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[Kyle Malenfant]

interesting theories presented in everyone's replies..

So clearly there's no way to pinpoint what was going on without actually looking at the wiring/specs of the system.  I agree though that it seems overall phase cancellation was at play rather than comb filtering of specific frequencies, especially since the problem was happening between every set of speakers. 

I suppose it's possible that the installer mistakenly swapped the + and - terminals accidentally on many of the speakers, though I'm shocked that after the system was completed no one walked the space and said "hey that sounds funny, let me see if I messed up somewhere"..!

[John Rutirasiri]

Kyle, I've had hot and neutral swapped on some circuits in my house.  Some light switches were switching the neutral instead of hot.  It would not surprise me to find some install speakers with +/- reversed.  In many cases, testing is done is once the install is complete: go under each speaker and "Yeah I hear it."

Cheers,
JR

[eric lenasbunt]

This whole thread tells me two things:
1) order another beer
2) enjoy some wings

[duane massey]

I'm with Ivan on this. I would suspect irregular coverage to be the culprit, as I've heard this very issue at a few installs here in town. Of course the average person will never notice, so no one says anything.

[John Roberts {JR}]

interesting theories presented in everyone's replies..

So clearly there's no way to pinpoint what was going on without actually looking at the wiring/specs of the system.

There are test sets that generate an asymmetrical test signal, then listen to the return to identify a reversed polarity.

The Galaxy Cricket (probably named that because of the sound it makes), and sundry other cheaper versions.
http://www.galaxyaudio.com/CRICKET.php

I agree though that it seems overall phase cancellation was at play rather than comb filtering of specific frequencies, especially since the problem was happening between every set of speakers. 

I suppose it's possible that the installer mistakenly swapped the + and - terminals accidentally on many of the speakers, though I'm shocked that after the system was completed no one walked the space and said "hey that sounds funny, let me see if I messed up somewhere"..!

The install market preferred floating output transformers over grounded auto-formers for 70-100V constant voltage systems because they were more tolerant of wiring faults (like accidentally grounding one leg). Installation wiring is not a precision exercise.

JR

[Tim McCulloch]

There are test sets that generate an asymmetrical test signal, then listen to the return to identify a reversed polarity.

The Galaxy Cricket (probably named that because of the sound it makes), and sundry other cheaper versions.
http://www.galaxyaudio.com/CRICKET.phpThe install market preferred floating output transformers over grounded auto-formers for 70-100V constant voltage systems because they were more tolerant of wiring faults (like accidentally grounding one leg). Installation wiring is not a precision exercise.

JR

We were beta-testers for the first and second generation of Galaxy Cricket testers and still have 2 sets in the shop.  The early versions were prone to misidentifying polarity when using too much receiver unit gain or when receiver positioning was inconsistent.  Galaxy largely addressed those issues.

An interesting bit of fun - we pointed the receiver at a 4ft square piece of plywood and hit it from behind, the unit indicated positive polarity.  When we hit the plywood from the front, the unit indicates negative polarity.  Brock J. has something interesting going on inside that box...

[Mike Caldwell]

I suppose it's possible that the installer mistakenly swapped the + and - terminals accidentally on many of the speakers, though I'm shocked that after the system was completed no one walked the space and said "hey that sounds funny, let me see if I messed up somewhere"..!

That's making a BIG assumption the installer actually knows what to listen for other than yep sounds are comping out of the speaker were all done here type of system test and tune.

I had some service work at a new school a couple years ago were the people at the school knew something was not right in their cafeteria system. The original installer had been back numerous times to check that system and gave it the ole everything is in spec seal of approval.
I go into the room make some noises into a microphone along with a couple check one two's and with in 30 seconds find half the ceiling speakers in the room are not even making any sound at all.
Turns out the speakers that were used have an off position in the tap selector located under the grill, they had never been turned on since day one.

No surprise that same school also had other issues with sound systems in other areas of the school.

I have also taken care of issues that same installer has left behind at other schools.

Yes there still in business and bidding on publicly funded jobs!!!!
 

[John Roberts {JR}]

Yes there still in business and bidding on publicly funded jobs!!!!

Unfortunately when people are not spending their own money they are less concerned about getting value for that money spent.

JR

[Ivan Beaver]

Unfortunately when people are not spending their own money they are less concerned about getting value for that money spent.

JR

I have run into this quite often, and to many people, as long as a system makes some kind of sound (you don't need to be able to understand it), then that is considered "working".

No wonder the bar is so low for our industry

[Mike Caldwell]

My above example was from installs where the electrical contractor formed an audio video install division with in their company so they can get the entire bid and not sub out the AV portion to reliable company...hopefully. 
There are many that do this.

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