Would it be a bad idea to make my room trapezoidal?

Trapezoidal and other irregular rooms may be much harder to model and predict but in practice typically have fewer strong modes as the non-parallel surfaces all but eliminate axial and tangential modes in that direction and also significantly distribute and reduce oblique modes.

It isn't a panacea, but it can be a huge benefit. I wouldn't be afraid of it because it is unusual. One thing to recognize in planning is that the remaining two axial mode directions become effectively more sparse (they don't change, but there are fewer other modes interspersed with those two) and thus dominant. While you have less chance of a tangential and oblique modes summing with an axial mode creating a huge localized problem, there is also less chance of adjacent tangential and oblique modes smoothing response around a dominant axial mode.

The point being, pay very close attention to positioning around the two remaining axial modes and make sure those are optimized as well as is possible. The rest is measure and correct with strategic room treatment and signal shaping after the room is built, which is the same in all cases.

skylarlove1999 said:

" if it works " or "it should work" are not things I want to hear when I am spending tens of thousands of dollars. The risk of it failing seem to far outweigh the benefit.

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Yup, that is exactly my concern!

SierraMikeBravo said:

I don’t mean to step on your theater designers toes, I know how I would feel if someone criticized my designs without knowing the full history and backstory on the decisions made. My advice would be this, ask if they could show you the numbers. What numbers? Acoustical analysis? If so, please show the seat to seat low frequency response analysis. Also ask how the surrounds and Atmos work with the tapering shape? If they don’t provide answers that are logical or make sense, then ask to go back to a more traditional shape. Rectangles are much easier to predict, and pretty much all models are based on that shape and make several assumptions. Very few models (I can think of only two off hand) will allow you to change to such a shape. Hope this helps!!

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Thanks. I just sent them an email and asked them to show me the numbers. I should have done that a while ago when they told me that they did an analysis and confirmed it would work. I'm not sure I'm sophisticated enough to determine if they've done everything correctly, but at a minimum, I'll feel better seeing they've modeled it out.

BIGmouthinDC said:

you could make the trapezoidal walls acoustically transparent (Fabric) for the looks of the room and acoustically it will model like a rectangle. Mount your acoustic treatments on the solid walls behind the trapezoid.

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That's a good idea. Currently the plan calls for a "False wall with 2x6 framing and maintenance access door built-in (acoustical treatment details TBD)". If we choose not to drywall it, then there will really only be the reflections from the framing materials to worry about.

Bigus said:

Trapezoidal and other irregular rooms may be much harder to model and predict but in practice typically have fewer strong modes as the non-parallel surfaces all but eliminate axial and tangential modes in that direction and also significantly distribute and reduce oblique modes.

It isn't a panacea, but it can be a huge benefit. I wouldn't be afraid of it because it is unusual. One thing to recognize in planning is that the remaining two axial mode directions become effectively more sparse (they don't change, but there are fewer other modes interspersed with those two) and thus dominant. While you have less chance of a tangential and oblique modes summing with an axial mode creating a huge localized problem, there is also less chance of adjacent tangential and oblique modes smoothing response around a dominant axial mode.

The point being, pay very close attention to positioning around the two remaining axial modes and make sure those are optimized as well as is possible. The rest is measure and correct with strategic room treatment and signal shaping after the room is built, which is the same in all cases.

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Thanks. Just to make sure I'm understanding correctly, the 2 axial modes would be front to back and back to front (since the sides aren't parallel)?

If non-parallel walls eliminate a lot of axial and tangential modes, is there some reason why more people don't use them? I was also thinking that having non-parallel walls would be better for the sound, but the fact that I've never seen anyone do this intentionally make me worry that they know something I don't!

dla26 said:

Thanks. Just to make sure I'm understanding correctly, the 2 axial modes would be front to back and back to front (since the sides aren't parallel)?

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The two remaining axial modes are front to back wall, and floor to ceiling.

If non-parallel walls eliminate a lot of axial and tangential modes, is there some reason why more people don't use them? I was also thinking that having non-parallel walls would be better for the sound, but the fact that I've never seen anyone do this intentionally make me worry that they know something I don't!

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A few reasons. Building materials and methods tend to have right angles. It's a bit of a waste of both material and space unless the room on the other side is also going to have an angled wall. Architects don't design theater rooms generally, they design houses with rooms that can fit common uses. HT designers/architects/integrators don't design houses, so they are typically starting with the rooms already there meaning rectangular. A wall can be added as in your case but it does waste a little space. Finally the resulting space is difficult to model, so even the person who might really consider adding such an angled wall (HT architect) may also be a little reluctant to design a space they can't analyze and predict on the front end.

So it can be done, but it isn't common. Sometimes it may follow from a specific homeowner request going for a certain aesthetic. Maybe it is an option considered in ambitious projects.

Big makes a good point that a fabric false wall can make the room visually symmetric and leave the room acoustically right angled. The space behind can put to use for absorption or diffusion as needed. So there is a Sonic benefit to making the wall fabric, and a Sonic benefit (probably) to making the wall solid.

I'm going the route Big suggested for the most part with fabric walls that can hide whatever treatments are needed. If absorption or diffusion isn't needed (diffusion is almost universally a good thing in small residential rooms), that treatment could consist of largish flat panels angled to help break up modes, though that isn't terribly effective as frequency drops.