## Odd Room Calculations

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### Odd Room Calculations

From: Jon Best <jrbest@x???xx.xxxx
Date: Wed Dec 1, 1999 3:25 pm
Subject: Odd Room Calculations

Hey- I'm building a new control room, and it's got a peaked ceiling. Now,
there are
acoustics calculators out there you can use to figure out your potential mode
stackups or lack thereof if the room is a six sided box, but does anyone know
of any
way of doing this with a room that's not exactly rectangular? The modal spread
looks
really good when I run the numbers with an averaged (by volume) ceiling height,
which
is the closest way I can think of to do it. Any ideas? Thanks!

Jon Best
Sales Weasel From Mars
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From: "CactusFire (A.S.)" <cactusfire@x??????xxxxx.xxxx
Date: Wed Dec 1, 1999 7:36 pm
Subject: Re: Odd Room Calculations

Jon, I'm doing exactly the same thing right now. I was told by several people
that this
is the best way to do it, just average the height (mine is 13 feet at the
lowest, and 16
at the highest, so 14.5 is the average), but I think that things can get a
little
strange is the angle is pretty steep.

I am still trying to get a handle on it, but here are a few things I have

The bass is going to compress into the corner (apex), and the inner ceiling is
two
layers of drywall. Since it is next to impossible to get a really good barrier
at the
apex, I imagine that this is going to be somewhat of a low-end absorber,
weakening some
of the vertical modes. Since there is 4 feet of insulation directly above the
apex, I am
wondering if this won't turn out to be a really powerful (perhaps too much so)
bass
trap...?

If you are framing the vertical walls and if the vertical walls are splayed
horizontally, cutting all those compound miters at the top of each stud will be
quite an
experience. Also, if the walls are splayed, the cuts on the ceiling drywall (if
you put
in a suspended ceiling within the room) will take some concentration.

Good luck, and keep us posted,

Andy
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From: SRF7@x??x.xxx
Date: Thu Dec 2, 1999 3:23 am
Subject: Re: Odd Room Calculations

Well ... doing axial mode calcs off the average height would seem pointless
given the nonparallel nature of the ceiling... of course room volume derived
from average ceiling height would still be valid for calculating critical
distance. I'd suggest running your room mode calcs using only the vertical
wall axial modes.

If you take steps to reduce the strength of ceiling to floor modes altogether
by using the ceiling as the primary location for you absorbtive treatments
(with particular focus on the ceiling peak) and double pad and thick
carpeting the floor, you might get away with initially ignoring their
import. Bare minimum I'd suggest a nice thick (4" to 6") 24" wide piece of
703 along the run of the peak, thereby creating a variable depth cavity in
lieu of a reflective long "corner." Maybe augment the treatment of the
ceiling peak with a random array of 703 "clouds" suspended from the ceiling
(you could probably damn near double the flush mount Sabin number for these
elements [if they are thick and hung low enough off the ceiling] as both
sides of the panel will be exposed to sound waves).

If you are digging this so far... then treat the actual corners of the room
in similar fashion (703 across the corner to form a variable depth cavity).
At this point your prior analysis using solely the axials between the two
pairs of vertical walls should start to become a rational rough estimate of
the rooms most troublesome modes. Such additional absorptive treatment as
called for by your calculations might best be effected by 703 panels along
the run of the wall interrupted by reflective (and some diffusive) elements
such that the reflective are in large part opposite absorptive elements on
the opposite wall.

Good Luck.. lets us know what works.

Scott R. Foster
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From: perrault@x????xxx.xxxxxxxxxxxxxxxxxxx)
Date: Thu Dec 2, 1999 12:13 pm
Subject: Re: Odd Room Calculations

I was trying to solve the resonance problems in a similar room a
couple of years ago. In my case the room was quite small, had peaked
ceilings, with the slope running along the long edge.

To make a long story short.... I felt that the floor-to-peak dimension
gave the most sensible explanation for the problem I was having - it
was interacting with the width and causing some problems.

FWIW, this rather small room had surprisingly good acoustics for a
small space. Due, I think to the bass absorbing qualities of the
plaster-lath construction and the peaked ceiling.

