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Author Topic: In-ceiling speaker comb filtering  (Read 5443 times)

Kyle Malenfant

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In-ceiling speaker comb filtering
« on: November 15, 2014, 12:43:32 am »

Hi All.  I was at a local Miller's Ale House tonight and noticed a pretty heavy amount of comb filtering between the ceiling speakers as I walked around the restaurant.  No logos on the speakers so not sure the brand/coverage pattern.  Ceiling height is 11' and the speakers were placed at 8' intervals.

I've done a few ceiling speaker installs using JBL products and following (more or less accurately) the layout as designed by their ceiling speaker software dictated and didn't get the interference I heard tonight.

As I understand it, we want the coverage to overlap slightly so the levels are consistent throughout the space. 

Question:  So why would the comb filtering happen in one case but not the other?  If the speakers were placed further apart so coverage is not overlapping, then the volume won't be smooth as one walks around the venue.

Can anyone shed some light on this?
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John Rutirasiri

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Re: In-ceiling speaker comb filtering
« Reply #1 on: November 15, 2014, 12:53:49 am »

Are you sure it was comb filtering and not phase reversal on one of the speakers?

Also, I have seen some 3-way install speakers that has the mid/hi assembly being adjustable.  I suppose one can adjust it too much that it affects the neighboring speaker.

Finally, could it be the speaker processor/zone controller programmed incorrectly?

JR
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Kyle Malenfant

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Re: In-ceiling speaker comb filtering
« Reply #2 on: November 15, 2014, 01:17:39 am »

Are you sure it was comb filtering and not phase reversal on one of the speakers?

Also, I have seen some 3-way install speakers that has the mid/hi assembly being adjustable.  I suppose one can adjust it too much that it affects the neighboring speaker.

Finally, could it be the speaker processor/zone controller programmed incorrectly?

JR

Could have been phase several but I heard the interference between every speaker (in the couple of areas I walked through).

The mid/high adjustments are a possibility..no way to know without knowing the model of speaker.

Same goes for the processing..no clue as I couldn't get any info by glancing at their AV rack by the bar.


I know there are many unknowns here which could affect the performance of the system..just hoping to get some insight.

Thanks for the comments.
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John Rutirasiri

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Re: In-ceiling speaker comb filtering
« Reply #3 on: November 15, 2014, 01:41:56 am »

I suppose a bad/different transformer on one speaker could cause what sounds like comb filtering.

This one I'm not sure...perhaps one of the install experts can chime in:  if a wrong tap was used on the transformer on just ONE of the speakers, would that only result in a different volume on that speaker, or would it have an impact on the freq/phase response that it sounds different than the rest of the speakers?
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Lee Douglas

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Re: In-ceiling speaker comb filtering
« Reply #4 on: November 15, 2014, 08:47:58 am »

If they were running stereo pairs (rather than 70V mono) and one of each pair were wired out of phase, because maybe the wiring convention was misunderstood, you would experience cancellation as you passed the mid-point of each pair.
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Ivan Beaver

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Re: In-ceiling speaker comb filtering
« Reply #5 on: November 15, 2014, 09:34:18 am »

It could be not so much combfiltering but a irregular coverage pattern.

Meaning that the highs are narrower than the mids which are narrower than the lows.

So what you hear is a lot of basic coverage issues.  The end result can sound the same-holes in the freq response.

With any speaker it is important to have the same pattern over as wide a range as possible.  If not, it is going to sound different at different places.

Back in my days of the install world-I have listened to many different ceiling speakers and heard VERY DIFFERENT coverage patterns-despite what the spec sheet simple numbers say.

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Craig Hauber

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Re: In-ceiling speaker comb filtering
« Reply #6 on: November 16, 2014, 12:58:56 am »

Out of polarity plain and simple. 
Ceiling rigs are such a mish-mash of interference, reflections and audio bouncing around everywhere (especially in modern "echo-chamber" style restaurant interior design) that if you heard obvious combing then it was a batch of out of polarity speakers.
It only takes a couple wrong ones to affect everything.

Many of these systems are put in by the electrical contractor using non-standard wiring (i.e. cat 5), and modern speakers use phoenix connectors with no color coding, it's amazing they get connected at all sometimes.

I've watched a master electrician wire "+" to the black wire and "-" to white -as they would normally for outlets and switching, yet an apprentice wiring a different string of speakers who is a car audio hobbiest wires it properly the opposite way!

Then a few weeks later the chain-wide AV vendor shows up with the rack and slams it into place, checks that it makes noise and leaves without ever knowing anything about polarity, let alone any EQ and limiting!

Not complaining too much as it keeps me gainfully employed "fixing" things!


