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Acoustic preparation of theater room questions.

post #1 of 45
Thread Starter 
I've tried to read as much as I can on acoustics for a home theater and, quite frankly, it makes my brain hurt. I've spent most of my life learning everything I can on a variety of topics from electronics to automotive to string theory, and I am always willing to try and learn more, but there is just too much information on acoustics and it all seems to be experience based and laced with too much opinion for me to get a firm grasp on. So, I'm defaulting the experience of the people here for some general guidance on building my next theater, cause from what I read, I got it COMPLETELY wrong when building my dad's theater, and I think it sounds pretty damn decent for being a few years old now.

1) primary acoustic treatment should be done from the floor to ear height on the side walls, and likely on the rear wall? What should be done above that treatment? I was thinking 2" acoustic treatment to ~48" with a chair rail then loose curtains above to the ceiling for a smooth and consistent look throughout the room, AT curtains around the screen on the false front wall. I'll likely have a 12-14" riser for the rear row and angle the lower wall treatment up at the stairs then level it off at the riser again to maintain the ~48" height from the top of the riser. What about carpet, just nice, pretty plush carpet?

2) the entire front wall should be acoustically treated including bass traps? I'm not sure I've actually grasped how a bass trap functions, or why it's necessary, but I've concluded it is, so I'll install them, I figure 1' squares of insulation cut diagonally and stacked in the corners behind my screen wall should suffice? Then the same insulation installed on the wall itself, R19 fiberglass? Other options? I've read foam = bad.

3) speaker location, THX 7.1 is outdated and not used, we went a little crazy in my dad's theater, but it sounds good so I'm thinking of emulating it, with some improvements. LCR will have two speakers per channel, just below and above the non-AT screen set just behind AT material on the false screen wall. Two speakers for the surrounds, one placed ~18" in front of each row of seating at ~7' from the ground, finally two speakers per channel of rear surround, two placed at the rear row of seating firing slightly forward and two ~6' apart in a 10' wide room on the rear wall. Subwoofers will be dual 12" at the front, ~6' apart behind the false screen wall and two 10" subs behind the rear row of seats roughly the same distance apart. Sound about right?

4)I'll be doing a 12" soffit around the room at the ceiling for running speaker wires and because I think they look nice, I'm hoping to mount the projector inside the rear soffit, but I was considering doing and angled soffit to look like a tray ceiling instead of a square one, any advantages or disadvantages of this style?

5) I don't mind the sound bleeding out into the entire rest of the house, or even outside a little, so long as it doesn't make it into my neighbors houses or piss anyone off, I just want to make sure it sounds good from inside the room.
post #2 of 45
Quote:


3) speaker location, THX 7.1 is outdated and not used, we went a little crazy in my dad's theater, but it sounds good so I'm thinking of emulating it, with some improvements. LCR will have two speakers per channel, just below and above the non-AT screen set just behind AT material on the false screen wall. Two speakers for the surrounds, one placed ~18" in front of each row of seating at ~7' from the ground, finally two speakers per channel of rear surround, two placed at the rear row of seating firing slightly forward and two ~6' apart in a 10' wide room on the rear wall. Subwoofers will be dual 12" at the front, ~6' apart behind the false screen wall and two 10" subs behind the rear row of seats roughly the same distance apart. Sound about right?

I think a bit more reading is in order ... stick to text books and stray from fairy tales and op ed pieces.
post #3 of 45
Multiple speakers for the same channels can result in undesired comb filtering.
post #4 of 45
Thread Starter 
Quote:
Originally Posted by Dennis Erskine View Post

I think a bit more reading is in order ... stick to text books and stray from fairy tales and op ed pieces.

Wow, thanks for the useful addition, I don't suppose you have any book suggestions? My dad's theater all ready has a similar layout to this and it sounds great, can you give me any useful information at all?
post #5 of 45
Quote:
Originally Posted by ArtosDracon View Post

Wow, thanks for the useful addition, I don't suppose you have any book suggestions? My dad's theater all ready has a similar layout to this and it sounds great, can you give me any useful information at all?

Titles, huh ? Try this LINK
post #6 of 45
I'd add "Sound Reproduction" by Floyd Toole. The "Handbook for Sound Engineers" as well. The problem is the significant notches as a result of path/phase differences between two identical signals coming from more than one source. The additional money spent on speakers and amps would be better spent on better speakers and better room acoustics.

As to your point 5 ... the intent behind sound isolation in playback rooms is to get the ambient noise level in the room down to a level lower than the softest sound recorded on a sound track ... (so you can hear it without loosing your hearing) ... While reducing sound "leakage" into adjoining spaces is useful (sometimes really desirable), it is an additional benefit but not the primary objective.

