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speaker placement up almost against A-T material or further back?

post #1 of 15
2/13/12 at 8:11am
Thread Starter 
Im just curious here as I hadnt really thought about it until I was behind the screen last night prewiring for some new additions; but I got to thinking about speaker placement behind A-T material. Specifically with speakers using CD's with horns and/or coaxial in my case, is it better to put the speakers right up, almost against the screen or set them a ways back so they can hopefully get around the screen as well as through it? I'd love to hear some opinions.
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post #2 of 15
2/13/12 at 8:47am
The main concern is Allison Effect with respect to the distance from the front of the cabs to the wall behind them. You don't want that at 1/4 wavelength in the pass band.
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post #3 of 15
2/13/12 at 9:28am
Thread Starter 
Quote:
Originally Posted by Bill Fitzmaurice View Post

The main concern is Allison Effect with respect to the distance from the front of the cabs to the wall behind them. You don't want that at 1/4 wavelength in the pass band.

rear wall is treated with 2inch thick acoustic panels, speakers at the moment are up against the material. Being about 12 inches deep, the triple 12's have at least another foot to 18 inches behind them, possibly a little more, but not much
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post #4 of 15
2/13/12 at 10:01am
I have my speakers within an inch or two of my AT screen. You don't want them flush up against them, touching. Close but not several feet away or anything either.
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post #5 of 15
2/13/12 at 11:23am
Thread Starter 
Quote:
Originally Posted by Scott Simonian View Post

I have my speakers within an inch or two of my AT screen. You don't want them flush up against them, touching. Close but not several feet away or anything either.

yea mine right now are about 4 inches or so from the material, I wouldnt think of putting them flush ON the screen, haha. I need to continue running sweeps and see what I come up with for most optimal distance, Just wanted to ask in case there was a general rule of thumb
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post #6 of 15
2/13/12 at 12:31pm
Quote:
Originally Posted by beastaudio View Post

Being about 12 inches deep, the triple 12's have at least another foot to 18 inches behind them, possibly a little more, but not much

Danger zone. 24 inches is 1/4 wavelength at 140Hz, 30 inches is 1/4 wavelength at 115Hz, and two inch acoustic panels will have a cursory effect in that bandwidth. I'd move them closer to the wall. Having them tight, so that there's about 14 inches to the reflective surface, moves the notch frequency to 240Hz, where the acoustic panels will be much more effective.
Quote:


You don't want them flush up against them, touching. Close but not several feet away or anything either.

They should be placed either close enough or far enough away to prevent a reflective response dip. Eliminating the potential for Allison Effect is why theatrical THX installations have the mains walled in.
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post #7 of 15
2/13/12 at 12:39pm
Thread Starter 
Quote:
Originally Posted by Bill Fitzmaurice View Post

Danger zone. 24 inches is 1/4 wavelength at 140Hz, 30 inches is 1/4 wavelength at 115Hz, and two inch acoustic panels will have a cursory effect in that bandwidth. I'd move them closer to the wall. Having them tight, so that there's about 14 inches to the reflective surface, moves the notch frequency to 240Hz, where the acoustic panels will be much more effective.
They should be placed either close enough or far enough away to prevent a reflective response dip. Eliminating the potential for Allison Effect is why theatrical THX installations have the mains walled in.

hmm, assuming my major dip right now is from 300hz all the way up to abou 1khz, perhaps I have other issues going on as well
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post #8 of 15
2/13/12 at 10:04pm
A dip that wide wouldn't be from wall proximity. Likely something else. You have measurements?
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post #9 of 15
2/14/12 at 5:32am
Quote:
Originally Posted by beastaudio View Post

hmm, assuming my major dip right now is from 300hz all the way up to abou 1khz, perhaps I have other issues going on as well

That wouldn't be rear wall reflection. Allison Effect is more accurately called a notch rather than dip, as much as 24dB deep but only about 1/8 octave wide.
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post #10 of 15
2/14/12 at 8:17am
While solving those pesky boundary problems, also measure response at fairly high resolution (1/24 octave or better) with and without the screen. If the screen results in rougher HF response or significant attenuation, it may be reflecting audio between it and the speakers. This can be a greater issue with perf type screens/scrim...fine woven, not so much. If it's a problem, you might try higher spacing from the screen to the speakers.
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post #11 of 15
2/14/12 at 8:50am
Quote:
Originally Posted by pnw View Post

While solving those pesky boundary problems, also measure response at fairly high resolution (1/24 octave or better) with and without the screen. If the screen results in rougher HF response or significant attenuation, it may be reflecting audio between it and the speakers. This can be a greater issue with perf type screens/scrim...fine woven, not so much. If it's a problem, you might try higher spacing from the screen to the speakers.

+1 to PNW.

Screen material affects the higher frequencies due to reflections causing comb filtering. The comb filtering effect is lessened if you increase the spacing of the AT fabric from the speaker. The distance used is ultimately a compromise of space and performance. I find that with an AT weave like SMX or Seymour the comb filtering is unnoticeable even if the speaker is very close the fabric, but it will measure a little better with more spacing. The good thing with a weave as opposed to a perf screen is that the effect is generally very uniform and is easily EQd out.

The questions about placement of your T12's relative to your boundaries are probably a good question for JTR themselves. It is a passive speaker that was designed for specific placement. They should be able to provide you with an optimal recommendation.
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post #12 of 15
2/14/12 at 12:04pm
Thread Starter 
Yea I'll get with Jeff about the t-12's specifically. My screen material is Stewart firehawk microperf so perhaps that isn't helping out. I'm hopefully going to have some time to mess around tomorrow night. I also might entertain the idea of moving the l/r's back outside the screen if I can't fix it. That would put them basically against the side walls, which are treated as well.
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post #13 of 15
2/14/12 at 12:28pm
Quote:
Originally Posted by beastaudio View Post

Yea I'll get with Jeff about the t-12's specifically. My screen material is Stewart firehawk microperf so perhaps that isn't helping out. I'm hopefully going to have some time to mess around tomorrow night. I also might entertain the idea of moving the l/r's back outside the screen if I can't fix it. That would put them basically against the side walls, which are treated as well.

With Microperf, you want a little distance (around 10" or 12") between the screen and the speaker baffle. With a woven screen you can be quite close and not make an audible difference.
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post #14 of 15
2/14/12 at 12:34pm
Quote:
Originally Posted by Bill Fitzmaurice View Post

That wouldn't be rear wall reflection. Allison Effect is more accurately called a notch rather than dip, as much as 24dB deep but only about 1/8 octave wide.

Bill's been saying this for 7 years now
http://forums.musicplayer.com/ubbthr...&Number=693960

A life of learning this hobby called Home Theatre/Audio/Acoustics/Video.....
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post #15 of 15
2/14/12 at 1:20pm
Thread Starter 
Quote:
Originally Posted by mtbdudex View Post

Bill's been saying this for 7 years now
http://forums.musicplayer.com/ubbthr...&Number=693960

A life of learning this hobby called Home Theatre/Audio/Acoustics/Video.....

I actually came across this article while reading up on the allison effect. seems that I have more than just that going on, but I am confident I will find a way out with minimal issue
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