And as far as I have ever experienced, different taps don't affect polarity, if there's any phase differences they are too subtle to hear -especially when you notice the volume difference first and are focused on that.



Hi All.  I was at a local Miller's Ale House tonight and noticed a pretty heavy amount of comb filtering between the ceiling speakers as I walked around the restaurant.  No logos on the speakers so not sure the brand/coverage pattern.  Ceiling height is 11' and the speakers were placed at 8' intervals.

I've done a few ceiling speaker installs using JBL products and following (more or less accurately) the layout as designed by their ceiling speaker software dictated and didn't get the interference I heard tonight.

As I understand it, we want the coverage to overlap slightly so the levels are consistent throughout the space. 

Question:  So why would the comb filtering happen in one case but not the other?  If the speakers were placed further apart so coverage is not overlapping, then the volume won't be smooth as one walks around the venue.

Can anyone shed some light on this?
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Ivan Beaver

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Re: In-ceiling speaker comb filtering
« Reply #7 on: November 16, 2014, 08:13:55 am »

Out of polarity plain and simple. 

And you know this without listening to the particular system? And just going on a simple description?

WOW-that's pretty good.

And they just happened to get enough of the right ones out of polarity to give the same impression around the room?  That would take some thought/planning.

You might be right-and you might be completely wrong.

There is no way to know without more investigation.

I would not be so quick to jump to conclusions-sorry.
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Craig Hauber

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Re: In-ceiling speaker comb filtering
« Reply #8 on: November 17, 2014, 01:17:58 am »

And you know this without listening to the particular system? And just going on a simple description?

WOW-that's pretty good.

And they just happened to get enough of the right ones out of polarity to give the same impression around the room? 

That would take some thought/planning.

You might be right-and you might be completely wrong.

There is no way to know without more investigation.

I would not be so quick to jump to conclusions-sorry.

Not trolling (even though I'm laid-up off work, it's 12-below outside and there's nothing else to do today.)
I've fixed these types of systems enough to know the odds are so high for a polarity issue that I don't bother with "maybe" and just state it definitely in hopes that something can just get fixed faster.

I also went with my answer because the only way to actually get such a clear and obvious example of combing out of the nasty audio-soup that is modern restaurant sound is to have a polarity difference.  Can't imagine any other failure that would stand out as that obvious to an experienced audio guy who is used to hearing ceiling rigs.

No by "polarity" I wasn't just thinking a single speaker, but I guess I wasn't clear beyond describing how entire strings of speakers in a system can be opposite ofothers just because of installer/electrician misconception, but there is also another common occurrence that could be at play also....
Do you realize how many people attempt to wire these systems in "stereo" (however misguided they may be it still happens way too often) Every other speaker L&R!
Gets even worse when there's series-parallel wiring involved instead of 70V.
-And what if the chopped 1/8" cable from the Muzak/DMX/Spotify/Pandora box was not corrected right to the phoenix connector on the back of the Zonepro? (not hard to do, even I've done it wrong in haste)
So if a channel is reversed before even entering the system?
Voila, instant consistent comb filtering over the entire venue!

Not trying to be a prick -even if that's how you interpreted my last post.  (I did disparage the whole modern restaurant sound concept so maybe I was asking for it.)

But I will still stand behind "blatant polarity issue" and Ivan, I'll buy all the rounds at whatever restaurant bar I'm installing in your neighborhood if I'm wrong (got some near Atlanta next spring, last one was Savannah)
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Tim McCulloch

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Re: In-ceiling speaker comb filtering
« Reply #9 on: November 18, 2014, 09:47:26 am »

(I did disparage the whole modern restaurant sound concept so maybe I was asking for it.)

"...modern restaurant sound concept...." deserves all the disparagement you can muster.  I absolutely hate having to shout and scream over the music and overall din of the noize.  I will take my dining dollars where my companions and I can converse without vocal strain.

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Kyle Malenfant

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Re: In-ceiling speaker comb filtering
« Reply #10 on: November 18, 2014, 10:56:53 am »

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"..!
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John Rutirasiri

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Re: In-ceiling speaker comb filtering
« Reply #11 on: November 18, 2014, 11:05:35 pm »

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
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eric lenasbunt

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Re: In-ceiling speaker comb filtering
« Reply #12 on: November 22, 2014, 03:56:42 pm »

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

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Re: In-ceiling speaker comb filtering
« Reply #13 on: November 29, 2014, 01:31:16 am »

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.
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Duane Massey
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John Roberts {JR}

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Re: In-ceiling speaker comb filtering
« Reply #14 on: November 29, 2014, 09:52:38 am »

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
Quote
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
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Tim McCulloch

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Re: In-ceiling speaker comb filtering
« Reply #15 on: December 15, 2014, 04:46:43 pm »

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...
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Mike Caldwell

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Re: In-ceiling speaker comb filtering
« Reply #16 on: December 23, 2014, 09:39:57 pm »

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}

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Re: In-ceiling speaker comb filtering
« Reply #17 on: December 23, 2014, 10:57:05 pm »


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
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Ivan Beaver

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Re: In-ceiling speaker comb filtering
« Reply #18 on: December 24, 2014, 09:39:59 am »

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 :(
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Mike Caldwell

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Re: In-ceiling speaker comb filtering
« Reply #19 on: December 24, 2014, 09:49:21 am »

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.

