dave weil wrote:

On Thu, 31 Mar 2005 08:07:34 -0700, "Tom" wrote:

"dave weil" wrote

BTW, did you fix the bad ratio of your main room? Unless you raised
the ceiling, I doubt it. You could have used my 9 1/2 foot ceilings.


What do you know about ceiling heights, Dave? I'm building a house
and planning a home theater in the basement. I'm struggling
with whether to spend extra for a 10 ft basement. The room is 12 ft
wide and will be from 17 to 22 ft long.

If you go here (see below), you can see some suggested ratios. If you
will be working with a fixed footprint, then you might need to alter
your ceiling height if you want an optimal ratio. If you are working
with a fixed ceiling height, then you will need to alter your length.
It sounds like you've got enough variation in length to hit one of the
suggested ratios by keeping your ceiling at 10 feet, although it
probably isn't necessary to pay extra for it. I would probably want at
least 9 foot ceilings, simply because I like the extra "space". 10
foot would be even cooler!

As Howard will tell you, you should also pay attention to the
furnishings. Also, if you can arrange it, you might want to toe in the
walls at one end as to break up a pair of parallel walls (and you can
also vary on of the short walls as well). I'm not sure how you would
calculate the optimal angle though. Most acoustic experts recommend
"uneven" walls. It shouldn't be all that difficult to do some drywall
framing that doesn't meet at 90 degree angles, even if it's only 5
degrees off. But I suspect that the more fanning you can do, the
better.

Here's the link:

http://forum.studiotips.com/viewtopic.php?p=5570

And here's a forum post that you might pay attention to:

http://hometheaterhifi.com/forum/arc...php/t-436.html

I can't find the webpage, so I will be forced to draw it pretty
crappy, but I recall this being the optimal shape for a dedicated
listening room, stereo or home theater. The M's are mains, C is
center, S's are surrounds, R's are rears, = is listener. Again, this
drawing is going to be off, but I think you can get the idea. The
shape is a hexagon, the rear wall is wider than the front wall. The
surrounds and listening position shouldn't be that close to where the
walls slant back inward, they should be up closer to the front, but I
only have so much control with this. Distance between each main to you
should be the same as the distance between the center and you, so the
center should be a little behind the mains considering you have the
mains 30 degrees off of you. You would want something like thick
drapes on the front wall to absord any reflections and pillars on the
side walls to break them up even more. Of all the home theaters I have
seen, this one comes the closest.

http://f1.pg.photos.yahoo.com/ph/ric...rc=ph&store=&a

Hope this helps.

.............._________
............./..............\
............/...M..C..M...\
.........../..................\
........../....................\
........./.S........=.......S.\
.........\....................../
...........\____R__R___./

EDIT: For reference, I read about this from an acclaimed acoustic
engineer who has published books on the matter, but it was a while ago
and for the life of me I can't remember his name or the website, I
will look for it though
----------
It's been said that Floyd O'Toole has "debunked" room ratios, but it's
clear that different ratios offer different room modes. He tacitly
accepts this with his "room calculator", which demands that you put in
room dimensions, admitting that they make a difference. Sure, the
room treatments that you put in can turn a sow's ear into a silk
purse, and vice versa, but why not start with a ratio that is already
optimized to cut down on unwanted room modes? That way, you reduce the
need for extensive room treatments, especially if you're building a
brand new room? O'Toole's advice is comfort for those working with
pre-existing rooms though.

Here's another bit of opinion about non-parallel walls worth
digesting, this time from Paul White a Sound On Sound:

It is a common misconception that building non-parallel walls will
improve the standing wave situation. In practice, this has minimal
effects at low frequencies: the low frequency modes will develop much
as before based on the mean distance between walls.
What is true is that splaying the walls by as little as 1:10 or even
1:20 will help reduce high-frequency flutter echoes caused by mid- and
high-frequency sounds bouncing between two facing walls or floor and
ceiling. However, this particular problem is solved even more easily
in most rooms with parallel walls by using small areas of acoustically
absorbent material.

You might want to read the full page:

http://www.soundonsound.com/sos/jul9...coustics1.html

And finally, O'Toole recommends tossing third octive eqs as a room
equalization tool (from what I've read). I'm sure that Howard simply
blocks stuff like that out...

Aside from this last paragraph, your comments and the
references are really quite OK. The problem is that the
ratios do not take into consideration wall flexing or the
amount of bulky items lining the walls, perhaps half-way to
the ceiling - such as full bookcases. When you factor in
those items all bets are off.

Regarding 1/3-octave analysis, here are two observations:

First, while he prefers resolution measurements out to 1/10
octave for speaker reviewing and design, Toole is hair
splitting when we get down to analyzing home listening
areas. For the most part, 1/3-octave analysis will do the
trick with typical home-listening room analyses.

Second, assuming Toole is correct about the need for
1/10-octave analysis (and I am not sure of this, but I will
cut him some slack for your sake), I can still assure you
that 1/3-octave analysis is still superior to NO analysis at
all, Dave.

PS: his name is not O'Toole.

Howard Ferstler