7.1 Speaker Placement THX vs. Dolby - AVS Forum
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post #1 of 6 Old 09-23-2012, 08:39 AM - Thread Starter
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New to this forum wanted to try my question on a diferent forum. First the room and system. The room is 30’x11’x8’6”(LxWxH) with only one door and no windows. Audio is Onkyo 9400 THX (7.1 THX cert), Video Epson 8500UB. Will be projecting a 98” screen which makes it 4’ high and 7’ long. Seating position will be 12’ from screen. Center as well as L&R speakers will be mounted on the front wall 40” from floor. This means the screen will be 2’ from top and start right above the center channel. The L&R will be 1’ from edge of side wall. L&R side surrounds will be 6’ high and at about 1’ behind the main listening position. The rear surrounds are the ones I’m having trouble with.

1. Can the front L&R be closer than 1’ from the side wall? I ask because if I can move them closer to the walls I can go from a 98” screen to up to a 112” if desired .
2. Rear Surround set-up- THX recommended of the rear surrounds together or traditional set-up Dolby recommended of the rears at 135-150 degrees?
3. Assuming I go with THX set-up, the rears will be 18’ from listening position. Is that too far back? Speaker specs below. Speaker stands are not an option with a 1 year old running around.
4. Assuming I go traditional, due to speaker to listening position distance the best I can get is 165 degree which is out of range. Is this too far out of spec? If not, is the 18’ distance to speaker to much? A solution to both is to place the rear surrounds on the side walls vs. the rear wall at 8’ back from listening position. This yields a 145 degree angle.

Thoughts on this?


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post #2 of 6 Old 09-23-2012, 09:52 AM
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Welcome to the forum!
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Originally Posted by devildog1679 View Post


1. Can the front L&R be closer than 1’ from the side wall? I ask because if I can move them closer to the walls I can go from a 98” screen to up to a 112” if desired .
The problem with L&R close to side walls is the walls will reflect sound from them, an "early reflection", which is detrimental. If you can put acoustic treatment on the side walls where those reflections occur, it can work.
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2. Rear Surround set-up- THX recommended of the rear surrounds together or traditional set-up Dolby recommended of the rears at 135-150 degrees?
Check out this link. The Dolby recommendation is 60 degrees apart, and at 135 - 150 from center. The close-spaced THX plan is intended to work with the THX ASA - Advanced Speaker Array processing. Checking your Onkyo manual, I see no reference either to that speaker plan or ASA, so I would have to assume it's not available.
http://www.thx.com/consumer/home-entertainment/home-theater/surround-sound-speaker-set-up/
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3. Assuming I go with THX set-up, the rears will be 18’ from listening position. Is that too far back? Speaker specs below. Speaker stands are not an option with a 1 year old running around.
These speakers are a little inefficient for the room size anyway, and the 18' isn't helping, so the limit will be maximum volume. Since you're closer to the LCRs that shouldn't be a big deal in your case, and with a 1 year old, you're probably not going to try for theater reference levels very often, if at all. Not a big deal, but the speakers are the limiting factor.
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4. Assuming I go traditional, due to speaker to listening position distance the best I can get is 165 degree which is out of range. Is this too far out of spec? If not, is the 18’ distance to speaker to much? A solution to both is to place the rear surrounds on the side walls vs. the rear wall at 8’ back from listening position. This yields a 145 degree angle.
Thoughts on this?
None of this is ideal, but none of it is all that critical either. Remember, those degree figures are angles from the front center, not the angle between speakers. Some people do ceiling mounts too, which is again a compromise, but you can hit your angles that way, they're just a little high. I wouldn't obsess about the exact angle, though. The idea is without THX ASA, get about 60 degrees between them, but give or take 10 -15 isn't a big deal. Spend more attention on the the front positions, and consider some side-wall acoustic treatment.
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post #3 of 6 Old 09-23-2012, 08:35 PM - Thread Starter
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Welcome to the forum!
The problem with L&R close to side walls is the walls will reflect sound from them, an "early reflection", which is detrimental. If you can put acoustic treatment on the side walls where those reflections occur, it can work.

So the acoustice fabric would go on the side wall edge?
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Check out this link. The Dolby recommendation is 60 degrees apart, and at 135 - 150 from center. The close-spaced THX plan is intended to work with the THX ASA - Advanced Speaker Array processing. Checking your Onkyo manual, I see no reference either to that speaker plan or ASA, so I would have to assume it's not available.
http://www.thx.com/consumer/home-entertainment/home-theater/surround-sound-speaker-set-up/.

Yep, and with my angle of 150 deg. b/w center channel, listening position and rear surround, both rear surrounds will make a triangle with 60 deg at each side. So I'm set here, haven't used geometry in years LOL
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These speakers are a little inefficient for the room size anyway, and the 18' isn't helping, so the limit will be maximum volume. Since you're closer to the LCRs that shouldn't be a big deal in your case, and with a 1 year old, you're probably not going to try for theater reference levels very often, if at all. Not a big deal, but the speakers are the limiting factor..

