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Height and width and length ratios to avoid

8951 Views 20 Replies 7 Participants Last post by  gobrigavitch
I have a home theater now that is open on one side. I'm thinking about closing the open side off so I can soundproof the theater. However I want to make sure I don't create some nasty room modes. I remember that I should avoid multiples of each other in the dimensions (ie length 2x width etc.) I remember there were some ratios that are good and some that are bad, but I can't remember what they are. I would appreciate some help. Thank you.
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Don't get too hung up about it. Every room no matter the ratios will have modal issues. You just try to avoid the particularly bad ones which overlap directly or within a multiple of 3 (like an 8' ceiling and 24' length).

And avoid a square room if at all possible, rectangular is much better.
That's good to know. I believe my length will be about 22, my width about 12-13 and my height about 7.5 feet. As long as the length isn't a multiple of the height I assume I'll be OK?
Well, you're still pretty close. 7.5x3-21.5 - very close to 22. Get as much width as you can. 7.5x14x20 would be nice if you can get the extra width.

The 1,68x rule has worked well for me in the past. I once had the opportunity to dimension a room more or less from scratch, and got close to that ratio. Sounded very nice, and had less modal issues than I'm used to.
The golden mean ratio (1.68) was one of the first functions that I found in trying to establish a better overall room. It has merit for certain.


Not specifically the above linked page, but the ratio as a function.

If you plug the room sizes in here:- http://www.bobgolds.com/Mode/RoomModes.htm you will get the main modes and an indication if they are good or bad

For a graphical display of room node ratios check out here:- http://forum.studiotips.com/viewtopic.php?t=3148
Or you could purchase a copy of F Alton Everest, "Master Handbook of Acoustics", and for the really adventurous: http://forum.studiotips.com/viewtopic.php?p=5570

I guess all contemporary roads lead to Eric Desart in this type of discussion.
Or you could purchase a copy of F Alton Everest, "Master Handbook of Acoustics", and for the really adventurous: http://forum.studiotips.com/viewtopic.php?p=5570

I guess all contemporary roads lead to Eric Desart in this type of discussion.
I agree, for the adventurous they are a good source. For most situations Bob Gold will suffice as it allows a quick and easy method to stay away from major room problems.
I have seen a similar link to the one you mention which has a secod "terrain" type plot of Louden values using two decimal places. (pm me if you want me to dig it up)
I played around with room sizes based on these graphs & found the best values are located in areas that do not have a sharp slope nearby and, in hindsight, this makes sense.
I think Bryan said it best for me "Don't get too hung up about it."
It is a good thing to be mindful of for new construction. If you can get a "good" ratio, that is fine.
But at the end of the day, you will most likely have to develop some type of acoustical plan for the woes of any room.
As you're finding out, height is usually the driving factor. Then, you try to get as much width as possible within the shell you have to work with. You can play with the width a bit to help but realistically, in most cases, length is about all you can really adjust much.

You'll find that taking a lot of time and care in using proper seating positions and sub locations will help a ton.

The height and length of my room are pretty much fixed. The three existing walls are all outside walls. In the interest of soundproofing I'm likely going to replace the suspension ceiling with a DD and GreenGlue ceiling and build a wall on the open side. The wall will be between 12 and 13 feet and really can't be moved much in either direction. I may end up with a multiple of 3 with height and length, but I really can't change this without chopping off a chunk of room, and the width I guess shouldn't be too problematic.
I don't have exact measurements so these may not be that helpful. The ceiling is currently a drop ceiling and is around 90inches. It will be 2 or so inches higher then it is now when I change it too a DD and clips and hat channel. The lenght is currently about 22.5 feet, however one end is below a bay window so this lenght is only for about a 6 foot width at the end of the protrusion. Their is a steel support beam along the length of the room that is currently wrapped in drywall. I am going to build a wall under this. That will make the width about 12.5 feet (150-154" depending on construction method). I don't think any of the dimensions are that flexible. I'm hoping the length and height don't enforce each other, but the only way I could change that would be to wall off some of the length and waste it which doesn't seem worth it. I'll have to use treatments to make the best of it.
As others have said, don't sweat it.
Those measurements come out pretty good when you plug them in to http://www.bobgolds.com/Mode/RoomModes.htm

The only thing it points out is that the height to length is a close multiple of 3.
this means you may need to install a bit of treatment in the ceiling area to tame it.
Otherwise it's fine. :clap:
This chart was developed long ago by Bolt of BB&N. It is still valid and can come in handy.

The example 1:1.4:1.9 shown is not the perfect ratio. It is shown as a guide to using the chart.

1, X, Y can represent any major room dimension.

Anyone really wanting to get a grasp on rectangular room modes would do well to study up on work by Bonello and Sepemeyer, as well.

It is quite possible to have a room with good modal distribution and still have problematic response due to placement of loudspeakers and listener(s). Never forget that all acoustical issues conisist of three parts...Source, Path, Receiver. Room mode calculations only result in a partial quantification of Path.
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Absolutely. Excellent point. You can do more to smooth response with speaker and listener locations than most anything else.

So I should be ok with what I have planned. I just may have to look at using bass absorption to tame problem areas once all is finished. I have a general question about what to expect. One side of my room was quite wide open and also unsymmetrical. However I don't think it had first reflections either. When I put a wall ther I will have symmetry, but also more reflection. Without treatments which room would likely have better sound. Should I suspect an improvement or worsening in pretreatment sound?
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The room with the symmetry will be much more coherent sounding. Without the wall, you still have reflections (unless it opens to the outside world) - they are just farther away and take longer to get back to you which skews the soundstage

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