quickly found and repaired the wire that a squirrel had chewed partially through up on his telephone pole.  ;D

arghhhh.

JR
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Greg_Cameron

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Re: Everything is hum. Or is it?
« Reply #9 on: February 27, 2014, 01:10:47 pm »

A few years ago, the Comcrap cable service that supplied our TV/phone/internet at home would go intermittent, mainly on the internet/phone service. Not during the rain, but a day after the rain was over and only at night when the outdoor temps dropped. Water was getting into a connection somewhere on a pole during the rain. But it wouldn't go intermittent until a partial dry out occurred and the connector's metal contracted in the cold of night. And it would last until the line would completely dry out, maybe a week or so. It would drive me bananas and I would constantly call Comcast and complain. They finally acknowledged other people on my street had reported a similar issue after I get the call escalated to the local office. By default, most service calls go to somewhere in the Philippines I believe. And the useless person on the other end with little technical expertise could never understand what I was talking about due to the very specific nature of the events that had to happen to make the connection get flakey. They sent a tech out once or twice, but only during the day when the problem wasn't present even after I explained the issue was only after sundown. Just useless customer service.

Finally, We were nailed with a big snow storm that downed many trees in our town, several on my street. Power, phone, and cable were out for days until they replaced all the lines and busted poles. Problem fixed.
« Last Edit: February 27, 2014, 06:32:50 pm by Greg Cameron »
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Re: Everything is hum. Or is it?
« Reply #9 on: February 27, 2014, 01:10:47 pm »


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