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Author Topic: Yup - there's the problem.  (Read 5488 times)

Rich Stevens

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Yup - there's the problem.
« on: June 13, 2007, 11:57:17 pm »

I have volunteered to administer, maintain, and upgrade our church's sound system.  Just so you know my modest SR skills mostly concern portable systems (frat houses and bars) and are 20 years out of date.

So, I've been picking through the tangle of cables piled on and around the mixer.  The system is a typical Carvin package: 24x4 Mixer, 15-band stereo equalizer, one big power amp, one small power amp, and a pair of 832 15-inch 2-way 90x45 black fuzzy speakers.  I'm positive it was purchased by someone who thought it would solve all the church's sound problems, oblivious to the fact that sound systems need to be installed.

The system feeds back at the drop of a hat, despite the fact that everything that can be turned down has been, most notably the equalizer, where many sliders are all the way down, and none are above about -8dB.

It's not hard to see where problem numero uno lies, but I made a drawing anyway to illustrate (see attached JPG).

index.php/fa/149/0/

The speakers are on the roof of the sacristy, and yes, that's a brick wall they're aimed at, between them and the congregation.  No one in the congregation is in their coverage area, but they do an excellent job of firing sound onto the roof.  

Don't let that lower light blue line fool you - if you look at the plan view, the vocal mikes are within the stage left speaker's nominal distribution. It's no wonder the system howls before the power amps' green "Signal" LEDs even think about lighting up.

OK, so I've read enough on this board and elsewhere to know the right way to go about this: hire a consultant or design/install firm, have them model the room and select speakers. Then install the speakers, analyze and tune the room with a 31-band EQ or DSP, then lock it up and throw away the key.

I agree with all that, especially if we have the budget (which we probably don't).  But I can't help thinking: the system is SO bad that whatever I (a semi-informed amateur) do, it would be such a huge improvement that most people wouldn't miss the additional benefits that could be gained by going the "pro" route.  I can't help it, I'm an 80:20 kinda guy - but does that line of thinking seem to make sense in this (rather extreme) case?  

Once I got the drawing done, I moved the speakers around on paper to look at some possibilities.  Actually the 832's aren't all that bad if positioned properly (as long as we're looking at colored lines on a piece of paper. If we're listening to them, who knows?).  I could post the results for discussion if you want, but I'm not really considering flying the 832's.  (But trust me - if I did, I'd hire a rigger - I may be an 80:20 kinda guy, but I'm not stupid.)

I guess what I'm looking for is some suggestions on a path forward.  Keep in mind that I'm an engineer, just not an audio engineer.  Is it possible (likely?) that I could use my cheesy little CAD drawings to lay out a system, fly some new mid-priced installable-rather-than-portable cabinets up there in a center cluster, maybe add a sub, and get decent results?  We might even spring for a 31-band, though I'm not sure about getting some one to analyze the room and set it up.

BTW, the system is presently set-it-and-forget-it with the board up with the band.  The Pastor wants to move it to the back and build a booth, but right now we have no one to run it, so that would be sort of pointless.  However, this is another discussion of another day.

Critique away!  (and thanks... I'm amazed at the level of knowledge, professionalism, and willingness to help on this board).
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Rich Stevens

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Re: Yup - there's the problem.
« Reply #1 on: June 14, 2007, 12:01:16 am »

Here's the plan view.
index.php/fa/150/0/
Can anyone tell me why the system feeds back so easily? Very Happy

P.S. Sorry about the size of the drawings - I tried shrinking them, but the lines start to disappear.
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Josh Rose

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Re: Yup - there's the problem.
« Reply #2 on: June 14, 2007, 03:51:30 am »

Well, if I am looking at that diagram correctly, the speakers are behind the stage. You put those speakers in front, and you will drop 75% of your feedback problems immediately.

