A church located in a residential area of a large city installed a new audio system to project the pastor’s voice on Sunday morning. The system has a short-range radio frequency connection between the pulpit microphone and the amplifiers. The sound company installed new speakers and wiring, and the system worked perfectly when tested. But on the first Sunday, the sound system started to squeal, squawk, crackle, and make other untoward noises at random.
Church officials called the sound company, their engineers, the manufacturers -- the whole nine yards. They tested and tested, and the equipment worked fine, except on Sunday mornings.
The church called an independent engineer. He trusted the tests on the system. He figured the source of the noise was external to the sound system.
The system was used primarily on Sunday mornings. Whatever the external source of the interference, it was powerful enough to generate some potent radio frequency disturbance. That eliminated the residential buildings near the church. There was a three-story factory about a block away, but that couldn't be the source of the interference. The factory wasn't open on Sunday mornings.
The next Sunday, the engineer visited the factory, just in case it might be the source of the problem. As expected, it was closed, except for a security guard. "Not us," the security guard told the engineer, so the engineer left. The guard got into the factory’s elevator and returned to the third floor to continue his rounds. The engineer went back to the church, and there it was -- the same crackling and squawking.
The engineer started to seek solutions. One possibility was encasing the entire audio system in metal -- building a huge Faraday cage around the pulpit. That certainly wasn’t an ideal solution, and the thought of the factory still nagged at the engineer.
Later that Sunday, the engineer and the pastor paid another visit to the factory. They asked the security people if they could avoid using the elevator on a Sunday morning, just as a test. Bingo -- no squawking. As it turned out, whenever the guard took the elevator, the electrical motor sparked at startup.
Since the church didn't often use the audio system at other times, it was a fine fix. Eventually, the factory insulated the elevator motor and ended the problem.
This entry was submitted by Tim Perper and edited by Rob Spiegel.
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I guess you could call this a case of elevated noise. I love it when the Sherlock Ohms mysteries end up being some oddball element outside the immediate area of the problem.
Our church had a noise problem on some Sunday mornings years ago, before wireless mikes were used. It was a random, distorted squawking sound, in short bursts with long pauses between. Usually it was weak and ignorable, but now and then it was loud. Some of us started to recognize it as distorted speech. Turns out that one member's husband often drove her to church, but sat in the car in the parking lot during services. Then he got a CB radio, with a (illegal) power amplifier, to pass the time. Depending on how close to the sanctuary he parked, his calls got into the amplifier system! Just asking got him to stop (or move a block away), but we had to add lots of RF chokes and bypasses to make sure that other stray RF stayed out of the system.
When I was in Technical Support in Michgan Bell, we got a series of Sunday morning crosstalk complaints from a neighborhood surrounding a particular church. Turned out the services were broadcast over a local radio station, they had leased a dry pair from Bell to get the audio. Investigation showed they had connected the pair to the 70 volt line terminals of the church's PA system. When the Rev got to the Turn or Burn part, everybody on the phone within a half mile got the message.
This story shows that you can not give up when troubleshooting. Once you have eliminated all of the causes, check again because you may have missed something.
This is quite interesting, moreso when you considr that the "short wireless link" was probably FM. A correctly functioning link would have the receiver in a full quieting mode, meaning that there was enough signal to saturate the limiter stages, which is what allows FM to provide the much desired noise elimination. The signal from the sparking elevator drive motor should not have been strong enough to get past the limiter stages, leading to my guess that the radio signal amplitude was not nearly as high as it should have been. So perhaps there was a failure there some place.
One more question, which is, why didn't they check the wireless link first thing? LInks are almost always a source of noise in an otherwise silent system,
On the other side, what kind of company has an elevator that is only used by a security guard on Sunday? That is a bit puzzeling, I think. Must business elevators get used more during the weekdays.
This sounds way too oversimplifed. I work in the area of pro audio and have helped thousands of customers find all sorts of problems. What's hard to believe is that, because wireless mics operate at VHF frequencies where wavelengths are only a few feet, just insulating a motor (presumably from surrounding metal work) would have any effect at all. Generally, to reduce emissions caused by arcing (brush motor in this case), some filtering has to be applied as close to the brush connections as possible ... sometimes the filtering is as simple as snapping some ferrite choke cores around the radiating wires. I'd like to know more about the solution.
I have a hard time understanding how emissions from the motor got picked up by the wireless system from that distance. I was thinking more along the lines of power line noise.
Me too, Noswad. At VHF frequencies, the interference (which originates with the sparking of the motor's commutator) will be severely attenuated as it propagates through the premises wiring. So it's very unlikely that it's conducted from source to victim. Instead, the VHF noise will be radiated from the first foot or two of wiring connected to the brushes ... and this is where clamp-on ferrite chokes are very useful at spoiling the efficiency of the inadverdent antenna.
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