How is 7.1 outdated? The two subs shouldn't be firing forward into the listening area. What documentation (or stated research) led you to that very specific subwoofer positioning? Any of the credible recent work on the topic would disagree with your research (or reading). The point here is your "reading" about acoustics and home theater until your brain hurts. It should hurt. I don't see much evidence of reading credible materials and that will certainly lead to pain. If you want to study this, do your own research, and move forward, that's great. I laud that; however, you need to be reading factual information and not unsubstantiated opinion.
post #7 of 45
Jeez Dennis, who pissed in your cornflakes this morning? Giving him actual suggestions for good reading on one hand is nice, but taking him behind the woodshed for not knowing about them in the first place is a rather harsh stance. And no, your first post suggesting he just read more was not very helpful.

If you want to help people here and share your knowledge of the subject, that's really great. But the condescending overtones are not needed.

-Suntan
post #8 of 45
Dennis has been here on AVS for a decade helping people. Over 6000 posts.

I don't thing your vulgarity is appropriate, courteous,
respectful or welcomed, frankly
post #9 of 45
Perhaps my comments were a little over the top, but it is a pretty common phrase in use.

As I said, the help is appreciated. I just don't think comments inferring that "his brain should hurt based on the material he has chosen to read..." are very appropriate, courteous,
respectful or welcomed either.

-Suntan
post #10 of 45
Quote:
Originally Posted by ArtosDracon View Post

1) primary acoustic treatment should be done from the floor to ear height on the side walls, and likely on the rear wall? What should be done above that treatment? I was thinking 2" acoustic treatment to ~48" with a chair rail then loose curtains above to the ceiling for a smooth and consistent look throughout the room, AT curtains around the screen on the false front wall. I'll likely have a 12-14" riser for the rear row and angle the lower wall treatment up at the stairs then level it off at the riser again to maintain the ~48" height from the top of the riser. What about carpet, just nice, pretty plush carpet?

True for the side walls, but corner bass traps should be floor to ceiling if you can manage it. If not, ear height will work, just not quite as well because you're covering half the surface area. Bass traps on the back wall (4" thick or more) at ear height will be fine.

Quote:
Originally Posted by ArtosDracon View Post

2) the entire front wall should be acoustically treated including bass traps? I'm not sure I've actually grasped how a bass trap functions, or why it's necessary, but I've concluded it is, so I'll install them, I figure 1' squares of insulation cut diagonally and stacked in the corners behind my screen wall should suffice? Then the same insulation installed on the wall itself, R19 fiberglass? Other options? I've read foam = bad.

If you can cut them so that you're covering 2' across the corners that would be better. That will leave you with a triangle that's 24" at the base and 17" per leg with about 12" between the apex and the center of the base. Or you can make 2'x4'x4" panels and just straddle the corner as well. That doesn't work as well or as low, but it's harder to mess up. Foam for bass trapping isn't a good option.

Frank
post #11 of 45
Thread Starter 
Quote:
Originally Posted by Dennis Erskine View Post

I'd add "Sound Reproduction" by Floyd Toole. The "Handbook for Sound Engineers" as well. The problem is the significant notches as a result of path/phase differences between two identical signals coming from more than one source. The additional money spent on speakers and amps would be better spent on better speakers and better room acoustics.

As to your point 5 ... the intent behind sound isolation in playback rooms is to get the ambient noise level in the room down to a level lower than the softest sound recorded on a sound track ... (so you can hear it without loosing your hearing) ... While reducing sound "leakage" into adjoining spaces is useful (sometimes really desirable), it is an additional benefit but not the primary objective.

How is 7.1 outdated? The two subs shouldn't be firing forward into the listening area. What documentation (or stated research) led you to that very specific subwoofer positioning? Any of the credible recent work on the topic would disagree with your research (or reading). The point here is your "reading" about acoustics and home theater until your brain hurts. It should hurt. I don't see much evidence of reading credible materials and that will certainly lead to pain. If you want to study this, do your own research, and move forward, that's great. I laud that; however, you need to be reading factual information and not unsubstantiated opinion.

Thank for the addition, I'll actually be heading to my local bookstore to find some of these later today.

For your point about keeping sound out, not necessarily in, I don't really see how they could be mutually exclusive, unless a barrier worked like a one way mirror, except for sound. Or maybe I'm just over-thinking the wrong details, I should just worry about keeping noise out and ignore sound leakage except as a bi-product of keeping noise out? I'm practically deaf as it is, too much time next to angle grinders, really loud speakers(concerts) and open headers, so I play music and movies too loud as it is.

