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theater dimensions

901 Views 17 Replies 11 Participants Last post by  proufo
I think these are simple questions, maybe not.

I'm considering putting a theater in a basement of a ranch structure that does not exist yet. It will be a modular construction build structure with a 10' deep Superior Foundation system, and the basement will be rectangular, and 26' wide unfinished. In order to get an idea of the "weather tight" structure cost, I need to know the ideal length. It can be between 40' and 66', in any of 2' increments (40', 42', 44', ..., 66'). Width has to be 26'. Height can be 8', 9' or 10'. These are all unfinished dimensions.

Is there some magic formula saying that the room length should be a certain ratio of the width? If so, I can't find it. Also, it would seem that the height may play a factor as well?

Let's assume that 24' will be the interior finished width of the theater, since essentialy I will be building a room within the basement structure. Also assume that on the screen end the floor to ceiling finished height will be 9' 6" or so, and that a "floating floor" system is desired to get away from the rigidity and acoustic problems of the concrete floor. There will be terraced seating, at least 4 levels total (including the bottom level), and the finished floor-to-ceiling height opposite the screen will be 7' 6" minimum

Given all of this, what is the ideal finished LENGTH of the theater?

Also, since I will be building a room within the room, I have total control, it doesn't have to really be a rectangle. For example, the screen wall could be narrower than the rear wall, resulting in a trapezoidal shape instead of a pure rectangle, if this beneficial? A lot of real theaters are shaped like this, but my understanding is that this was done for natural acoustical amplification. However, with speakers on all sides, is this really needed? Is there any real advantage to having this shape room? Or does it just make things harder to deal with? For example, all of the articles and books I've read give acoustic formulas for determining "sweet spots" for seating purely for rectangular (never square) rooms. Should I just stick with that? Or is there some benefit to a trapezoidal room, and if so, can anybody point me to formulas?

Thanks in advance for any info!

- Rhino
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This seems to be a very common question. In fact I have the same question, except my house isn't as impressive :)

There is a spreadsheet on Stereophile Guide to Home Theater. Here is the spreadsheet:

And metric version for all you non-Yanks

And here is a link to the article

I havn't sat down with it yet, so I know nothing.

As for trapazoidial rooms, I think it is harder for standing waves to form when the room isn't a rectangle. All the sound bounces off at an angle and doesn't bounce back to where it came from. I think In this month's Home Theater Magazine or maybe Stereophile Guide to HomeTheater, there is a "budget" $35k home theater that isn't even trapazoidial; all the angles are different. I'm thinking of making my theater slightly non rectangular, and would love someone elses opinion.

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For the best sound you do not want any two walls to be parallel. Less reflections and dead spots. A perfectly square room has too many challenges. A rectangular room still has some, but obviously not as much. But then that all depends on how picky you are also. I think 99% of the people out there wouldn't even know when there is a dead spot in a room and 50% of them wouldn't care even if someone showed them.

My 2 cents.


The Dolby website explains this a little better.
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Rhino, Ron is right....I was "THIS CLOSE" to framing my side walls at a slight angle for what he just mentioned (In fact my original design had it that way)...but then I thought, good gravy what if for some reason this room is used for something else down the road, we move, etc....so straight walls they were. My room is rectangular but still the best would be just a degree or so off center and you'd clear away many a problems. But like most things in life...we must live in reality! :D

The problem with a non-rectangular room is that it is almost impossible to figure out what it will sound like. The standard calculations just don't work.

George Cardas and several others recommend a ratio of H-W-L of 1:1.6:2.6. ie 10' ceiling, 16 feet wide, 26 feet long. This is based on the Fibanacci sequence of numbers - tool long to go into here. Check out his web site. He also generates some interesting theories on speaker placement within a room for nearfield and farfield listening.

I have heard a room and setup done this way with minimal acoustic treatments and it sounded fabulous. Wide sweetspot for imaging, few nodal problems, smooth bass, etc.

There are 1000 other theories on how to do things. Some work, some don't, some kind of work but need help. Again, at least with a rectangular room, you can begin to predict a bit and minimize the tweaking later.

There are many factors that come into play when trying to determine the "perfect" room. While the so-called "golden ratios" will get you in the ballpark, things such as seating type and location, desired screen size (which affects seating placement) acoustical treatments, etc. all play a role.

Given that you have basically a blank slate to work with I would consider enlisting the help of an experienced theater designer in your plans. I think you'll be surprised at how reasonable the cost can be, especially when weighed against your entire investment.

Good luck with your project. You seem to have the type of opportunity most of us only dream of. Take advantage of it and keep us informed of your progress. We love to live vicariously!
Chip is giving you good advice.

I would also chime in with the thought that you room length will also be factor of the projector and screen you plan to use and viewing distances and row depth. You mentioned 4 levels of seating which I assume means 4 rows. Even at a minimum of 3-4' per row and say 12' away from the screen you are looking at a depth of 21-24' but at a more comfortable 13-14 viewing distance and 5' between rows you are at 29' plus a couple of feet behind the rear row. Add in a stage and prosecium wall 3' now you need 32' total. BTW - depending on projector and screen how will that image look at 29'.