DP
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From: "Mucha, Kevin S (Kevin)" <kmucha@x???xx.xxxx
Date: Thu Dec 2, 1999 3:42 pm
Subject: RE: Odd Room Calculations

I'm also facing a peaked roof situation for my control room and tracking
room. 9 foot walls to a height of 13 at the peak. I have collar ties
(rafters) that run a span at about a 10 1/2 foot height. I plan to make a
panel resonator by attaching either masonite or plywood on the bottom of the
collar ties thus forming a triangle with the peak of the roof. Inside the
triangle I am putting 4" of 703 on all faces. In the control room directly
behind the mixing position is a corner of the room. Not optimal at all but
I had to make do with the space that I had and this was the best option
(unique layout). I am building a corner bass trap/diffuser out of a
hemisphere of masonite bowed and suspended on the vertical edges. The
structure I am building in is 19x23 overall. The center wall for the rooms
also follows a unique shape to allow the most space for each room possible.
I'll try to get a drawing uploaded to the site once I can. Curious as to
any comments that anyone may have. This is an excellent forum with great

Kevin S. Mucha
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From: Jon Best <jrbest@x???xx.xxxx
Date: Thu Dec 2, 1999 4:05 pm
Subject: Re: Odd Room Calculations

>
> If you take steps to reduce the strength of ceiling to floor modes altogether
> by using the ceiling as the primary location for you absorbtive treatments
> (with particular focus on the ceiling peak) and double pad and thick
> carpeting the floor, you might get away with initially ignoring their
> import. Bare minimum I'd suggest a nice thick (4" to 6") 24" wide piece of
> 703 along the run of the peak, thereby creating a variable depth cavity in
> lieu of a reflective long "corner."

This is actually what I was planning on doing anyway- does anyone know if
shutting off a
corner with absorption is more or less effective with one end open? I was
lining all three sides of this cavity with 4-6" 703 and stopping 8 to 10 inches
off the
front wall, as it struck me as a nice way of getting some low end absorption as
well as a
nice high-volume/low velocity HVAC return. I was also going to do this with
four 8x12ish
box shaped duct/traps running from four ceiling vents down to within a foot or
so of the
floor as the HVAC send. Any comments? BTW, room is roughly 15.5' x 19.5' x
12' peak.

Jon Best
Sales Weasel From Mars
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From: SRF7@x??x.xxx
Date: Thu Dec 2, 1999 7:03 pm
Subject: Re: Odd Room Calculations

In a message dated 12/2/99 11:05:04 AM Eastern Standard Time,
kmucha@l... writes:

> I'll try to get a drawing uploaded to the site once I can. Curious as to
> any comments that anyone may have. This is an excellent forum with great
>
>
>
> Kevin S. Mucha

Do we have a digital drafting standard we can all agree on? This might
simplify things if we can identify a common format, or even better an URL to
a simple freeware/shareware drawing program. I have ACAD 14, and can export
bmp's, gif's etc., but I realize that this just isn't going to work for
making revisions to scaled drawings without something simple for everybody to
use... MS Paintbrush would work I guess... anybody got a better idea?

Scott R. Foster
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From: SRF7@x??x.xxx
Date: Thu Dec 2, 1999 7:48 pm
Subject: Re: Re: Odd Room Calculations

In a message dated 12/2/99 11:40:03 AM Eastern Standard Time,
jrbest@e... writes:

> This is actually what I was planning on doing anyway- does anyone know if
> shutting off a
> corner with absorption is more or less effective with one end open? I was
> lining all three sides of this cavity with 4-6" 703 and stopping 8 to 10
> inches off the
> front wall, as it struck me as a nice way of getting some low end
absorption

If open, then I'd reckon it'd absorb slightly better at the resonate freq.s
of the cavity... but if you are using 6" 703 with a maximum cavity depth of
12"s you are doing pretty good on the low end even if you skip treatment on
the other two side of the triangular cavity.

> as well as a
> nice high-volume/low velocity HVAC return.

Neat idea, but If you use three 24" wide 6" thick pieces of 703 to make a
triangle, the hole in the middle aint very big... about 12 sq. inches (3" x
4" duct)... If you use 4" it'd be about 40 sq."s which is still too small for
a return register (5" x 8") for your room... If you skip the interior 703 the
return might still be too small, even given that it will draw from both sides
(144 sq."s per side = a 2 sq.' total area)... You could do some light framing
to increase the width of the base, but seems like a lot of work... why not
just oversize the flexi-duct in the attic and run it in a big ess curve along
the way.... that might cure the problem without any special ceiling fixtures.