I think you mis-understood my post, I meant that the THX standard for speaker placement in 7.1 with the rear surrounds right next to each other is out-dated based on the fact that it's based on their 6.1 standard, with two speakers instead of 1 in the rear and with the left and right surrounds moved forward a bit. 7.1 in general is not outdated, it can't be, there's nothing better yet. I meant no harm or foul, was just hoping for this exact kind of information over a basic "you're stupid, try again" post. I do really appreciate any creative input, including, go read this or that book.

EDIT:Oh, and as to the subs, that positioning was based on the THX and DD room layout examples, my dad's set-up(one of the only I've heard in person) and the theater build threads that seem to mostly have the subs firing into the listening area from the front wall. Let me re-state that I'm no expert and not hoping to achieve acoustic perfection, I'm just hoping to make sure I've got the right general idea, if they should be firing backwards and you can back that up with something, I'll likely have them fire backwards.
post #12 of 45
Thread Starter 
Quote:
Originally Posted by BIGmouthinDC View Post

Multiple speakers for the same channels can result in undesired comb filtering.

Just to make sure I understand you, comb filtering is sound arriving at the listener with a delay effect because of the different distances from the speakers, right? If so I know exactly what you mean, but I have to admit I've noticed it more in actual movie theaters than in my dad's set-up, which I'll admit is still different than what I was proposing, but that's why I listed the exact distances I was thinking of putting them, because I was trying to center the audio around a point in the center of the room and right between the two rows of seating to prevent that echo effect that I have noticed before. Am I just all wet in my thinking?
post #13 of 45
Thread Starter 
Quote:
Originally Posted by Weasel9992 View Post

True for the side walls, but corner bass traps should be floor to ceiling if you can manage it. If not, ear height will work, just not quite as well because you're covering half the surface area. Bass traps on the back wall (4" thick or more) at ear height will be fine.



If you can cut them so that you're covering 2' across the corners that would be better. That will leave you with a triangle that's 24" at the base and 17" per leg with about 12" between the apex and the center of the base. Or you can make 2'x4'x4" panels and just straddle the corner as well. That doesn't work as well or as low, but it's harder to mess up. Foam for bass trapping isn't a good option.

Frank

You're right, I didn't specify well, I was intending on doing the entire front wall with R19, between the bass trap, I was intending on cutting the 2' widths of insulation into 2' squares, then cutting those diagonally so that the legs would be a full 2' and stacking them up both corners, to the full 8' ceiling height.
post #14 of 45
Thread Starter 
Quote:
Originally Posted by Suntan View Post

Perhaps my comments were a little over the top, but it is a pretty common phrase in use.

As I said, the help is appreciated. I just don't think comments inferring that "his brain should hurt based on the material he has chosen to read..." are very appropriate, courteous,
respectful or welcomed either.

-Suntan

"pissed in your cereal" is a pretty common expression amongst military of my fathers generation from best I can tell, I've heard it and do tend to use it from them, but I can also see from the perspective of someone who's never heard it before not understanding it's inference.

As for my brain being supposed to hurt, it doesn't surprise me, in college I would get similar headaches studying for astrophysics exams, and I thusly know better than to be an astrophysicist, I have no hopes or intentions of ever being an expert at this, I just want to make sure my theater doesn't sound like crap.
post #15 of 45
Thread Starter 
Apparently everyone, myself included was a little grumpy yesterday, let's try again, shall we. I'm not smart with this stuff, my first post lists a bunch of assumtions I've made based on my limited reading thus far, which apparently has been the absolute wrong things to read, I'm going to attack better reading as I can, which isn't too limited for time.

I really appreciate the book suggestions, thanks.

I really appreciate Big's post about comb filtering, that's exactly the kind of post I was hopping for, I know my ideas are swiss cheese, I just don't know where the holes are.
post #16 of 45
Your understanding of the effects of comb filtering and the comments on isolation ... I'll get to those tomorrow. In the meanwhile, Big usually is full of helpful hints when he can see a floor plan.
post #17 of 45
Hopefully I'm not speaking out of turn, but I'll try to address some of your questions.

Quote:
Originally Posted by ArtosDracon View Post

For your point about keeping sound out, not necessarily in, I don't really see how they could be mutually exclusive, unless a barrier worked like a one way mirror, except for sound. Or maybe I'm just over-thinking the wrong details, I should just worry about keeping noise out and ignore sound leakage except as a bi-product of keeping noise out? I'm practically deaf as it is, too much time next to angle grinders, really loud speakers(concerts) and open headers, so I play music and movies too loud as it is.