I would definately go with the 10' height. That really helps with multiple rows.

Lots to consider.

BTW-My dream theater is a sort of pie shaped room. At least in my mind. Implementation of such a room may be harder but I think if money was no object that would be my goal.
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Thanks for all the great advice folks!

Ed - thanks for the spread sheet link, it has already come in very handy.

Ron - that makes a lot of sense, I just spent a while reading up at the Dolby site, I didn't even think of looking there. As you know, the floor and ceiling aren't going to be parallel. From what I read, even a small amount off of parallel would make a difference.

E - You raise a very good point, but I don't have the same concerns, this room will have 1 purpose forever, that is to be a HT. In your "original plans", how much of an angle were you planning on the side walls off of perpendicular? We're you going to try to solve the front and rear walls from being parallel as well?

Chip and Jim, thanks for the advice. I do actually plan on going with a designer at some point. However, at this very (very) early stage I'm just trying to see what the addition costs will be, and need rough exterior dimensions for the builder to give the estimate. First will come the "mandatory" things that we reap out of the addition (office, extra bedroom, extra bathroom, etc), and later (probably 2-3 years) will come the "not-quite-so-mandatory" things, like the HT which the ENTIRE basement is being reserved for.

I just wanted to make sure I didn't go with something that is too small in length given the width. Since the width is fixed at 26', and the min length is 40', it sounds like that's probably not possible. From those spreadsheets it seems 36x26x10 isn't half bad.

Thanks all,


Based on the advice here and more research, it sounds like 42x26 will be plenty big enough. I can always make the theater SHORTER than the basement, not longer, so maybe even 44'. If it turns out I have enough room, the snack / lobby room can go downstairs, although I was planning upstairs. Still, it would be nice to not have to walk up a flight of stairs to get a soda or some snacks.
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If you are going to pursue "splayed" walls, I believe the minimum angle to be effective is quoted at 3 degrees each (6 degrees included angle).

Since you have the vertical height, you might also want to consider tapering the ceiling from front to back. Say 9' in front and 10' in back. Again, this will depend on how long you make the room.

Whatever you do, just remember that most concert halls and other "acousitcally correct" spaces are designed for sound to go one way - toward the audience. In the case of a home theater with surround sound, you have sound coming from all directions so the design parameters need to be a bit different.

Good luck and have fun. This is the fun part.
To trapezoid or not to trapezoid... that is the question.

There have been many arguments going both ways on this Forum. I would probably stick to the rectangle. There are many elements that a good theater make. If you want four rows of chairs, there will be sightline considerations to think about. With a very big screen (lets say a 135" Diagonal 16:9) you may not have enough rise to fit four rows. How many people do you want to seat? Any particular architectural style you would like to re-create?
Rhino, in my original design I was gong to put them somewhere between 3-5 degrees...larger in front/smaller in back of room. And the front and back walls were going to remain parallel cause they already were! Well if any house has true 90 degrees I'd be shocked! So I guess, in reality unless we use lasers to align, no one has true parallel walls huh? LOL...

I say do what you want and enjoy...that's what matter right?:D

I'm not too concerned about splayed walls now the more I've researched. It seems that a whole lot of professional designs out there work great with rectangular rooms. It seems to me that the use of columns for dispersion would have just as great of an effect if not greater effect than splayed walls.

John, when doing up some drawings last night I came to that conclusion as well...I need more headroom! I know I'm "blessed" to have a 10' ceiling option to begin with, but man I really could use a 12' or 13' for the size of the theater I was thinking of. This isn't out of the question, I talked it over with the builder, but the cost/sq ft is going to shoot up dramatically, not even considering the "theater" cost. The problem is have to go from the pre-cast Superior system, to a custom pour, and the custom pour is WAY more expensive when you start talking that much concrete. The foundation cost is actually a significant part of the "starting" budget, since the main story-and-a-half (it's a cape style addition) is going to be weather tight only, it won't even be insulated when I get it, and no wiring run, no interior partitions, etc. I'm doing all that work myself with my cousin who is a certified plumber/heating/electrician jack-of-all-trades.

The stereophile spreadsheet is none too kind to a room which is say anywhere from 28 to 38 ft long, 24 wide, and 9' 6" high. It simply doesn't seem to be enough ceiling to make it happy. :) I tried a whole bunch of combinations, even using odd and even dimensions, and could never get anything better than 11 or so bad frequencies (but maybe this isn't too bad?). Am I possibly mis-using this?

30x20x9.5 doesn't look too bad actually. 9 problem frequencies, no sitting position peaks, only a single double null. 27x20.9.5 isn't too bad either, two single nulls, also 9 problem frequencies, no peaks. I'm assuming two single nulls are better than a double null...