Alternatively you could hang the 703 along the run of the peak down a
distance of 16"... given a 12-12 ceiling slope this would give a 4" gap all
the way around the "cloud" which would be ample for air flow while retaining
most of your ceiling height. Pick a nice fabric to upholster the panels with
and it's look pretty cool as well.

>I was also going to do this with
> four 8x12ish
> box shaped duct/traps running from four ceiling vents down to within a
foot
> or so of the
> floor as the HVAC send. Any comments? BTW, room is roughly 15.5' x 19.5'
x
> 12' peak.

Maybe just oversize the flexi-duct and run in an ess in the attic instead...
though I was thinking of running flexi-duct down one corner (covered by a 703
panel) so I could blow cold air on the back of my rack and PC.

> Jon Best
> Sales Weasel From Mars
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From: Dan Nelson <dprimary@x??????xxxxx.xxxx
Date: Fri Dec 3, 1999 5:04 am
Subject: Re: Odd Room Calculations

I would add that the walls that run up to the peak have greater area to support
its modes so they could be the strongest. You may also want to calculate the
axial modes from the peak to the floor - the lowest point of the ceiling to the
floor and maybe a few points in between. To get an idea of the range of modes
and to see if any of those will overlap the wall axial modes.
If the peak height is only a few feet higher than the sides I might just average
it.

The second problem with a peaked ceiling is it is going to reflect back into the
mix area so you will want to do more treatment on the ceiling than on a flat
ceiling or an expansion ceiling.

You should be able to treat any mode and reflection problems, its just a little
more work then with a flat ceiling, but at least you shouldn't have to worry
about any flutter echoes off the ceiling.

Dan Nelson
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From: Brian Cassell <bcc2a@x???xx.xxxx.xxxx
Date: Fri Dec 3, 1999 6:03 am
Subject: Re: Drawing formats was Odd Room Calculations

I don't know of any way to save files as a gif directly from ACAD. I'm
referencing ver. 14.01 if it makes any difference. I think it'd be
appreciated by those of us who have it if you do post in dxf as well as
gif. Thanks,
-Brian
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From: perrault@x????xxx.xxxxxxxxxxxxxxxxxxx)
Date: Sat Dec 4, 1999 11:30 pm
Subject: Re: Odd Room Calculations

>,,,,You may also want to calculate the axial modes from the peak to the floor
- the lowest point of the
>ceiling to the floor and maybe a few points in between. To get an idea of the
range of modes
>and to see if any of those will overlap the wall axial modes.,,,,,,
>Dan Nelson

Don't you think applying axial mode calculations to these dimensions
would be a bit misleading? What I mean is that it would seem to me
that any dimensions from the middle of the slope would be better
handled by tangential mode analysis.

I think the only relevant vertical dimension in a peaked ceiling room
would be the floor-peak dimension. Granted the geometry of a peaked
ceiling could probably be analyzed, but not with typical axial mode
expressions.

That said, there is probably an equally valid point to be made for
considering just how little profile (area) the peak has. Meaning, the
peak-floor dimension is probably not all that relevant from an axial
mode standpoint anyway.

DP
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From: Dan Nelson <dprimary@e...>
Date: Sat Dec 4, 1999 8:54 pm
Subject: Re: Odd Room Calculations

Well they would still be axial modes, But I would treat them as a tangential or
oblique mode as the don't
have much to support the modes, maybe I should have been a little clearer on
what to do with them. So I
still think it would be helpful to look at them to see if any of them overlap
the true axial modes.
Say you have an axial mode 65 hz and then the peak has a mode at 60 hz as you
go down the ceiling you will
have a mode at 65 hz. This will cause the axial modes to have a greater effect
than expected. The slope
will cause the modes to be spread across a range. I think it is better to take
this range into account
rather then leaving the ceiling modes out of the calculations.

