3rd wall and ceiling question

[byrd62au]

I'm building a control room. 2 of the dimensions (length height) are dictated by the space I have but I have the luxury of adding the last wall which will determine the width. My decision is complicated by the fact that currently the ceiling slopes along the width axis.
I have a length of 3250 mm. The height varies from 2300 at the wall to 2550 at the midpoint of the space where I'm going to put the 3rd wall. My partner doing the building says it's easy for him to make the ceiling horizontal from the lowpoint but I"m wondering
if it will help the sound of the room if I leave the slope there or go with a flat roof?
In either case what would be a good width considering the other 2 dimensions? I guess I can't decide on the wall location till I decide on whether to keep the slope.

This is a great community, I appreciate anyone taking the time to answer.
Rob

[jazzymike]

Hi Rob, I'm by no means an expert, but just a thought:
why not work with the slope(height) as front wall (width becomes length and vice versa).
Then determine using something like the ratios described here:
http://www.silcom.com/~aludwig/Room_acoustics.html

or this calc:
Home Theater Room Dimension Formulas & Calculator

the best possible ratios for the width(length) respective your current length(width) x height
to avoid certain modes/peaks.
And when you've already the luxury to build the rear wall. Why not choose materials/finish the wall with best possible low absorption in mind.
I'm sure you will find loads of data on what materials to use in the other forum section or google.

just my2c..

[byrd62au]

Thanks for your reply jazzymike.
Because of the limited space available, if I was to take your suggestion, the length of the room would be the shorter than the width. For some reason I didn't think was desirable?
Presuming it's ok do I use the low point or high point of the ceiling for the calculation. That's the part I'm finding confusing.

Thanks for the links! I can't get the calculator on the home theatre site to work with Safari on Mac so I'm trying to dust off my brain to cope with spreadsheets. I"m drawing a blank on how to work out ratios from my dimensions. My left brain is very rusty! I love geeking with computers and audio gear but construction does my head in.

[jazzymike]

indeed it would be undesirable to have the length shorter than the width.
You'd have to ask yourself what the maximum distance from current short wall you could achieve with the fake wall.

The ratio calcs are done the following way. height = 1 (or constant 'C')
Then width = 1.14 x C, length = 1.39 x C

1.14 and 1.39 are just multiples of your given height.
There are other ratios, just see which one comes closest to your given dimensions. As to what slope value to use for height, that depends on where you place the 3rd wall, and whether the 3rd wall establishes room length.

The benefits of rear wall absorption are clear I think, but in case you'll have to go with the 3rd wall being one of the side walls you could maybe build it assymetrical to the other side wall. However that's where things like absorption symmetry comes in (f.e. "fake" side wall is treated to the max, how to determine amount of needed absorption/reflection for "real" opposite wall) and that's where my knowledge ends.

[byrd62au]

So if the 3rd wall is to is to determine width and we put in a false ceiling at the low height of 2300 I then have 2 dimensions.
H= 2300
L = 3350

I'm embarrassed to admit that my maths isn't even good enough to work out what the 3rd should be with the given data

[byrd62au]

Although from the spreadsheet if I divide the length by the height I get 1.41 which means I can use 1 1.18 1.41 ratio so my width is 2300 X 1.18 = 2714

So
H = 2300
L = 3250
H = 2714

As to whether to keep the slope in the roof or build the 3 wall asymmetrical that's another question.

[jazzymike]

1.39 / 1.14 = 1.219298245614
3350 / 1.219298245614 = 2747.4820143886


again don't forget, that's just ratios. there are many different ratios, many formulas, and all of them are applicable to rigid surfaces. When a new "fake" wall comes into play with absorption and what not, it gets complicated and far beyond my knowledge. Also closing the slope on one side and treating it means you'Re gonna have to compensate for the other rigid side.

I'd really suggest pro consultation here, since the choices and possibilited you have now might outweigh the limitations you're gonna find yourself in once the structure is completed.

[Græmatter Audio]

I'd say that given the constraints of the existing dimensions your best bet is to make your 3rd dimension 16ft (approx 4.9m) Make this your length, and make the ceiling flat. With a ratio of 1:1.4:2.1 you'll have decent (for a small room) modal distribution ... assuming rigid boundaries - and this arrangement will enable listener placement at least 10ft (3m) from the rear wall. First-reflection absorbtion/bass trapping, etc. etc. and good times will be had by all (or at least by whoever gets to sit in the miniature sweet spot.)

G.

[byrd62au]

Thanks guys, I'll check today whether we have enough room to make the 3rd wall the long one.
Cheers
Rob