One way I'm thinking of dealing with the height limitation is to eliminate the need for one riser level by making the last row of seats have a bar rigth behind it with stools. This way you still get 4 "rows" to hit my goal of seating 16+, yet allows the theater to be a few feet shorter, and doesn't gobble as much vertical height.

Another requirement which i have which I think may mess things up a bit is I definately want a concealed projector, and I definately want a DLP unit. I was hoping to have it concealed in the rear wall for simplicity and noise control. But I'm not sure if DLP projectors will throw that far, we're talking like 28 or so feet to the screen. Ideally I'd like it so that anybody 6' 2' or shorter walking on the back platform, or sitting at the bar stools, won't affect the picture. Later today I'm going to start looking at various projector specs to see if they can throw that far, and what size range the screen would be. I need to start doing some line of sight analysis for the various seating positions.

The one thing I've learned quickly is that you have to take EVERYTHING into account in the design phase, it is hard to isolate one area of design to work on, since all things are inter-related. It's hard to componentize the solution into individual black boxes, which is something I'm used to doing as an engineer. It all has to work together in unison, push here and pull there. Definately fun!

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Remember that the spreadsheet is based on a perfect room. It will get you in the ballpark and help you avoid major problems but does not predict the magnitude of the problems, only the location.

On another note, I try to avoid nulls as much as possible around the seating positions. Peaks can be dealt with through acoustic treatments, etc. Nulls are tougher. You can't create something that is not there. You can reduce something that is too much.

You can help quite a bit with diffusion but usually those are more expensive treatments and usually more appropriate in smaller rooms - which yours is certainly not!
RhinoDude: don't know if this will work, but could you cut some of the cost by having two levels of basement height? i.e. theater room flour being lower then the rest of the floor. You could also get the benefit (assuming back entrance) of not having to go up some stairs to get into the theater.
I wish these spreadsheets would go away and never come back. Not that they don't have some interest and value, it is simply that many have taken them as gospel or the end all in determining room sizes.

As I have posted before, the spreadsheets are based upon assumptions which do not exist in the real world. Your room is not a perfect rectangle, nor are the walls perfectly reflective. At the same time, they completely ignore a far larger problem in residential sized rooms ... reverberation time. Equally to the point, as was alluded to above, the spreadsheet will predict the frequency at which a mode *may* occur but are totally mute to the real concern...the amplitude of any predicted mode. If the change in amplitude is outside the audible range, why would you care if you have one, two, or three nulls? The basic problem with the spreadsheet is it places focus on modes whilst ignoring screen size, reverberation, resonance, and a long, long list of other factors that will have an equal, and many times greater, negative impact on room sound quality.

In many rooms with sofit, columns, risers and stages, you don't have a rectangle. Tangential and Oblique modes are almost entirely broken up and many axial modes cannot live long enough to do any damage (assuming their amplitudes are in the audible range).

Likewise "best" seating positions in most residential sized rooms are about 6-7" away from the "bad" ones...so don't buy recliners, don't lean in your chair, and don't move your head.

Splayed walls are of value assuming you have more than a 6 degree angle *and* the walls have been treated for reverberation time.

If you're going to use a spreadsheet, search for Pablo's posts and download his...it is several orders of magnitude better than the one available from guidetohometheater.
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Thanks for the replies folks.

Anthony, I had thought of the same idea. The builder I am planning on using has never done this with Superior Walls, but he thinks in theory it's sound but is checking with the manufacturer. It will probably require pouring a traditional footer. I was actually thinking I could go 3 levels, since Superior makes 3 wall sizes: 8', 9' and 10'. That would give me a row of seats at the 10' level (stage would be 6" higher than that), a row at the 9' level, a row at the 8' level, and the bar with stools behind that. No risers to build. I'll post some of my concerns for discussion in a new thread.

Dennis, thanks a bunch for putting things into perspective. As I noted in my last paragraph, it's becoming entirely obvious that this is a balancing act, and I'm already starting to diminish the importance of the "perfect" room size. I believe it's going to be the sum of the little things that make this work, not one major thing (like room size, or equipment) is going to do it. Sure, if I have a HORRIBLE room to start with that will pose a lot of challenges, but it sounds like I've learned enough to avoid that happening.

Thanks all,

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Rhino...it is a balancing act and a good example of the art of compromise. In the course of balancing, weights need be assigned to each of the little things for the balance to work. In rooms with raised platforms, etc., I'd give reberberation time and good wall/platform construction the weight of a cannon ball and the spreadsheet results the weight of a marble. In a perfectly rectangular room, the cannon ball becomes lighter and the marble heavier.
Originally posted by Dennis Erskine
If you're going to use a spreadsheet, search for Pablo's posts and download his...it is several orders of magnitude better than the one available from guidetohometheater.
Many thanks Dennis. You are so kind.

You can download the Mediaroom package from the link in my signature. It includes the spreadsheets and a ReadMe file with bibliography.

Hope it is useful.
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