Dan Nelson
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From: Jon Best <jrbest@e...>
Date: Sun Dec 5, 1999 2:01 am
Subject: Re: Odd Room Calculations

>
> Message: 4
> Date: Thu, 2 Dec 1999 14:48:01 EST
> From: SRF7@a...
> Subject: Re: Re: Odd Room Calculations

<trim>

> Neat idea, but If you use three 24" wide 6" thick pieces of 703 to make a
> triangle, the hole in the middle aint very big... about 12 sq. inches (3" x
> 4" duct)... If you use 4" it'd be about 40 sq."s which is still too small for
> a return register (5" x 8") for your room... If you skip the interior 703 the
> return might still be too small, even given that it will draw from both sides
> (144 sq."s per side = a 2 sq.' total area)... You could do some light framing
> to increase the width of the base, but seems like a lot of work... why not
> just oversize the flexi-duct in the attic and run it in a big ess curve along
> the way.... that might cure the problem without any special ceiling fixtures.

Yeah, good points. The angles up to the peak are both 30 degrees, and I was
really thinking
of a wider base- if I used 2" on top, and a 4' piece on the bottom (have to
figure the
hanging out, no big deal), that should give me roughly 250ish square inches.
I've got a
really limited attic space, and while I am going to use some piled up flex
duct, I figured a
bigger, absorptive passage would finish off the quieting job.

<snip>

> Maybe just oversize the flexi-duct and run in an ess in the attic instead...
> though I was thinking of running flexi-duct down one corner (covered by a 703
> panel) so I could blow cold air on the back of my rack and PC.

That's a good idea, until winter and you're using the heat... :)

I was thinking the same thing in my back corners, but *not* using the
flexiduct, just the
corner blocked off. More damping of sound from the HVAC.

Here's a totally unrelated idea. I was just thinking about my staggered stud
walls, and the
fact that it's hard to block between the studs with anything thick, because of
the lack of
clearance between one set of studs and the other. Then I started thinking
that every construction site has some kind of scrap OSB, plywood, MDF, etc.
lying around,
which is pretty stiff laterally. Wouldn't it help in terms of stiffness if you
set a table
saw to rip all your scrap to about 2", and just ran 2 to 7' lengths of whatever
you've got up

Jon Best
Sales Weasel From Mars
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From: SRF7@a...
Date: Sun Dec 5, 1999 6:20 am
Subject: Re: Re: Odd Room Calculations

In a message dated 12/4/99 9:02:04 PM Eastern Standard Time, jrbest@e...
writes:

SRF7: previously

>> Maybe just oversize the flexi-duct and run in an ess in the attic
>> though I was thinking of running flexi-duct down one corner (covered by a
703
>> panel) so I could blow cold air on the back of my rack and PC.

>That's a good idea, until winter and you're using the heat... :)

That doesn't happen here (NE Florida) .... not in a room full of glowing
tubes and no windows.

snip

>Here's a totally unrelated idea. I was just thinking about my staggered
stud walls, and the
>fact that it's hard to block between the studs with anything thick, because
of the lack of
> clearance between one set of studs and the other. Then I started thinking
> that every construction site has some kind of scrap OSB, plywood, MDF, etc.
lying around,
> which is pretty stiff laterally. Wouldn't it help in terms of stiffness if
you set a table
> saw to rip all your scrap to about 2", and just ran 2 to 7' lengths of
whatever you've got up
> the sides of your studs?

Are you trying to isolate the cells demised by each set of studs? if so, why?

If the intent is to dampen cavity resonances, maybe just snake thin
fiberglass batts horizontally through the offset studs ... when you put the
drywall up it will compress a bit at each offset stud. That would dampen
cavity resonances ... it would also isolate the cells I guess, but why worry

Scott R. Foster
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From: Jon Best <jrbest@x???xx.xxxx
Date: Sun Dec 5, 1999 1:34 pm
Subject: Re: Digest Number 35

Nah, just thinking that A) stiffer is better, wall-wise, and B) it seems like a
decent way of stiffening the walls and using all the scrap laying around. Just
running (3", not 2") strips up the studs should
reinforce the wall some, and perhaps make it more effective at blocking lower
frequencies. Probably worth the couple of hours and no materials investment!