I don't think that Dennis is saying that they are mutually exclusive. My understanding is that he's saying is that sound isolation should be used to keep unwanted sounds out of the theater. That it also keeps the sounds in the theater is a welcome side-effect.

Quote:
Originally Posted by ArtosDracon View Post

EDIT:Oh, and as to the subs, that positioning was based on the THX and DD room layout examples, my dad's set-up(one of the only I've heard in person) and the theater build threads that seem to mostly have the subs firing into the listening area from the front wall. Let me re-state that I'm no expert and not hoping to achieve acoustic perfection, I'm just hoping to make sure I've got the right general idea, if they should be firing backwards and you can back that up with something, I'll likely have them fire backwards.

Based on the reading that I have done, generally speaking, the best layout for subwoofers is to have 4 of them centered on each of the four walls. After that would be 4 subs in each of the corners, followed by two centered in each of the walls (either front and back or sides). That said, many rooms don't allow for the best subwoofer layouts.

CJ
post #18 of 45
Quote:
Originally Posted by CJO View Post

.............
Based on the reading that I have done, generally speaking, the best layout for subwoofers is to have 4 of them centered on each of the four walls. After that would be 4 subs in each of the corners, followed by two centered in each of the walls (either front and back or sides). That said, many rooms don't allow for the best subwoofer layouts.
CJ

Please realize that the aforementioned subwoofer layouts are for very specific seating regions and that further the metric used for "best" is minimizing seat to seat variation. Therefore, when utilizing one of these setups, even assuming that the seating area is the same, you will likely need to provide proper EQ and attendant calibration efforts to pull the diamond from the rough.
I also bet you really meant 1 sub at each midwall (or corner), and finally 1 sub at the midpoints of only two of the facing walls. Course 16 subs in the room would give some headroom!
post #19 of 45
Thread Starter 
Quote:
Originally Posted by Dennis Erskine View Post

Your understanding of the effects of comb filtering and the comments on isolation ... I'll get to those tomorrow. In the meanwhile, Big usually is full of helpful hints when he can see a floor plan.

My verbage is clearly lacking lately, I'm having a very hard time putting what I think I understand into a coherent thought. Please be patient with me, I'm not trying to be an ass or be blatantly dense, this stuff is just WAY outside my normal realm of understanding. I thought I did pretty decently on the comb filtering though
post #20 of 45
Thread Starter 
I don't quite have the budget for 16 subs, but I all ready have four specifically for the theater, on one each wall seems like it would get in the way pretty badly, but in the corners at the rear of the room would be quite do-able, in the very corners at the front should be fine as well, I just want to make sure I don't get them too close to my front left and right channel though, that was the reason for moving them in from the side walls slightly, as I can't afford an AT screen to put the speakers behind on my budget.
post #21 of 45
Quote:
Originally Posted by jamin View Post

Please realize that the aforementioned subwoofer layouts are for very specific seating regions and that further the metric used for "best" is minimizing seat to seat variation. Therefore, when utilizing one of these setups, even assuming that the seating area is the same, you will likely need to provide proper EQ and attendant calibration efforts to pull the diamond from the rough.
I also bet you really meant 1 sub at each midwall (or corner), and finally 1 sub at the midpoints of only two of the facing walls. Course 16 subs in the room would give some headroom!

Thank you for the caveats. Also, you are right about the number of subwoofers, though I that the original research actually did recommend 16 subs!

CJ
post #22 of 45
Subwoofers -
EQ will be required plus your room design should allow the ability to move and relocated the subs during the calilbration process. Generally, you should avoid having the drivers point directly into the seating locations. If you do, an 1" of fiberglass board ( like OC703) can be placed in front of the sub's driver. These drivers create HF artifacts as a result of their operation. These artifacts can lead to subwoofer localization ... something you want to avoid.

Comb Filtering:
The audible impact of comb filtering is very unequal frequency response within critical frequency ranges. The same sound from two different speakers will cross paths, and, when they do, you'll have some frequencies cancelled (180 degrees out of phase) and other frequencies augmented (in phase) plus all the variations in between. These cancellations/augmentations will be inconsistent throughout the room.