John Roberts {JR}

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Re: In-ceiling speaker comb filtering
« Reply #20 on: December 24, 2014, 10:49:19 am »



No wonder the bar is so low for our industry :(
What I was referring to is not so much an audio industry problem but whenever there is weak accountability for spending "other people's money". Fraud is rampant in areas of our economy where government spending is significant. Medicare billing is often in the news as they catch the most blatant high profile over-billing abuses, but for every one they catch, how many less obvious frauds get away with it? And these are mature programs with established anti-fraud divisions dealing with trusted professionals like doctors.

I'm sure 2015 will be better (/sarcasm)...  8)

JR

PS: FWIW we individuals are rarely competent to second guess professionals we hire to do specialized work for us, otherwise we would just do the work ourselves. The best mechanism to deter bad actors is sharing information with other customers in the market for their services. If we tolerate bad performance in silence we make it easier for them to do the same to others.
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Ivan Beaver

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Re: In-ceiling speaker comb filtering
« Reply #21 on: December 24, 2014, 02:04:20 pm »

What I was referring to is not so much an audio industry problem but whenever there is weak accountability for spending "other people's money". Fraud is rampant in areas of our economy where government spending is significant. Medicare billing is often in the news as they catch the most blatant high profile over-billing abuses, but for every one they catch, how many less obvious frauds get away with it? And these are mature programs with established anti-fraud divisions dealing with trusted professionals like doctors.

I'm sure 2015 will be better (/sarcasm)...  8)

JR

PS: FWIW we individuals are rarely competent to second guess professionals we hire to do specialized work for us, otherwise we would just do the work ourselves. The best mechanism to deter bad actors is sharing information with other customers in the market for their services. If we tolerate bad performance in silence we make it easier for them to do the same to others.
One of my projects this next year is to look at a room (9,000,000.00) budget that was designed by one of the most respected names in the industry, complete with physical variable acoustics.

It is a performing art space.

The "caretaker" of the space said it includes a line array that was installed sideways and has every other speaker tied together.

The space is only 500 seats- so a line array is the "perfect choice"-especially when flown sideways.

He said the "design firm" came into the room and listened and said it was fine.  He said (as far as he knows) they never measured anything or even got into the DSP to make any on site adjustments.

The reason I am being called in is because they are not happy with the system.

So just because somebody hires "the best in the business" and spends lots of money does not mean they will get the results they were looking for.

I have not been on site yet-so it should be an interesting "documentation" and adjustment experience.
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John Roberts {JR}

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Re: In-ceiling speaker comb filtering
« Reply #22 on: December 24, 2014, 03:45:25 pm »

One of my projects this next year is to look at a room (9,000,000.00) budget that was designed by one of the most respected names in the industry, complete with physical variable acoustics.

It is a performing art space.

The "caretaker" of the space said it includes a line array that was installed sideways and has every other speaker tied together.

The space is only 500 seats- so a line array is the "perfect choice"-especially when flown sideways.

He said the "design firm" came into the room and listened and said it was fine.  He said (as far as he knows) they never measured anything or even got into the DSP to make any on site adjustments.

The reason I am being called in is because they are not happy with the system.

So just because somebody hires "the best in the business" and spends lots of money does not mean they will get the results they were looking for.

I have not been on site yet-so it should be an interesting "documentation" and adjustment experience.
One would think that the architectural spec for a $9M install might include some performance metrics describing frequency response and SPL over some pre-defined coverage pattern. If the job doesn't meet spec no $9M payday.

JR

PS: Ivan you should be happy, you are getting paid to fix other people's mistakes. 
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Ivan Beaver

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Re: In-ceiling speaker comb filtering
« Reply #23 on: December 24, 2014, 03:59:22 pm »



PS: Ivan you should be happy, you are getting paid to fix other people's mistakes.
And there is A LOT of that type of work out there.  Good in one case and embarrassing for the industry.

It is AMAZING how many screwed up systems are out there.

And the really sad thing is that the person who put it in actually thinks that is it "fine" or doesn't realize there is a problem with the install.

Oh how low the bar is in so many cases.
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