No ammount of EQing will dissolve your feedback problems. The mains simply cannot sit behind the stage and shoot audio right back into the mics it's recieving it's feed from. It seems like whoever set the system up was not too informed.
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Chris Penny

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Re: Yup - there's the problem.
« Reply #3 on: June 14, 2007, 05:45:43 am »

I agree with Josh. One of the first rules of speaker placement in any setting, portable or installed, is the speakers go in front of the microphones. Move these and most of your problems should disappear.

Personally I would just for now move them down somewhere near the front of the stage centered as much as aesthetically permitted.

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Chris Penny
Lead Sound Person
Caringbah Anglican Church
http://www.stphils.org

Ira White

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Re: Yup - there's the problem.
« Reply #4 on: June 14, 2007, 08:27:59 am »

I'll third that short of a pro install. Since your seating is only eight rows deep, you should be able to get acceptable coverage for now from the floor, and that would get the dispersion down where it isn't all bouncing off that huge brick wall facing. Just get the stands up at least 6' high. The church would have to live with the more conspicuous placement until they can afford to do it right. For now it's a choice of either acceptable sound or looks. Take your pick.

You can probably set them to each side of the stage, or spread them farther out to each wall. Just see which works best for your coverage needs and cable runs. With proper hardware and mounting, you might even be able to safely secure them to the walls if that is more aesthetically pleasing. With a pro install, speakers with a proper dispersion characteristic for such a small area can be suspended and directed down into the seating to avoid reflections off the walls.

Oh, I forgot, you might buy an inexpensive wireless hearing-assistance system and ask everyone to buy their own receivers. Then you could get as loud as you want with no feedback or reflections! Laughing

(That was meant to be goofy. Sometimes I just need the release.)
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Ira White
Sanctuary Sound, Inc.

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Gary Creely

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Re: Yup - there's the problem.
« Reply #5 on: June 14, 2007, 08:28:44 am »

in terms of the placement of speakers they should be just infront of the front end of the stage. As I say on here all the time, call a contractor.

You do not need to have a contractor model the room in order to have a dramatic improvement in your systems performance. You could find a contractor that could come in and fly an new set of speakers for like 3k or so.
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Gary Creely
STEEPLE SOUND
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Rich Stevens

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Re: Yup - there's the problem.
« Reply #6 on: June 14, 2007, 10:14:53 am »

First, a shout out to Gary from an old Philly boy; raised in Abington, educated at Drexel, spent my young adult years in West Chester, lived for a while in Allentown.  Don't get me started on the Houston vs. Philadelphia thing.

Thanks all for your input. I'm thinking 80:20 isn't such a bad approach to take at this point, and we might be able to afford new cabs & installation.

What are your suggestions on the subject of EQ?  Assuming for the time being that I'm going to tune it by ear, is it worth going to a 31-band?  


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Ira White

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Re: Yup - there's the problem.
« Reply #7 on: June 14, 2007, 12:18:55 pm »

Definitely a 31-band if not a digital processor with all the reflective surfaces you're dealing. Just make that part of any speaker upgrade and install services, and the integrator can aid in tuning the system with an analyzer.
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Ira White
Sanctuary Sound, Inc.

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Rich Stevens

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Re: Yup - there's the problem.
« Reply #8 on: June 15, 2007, 10:06:02 am »

Where do we draw the line between a 31-band and a DSP though?

Now that I've asked the question, let me try and answer it myself: If we're dealing with a single mono cluster with passive crossovers and one big power amp, I'm gueesing a graphic is good enough.  For any level of complexity beyond that (in my case bi-amping would be the next logical step) jumping to DSP is preferrable to adding more discrete processors.

Am I close?
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Ivan Beaver

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Re: Yup - there's the problem.
« Reply #9 on: June 15, 2007, 04:31:18 pm »

So how is a DSP better than a 31 band because of reflective surfaces?  I fail to see how the surface types have anything to do with the choice of system eq.

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Ivan Beaver
dB Audio & Video Inc.

ProSoundWeb Community

Re: Yup - there's the problem.
« Reply #9 on: June 15, 2007, 04:31:18 pm »


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