Isolation:
Reducing noise entering a space and reducing noise leaving that space to adjoining rooms is NOT mutually exclusive, nor did I say that. My point is the REAL reason sound isolation should be employed in dedicated rooms is to reduce the noise floor IN the room. The fact those efforts assist in reducing noise levels OUTSIDE the room should be considered a bonus. There are some differences between the two efforts. First, a significant cause of high noise floors in a room are a result of the noise brought into the room by the A/V installer or home owner ... the equipment. Other in room sources of noise include HVAC noise. The remainder is noise entering the room from the outside which needs to be eliminated. Second, if the sound track is at 75dB in the room, and ambient noise levels outside the room are at 35dB, you have a 40dB problem to solve. If the ambient noise in the room (from outside sources) is 35dB, you have a 13dB problem to solve. Don't get excited ... that sounds easier than it is and when we say 'noise floor' the metric is either NC or NR which are weighted curves across a broad spectrum of frequencies.

The desired standard for noise transmission from inside a room to the adjoining spaces is the theater should not increase the ambient noise level outside the room by more than 3 dB SPL
post #23 of 45
Quote:
Originally Posted by ArtosDracon View Post

You're right, I didn't specify well, I was intending on doing the entire front wall with R19, between the bass trap, I was intending on cutting the 2' widths of insulation into 2' squares, then cutting those diagonally so that the legs would be a full 2' and stacking them up both corners, to the full 8' ceiling height.

Cool...that'll work.

Frank
post #24 of 45
[quote=Dennis Erskine;16674662]Subwoofers -
EQ will be required plus your room design should allow the ability to move and relocated the subs during the calilbration process. Generally, you should avoid having the drivers point directly into the seating locations. If you do, an 1" of fiberglass board ( like OC703) can be placed in front of the sub's driver. These drivers create HF artifacts as a result of their operation. These artifacts can lead to subwoofer localization ... something you want to avoid.

I plan on placing my subs at the midpoints of the four walls. However, instead of pointing them at the seating positions I will build a manifold and mount two 15" drivers opposite ea other inside the manifold. This should minimize localization, no?
post #25 of 45
btw, these will be ib subs, with the backwave seperated from the listening area.
post #26 of 45
This book is available on Gedlee.com

This should get you more than up to speed on all of the issues. Also has some insightful alternative ideas to the ubiquitious "bass trap" recommendations, etc.
The first 4 chapters are posted for free even.

Read chapter 4: Room acoustics.

(Electronics information is a bit outdated however.)

-Tony
post #27 of 45
Quote:
Originally Posted by traderxfan View Post

This book is available on Gedlee.com

This should get you more than up to speed on all of the issues. Also has some insightful alternative ideas to the ubiquitious "bass trap" recommendations, etc.
The first 4 chapters are posted for free even.

Read chapter 4: Room acoustics.

(Electronics information is a bit outdated however.)

-Tony

Does anyone else have experience with this book?

CJ
post #28 of 45
Quote:
Originally Posted by CJO View Post


Based on the reading that I have done, generally speaking, the best layout for subwoofers is to have 4 of them centered on each of the four walls. After that would be 4 subs in each of the corners, followed by two centered in each of the walls (either front and back or sides). That said, many rooms don't allow for the best subwoofer layouts.

CJ

Actually there is an alternative to this, (by Geddes as well). The setup you describe, I understand, came from Welti. Geddes' approach is similar that it uses multiple subs, but isn't so restrictive in placement.

He recommends using 3. One of them to pick up where your mains leave off and go to 20hz. This would be main sub. But then also to use 2 other subs, in asymmetrical locations, to fill in the room response.

Here is a quick description of that approach:
try this {-I can't post the link because of spam restriction on my newbie status}
//mehlau.net/audio/multisub_geddes

-Tony
post #29 of 45
Quote:


I plan on placing my subs at the midpoints of the four walls.

Understand the Welti approach provides consistent bass response in the seating area ... not necessary smooth bass response. I've been using two primary subs in the front of the room and three smaller asymetrically placed smaller subs for the past 15 years. The positioning of the three smaller subs is determined by estimating the interaction of the subs (ala Gerry Lemay's Virtual Subwoofer process) within the planned seating area. Several of those rooms have not required EQ ... Jamin has experience in one of them.
post #30 of 45
Quote:
Originally Posted by Dennis Erskine View Post

Understand the Welti approach provides consistent bass response in the seating area ... not necessary smooth bass response. I've been using two primary subs in the front of the room and three smaller asymetrically placed smaller subs for the past 15 years. The positioning of the three smaller subs is determined by estimating the interaction of the subs (ala Gerry Lemay's Virtual Subwoofer process) within the planned seating area. Several of those rooms have not required EQ ... Jamin has experience in one of them.

I plan on building four boxes to test placement before I cut into the walls.
My furniture plans seem to be in a state of flux. Will placing the four boxed woofers in an empty room give me the placement that I need, or do I have to have the furniture in its final resting place to get the smooth response that I crave?
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