Yea, upgrade later on, once the wife approves smile.gif
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Originally Posted by has7738 View Post

None of this is ideal, but none of it is all that critical either. Remember, those degree figures are angles from the front center, not the angle between speakers. Some people do ceiling mounts too, which is again a compromise, but you can hit your angles that way, they're just a little high. I wouldn't obsess about the exact angle, though. The idea is without THX ASA, get about 60 degrees between them, but give or take 10 -15 isn't a big deal. Spend more attention on the the front positions, and consider some side-wall acoustic treatment.

Got it, Thanks
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post #4 of 6 Old 09-24-2012, 07:54 AM
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So the acoustice fabric would go on the side wall edge?
One way to do this is to use a mirror that you can temporarily stick to the wall, or have someone help you by holding it. Sit in a seat, or seat position, with either the actual LCR speakers in place or with something marking their position. Looking at the mirror, have someone hold it against the wall and move it around until you can see a reflection of the nearest speaker (or place-holder). Mark that mirror position on the wall, and try the same thing sitting in all seats. You'll end up with a group of spots on the wall. Place an acoustic absorber such that it covers all spots plus about 30% more area around them. There are many absorbers available, but the rule of thumb here is the thicker they are, the lower frequencies they will absorb. Everybody likes 1" thick because it is thin and looks good, but thicker is better because it can absorb lower frequencies, so I never recommend much less than 2". Mounting the absorber slightly off the wall will extend the low frequency capability making it act as though it was thicker. No absorber panel will never really do much to absorb bass, though. Think wave lengths. 60Hz has a wavelength of about 18 feet, so a 3" panel is invisible to it. Bass treatment is a different issue, and as much as it should be addressed, it's not the same as the early reflection issue we're talking about here, and needs a different type of treatment. But killing pronounced early reflections of the L and R speakers is very important to sonic imaging, so go for that first. If the position ends up at a point where a picture or decorative graphic might look good, you can get absorbers custom printed with colors, and images, even your own photos. Sometimes that's the best compromise between having an absorber that conflicts with aesthetics and not having one at all.

Some manufacturers, the best ones, will help and consult with you on what you need. The owner of this company hangs out here, good guy, and his stuff is first quality yet affordable. Call and ask them to help you: http://www.realtraps.com.

One last thing, don't lock yourself into a subwoofer location until you have your system up and running so you can hunt down the best sub location. You do this by placing the sub in the best seat, then the system playing with good bass content, crawl around the room finding the spot where the bass is best (smoothest, not just strongest), and put the sub there.
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post #5 of 6 Old 09-24-2012, 09:07 AM
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THX makes it confusing.... The 7.1 with the rear surrounds placed together was pre HD audio pre True 7.1 or even PLIIx...... If you scrool down on the THX website page with that setup on it shows a different setup when Dolby TrueHD and DTS HD MA is used. It shows the surround speakers spread apart at a 60 degree angle.

Shawn
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post #6 of 6 Old 09-24-2012, 10:07 AM - Thread Starter
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Quote:
Originally Posted by has7738 View Post

One way to do this is to use a mirror that you can temporarily stick to the wall, or have someone help you by holding it. Sit in a seat, or seat position, with either the actual LCR speakers in place or with something marking their position. Looking at the mirror, have someone hold it against the wall and move it around until you can see a reflection of the nearest speaker (or place-holder). Mark that mirror position on the wall, and try the same thing sitting in all seats. You'll end up with a group of spots on the wall. Place an acoustic absorber such that it covers all spots plus about 30% more area around them. There are many absorbers available, but the rule of thumb here is the thicker they are, the lower frequencies they will absorb. Everybody likes 1" thick because it is thin and looks good, but thicker is better because it can absorb lower frequencies, so I never recommend much less than 2". Mounting the absorber slightly off the wall will extend the low frequency capability making it act as though it was thicker. No absorber panel will never really do much to absorb bass, though. Think wave lengths. 60Hz has a wavelength of about 18 feet, so a 3" panel is invisible to it. Bass treatment is a different issue, and as much as it should be addressed, it's not the same as the early reflection issue we're talking about here, and needs a different type of treatment. But killing pronounced early reflections of the L and R speakers is very important to sonic imaging, so go for that first. If the position ends up at a point where a picture or decorative graphic might look good, you can get absorbers custom printed with colors, and images, even your own photos. Sometimes that's the best compromise between having an absorber that conflicts with aesthetics and not having one at all.
Some manufacturers, the best ones, will help and consult with you on what you need. The owner of this company hangs out here, good guy, and his stuff is first quality yet affordable. Call and ask them to help you: http://www.realtraps.com.
One last thing, don't lock yourself into a subwoofer location until you have your system up and running so you can hunt down the best sub location. You do this by placing the sub in the best seat, then the system playing with good bass content, crawl around the room finding the spot where the bass is best (smoothest, not just strongest), and put the sub there.
Those are both good tricks, thanks.
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THX makes it confusing.... The 7.1 with the rear surrounds placed together was pre HD audio pre True 7.1 or even PLIIx...... If you scrool down on the THX website page with that setup on it shows a different setup when Dolby TrueHD and DTS HD MA is used. It shows the surround speakers spread apart at a 60 degree angle.
I think I'm going traditional at 150 deg from list position which yileds the same 60 deg b/w the rear surrounds. I will also wire for rear surrounds on the back wall just in case.
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