Jon Best
Sales Weasel From Mars
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From: SRF7@x??x.xxx
Date: Sun Dec 5, 1999 5:05 pm
Subject: Re: Digest Number 35

In a message dated Sun, 5 Dec 1999 8:34:47 AM Eastern Standard Time, Jon Best
<jrbest@e...> writes:

> From: Jon Best <jrbest@e...>
>
> Nah, just thinking that A) stiffer is better, wall->wise, and B) it seems
like a decent way of stiffening >the walls and using all the scrap laying
around. Just >running (3", not 2") strips up the studs should
>reinforce the wall some, and perhaps make it more >effective at blocking lower
frequencies. Probably >worth the couple of hours and no materials investment!
>

I'm lost... you'd be running the OSB (or whatever) rips parallel to each stud
to make them stiffer??? is this needed, helpful? What am I missing? Wouldn't
a stiffer wall be more reflective (less likely to absorb low end)?

To be clear, you are not proposoing connecting one side of the drywall to other
on the same studs by the addition of rips of panel scraps are you?? If you did
that it would of course make the whole double stud thing pointless since the
idea is to isolate the two sides of drywall (except at the floor plate and top
plate).

If additional stiffness of the stud members is your goal, I would point out
that an 8" O.C. 2x4 wall (offset so that each side is really 16" O.C.) with 2x6
top and floor plates is already a fairly stiff frame without modification, even
more so once the drywall is up, and if you upgrade to 3/4" drywall on one side
(I assume you were planning on 5/8")it'd be stiffer still, more importantly,
this would increase the STC for both reason of greater density (mass per SF of
wall) due to the increased weight of 3/4" over 5/8", but will also reduce sound
transfer because of the dissimilarity of resonances between the two weights of
drywall.

I suggest that cavity resonance is more of a concern (than stud stiffness), and
best dealt with a little of the pink stuff (noooo not that pink stuff ...
fiberglass ... get your mind out of the gutter).

You don't need to stuff the voids full, a thin layer would do the trick ...
save the money for 703 panels. My \$0.02 worth

Scott R. Foster
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From: Dan Nelson <dprimary@e...>
Date: Sun Dec 5, 1999 5:33 pm
Subject: Wall stiffness /was digest 35

Fletcher from RAP mentioned running perpendicular 2 by 4 at different heights
between the vertical studs to give different cavity resonances (much like a
fire stop) I have heard of other people doing this as
well. I think this would be more effective at making the wall stiffer.

Dan Nelson
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From: Jon Best <jrbest@x???xx.xxxx
Date: Mon Dec 6, 1999 12:54 pm
Subject: Re: Digest Number 36

Yeah, running it alongside various studs to stiffen them individually. That
would affect cavity resonance as well, wouldn't it? Mostly just idle thought
about all the plywood and OSB scrap I've produced along the way. As far as
mass, I've got 3/4" plywood on the outside wall (matches the existing part of
the building), and for the inside I'm probably going to have 3/4" MDF with 1/2"
or so drywall as a finish layer. R19 in there as well- cavity resonance as
well as the fact that it's 3 outside walls...

Doesn't 'less stiff = more likely to absorb low end' also kind of mean 'less
stiff= more likely to transfer low end through the wall?'

Sorry if I'm confusing- I tend to ramble occasionally.... :)

Jon Best
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From: SRF7@x??x.xxx
Date: Mon Dec 6, 1999 11:01 pm
Subject: Re: Digest Number 36

In a message dated 12/6/99 7:54:30 AM Eastern Standard Time, jrbest@e...
writes:

> Doesn't 'less stiff = more likely to absorb low end' also kind of mean
'less
> stiff= more likely to transfer low end through the wall?'

Yes I would think so .... but only to the extent the two walls are coupled
(either mechanically, or through the vibration of the cavity's air mass.
Given double studs, interior fiberglass batts, and the fairly high mass per
SF of the two wall coverings, I'd think that the two sides would be largly
independant (low transference).

Dan makes an interesting point about making the cells defined by the studs
different sizes by installing horizontal members to break the cells up into
different sizes, so that such interior air mass vibration that does occurs
will be spread across differing freq.s ... this seems rational, but I wonder
how much difference it would make in the system's STC vs. the trouble
involved.

I bet the system you have designed is already pushing 65

FWIW here's a nutty one. Rip the material to 6" and hang it between the
studs as free floating interior panel absorbers

Scott R. Foster
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