Recording - January 2017 - 8

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Choosing room dimensions: one acoustician's view
Dear Bruce Black: Thanks so much for your help and info
about building Helmholtz Resonators for my new control
room (Talkback, November 2016). I'm building my studio
from the ground up; we have the entire building up and are
currently going to work on the inside, laying out dimensions
for the various rooms before we build. That leads to another
question I hope you can help me with!
I have been reading and reading, trying to get everyone's
thoughts on the best possible control room dimensions. I've
read that 8 foot height is the kiss of death and I've been
encouraged not to do that.
So my thinking was to build a control room that was 9' H
x 14' W x 20' L. I have been reading about golden ratios and
everything. One source I read on the Internet said that this
fits in the golden ratio, but then another place said that those
dimensions would have about 600 standing waves. It suggested instead that a room of 9' H x 13'4" W x 19'8" L would
have only 6 standing waves! What are your thoughts on a
great control room dimension? Thanks,
Chad Grimes

Bruce Black replies:
Hi Chad-Well, you've hit on one of the two most
important things in getting a room with accurate, goodsounding acoustics. But you've also found that there is a
lot of very questionable information on the Internet.
Let's start with a little background. Every single room
has standing waves, also known as room modes-tons of
them. I like to refer to them as resonances because I feel
that most accurately communicates what they are, in a
simple and intuitive way.
For good acoustics, the first of your two important considerations is that you don't want resonances to shift frequency or distort from non-parallel walls or ceilings. That's
right, you want parallel walls, because the issues caused
by parallel walls are fixable, while the damage from nonparallel walls can only be band-aided at best, but never
fixed. Room EQ is a bad way to fix poor acoustics if you
have the option to get them right the first time!
Also, there is no such thing as only 6 standing waves.
By the time you count axial, tangential, and oblique
modes, and include all the audible frequencies, every
room has hundreds, even thousands. The thing is, once
you get to a certain number per octave, there are so
many that are so close together that one resonance's
impact on the overall sound is negligible. This is why
large rooms sound good naturally-you don't have to go
very far up into the audible spectrum to get to where
there is a smooth, pleasing density of resonances.
The reason resonances affect low frequency the most is
because that is where there are the fewest, and you are
much more likely to have isolated resonances. This is

where one resonance is separated from the others by a
particularly large "distance", making it act like a peaking EQ at that frequency.
This leads to the other important thing when considering room dimensions: you want the resonances to be as
evenly distributed as possible in the frequency domain. So
you need to calculate them and then plot them on a frequency graph. Forget Bolt ratios, golden ratios, Fibonacci
numbers, and all the rest. I've tried them, and sometimes
they work, sometimes they don't. I keep finding that the
best, most reliable method of creating good acoustics is
having the resonances evenly distributed in the frequency
domain. Period. The only way to do that is to calculate and
plot them, one set of dimensions at a time. You can set up
a spreadsheet to do the number crunching for you, and
there are calculators available online, too.
I prefer ceilings taller than 8 feet, but they're not the
"kiss of death". You just have to calculate that dimension's resonances along with the others until you get that
desired even distribution of resonances.
There are a couple of other things you can do to insure
good acoustics. One is to install your drywall with the
screws no more than 6 inches apart. It's a lot of screws and
a lot of work, but it'll make your room's sound a lot tighter.
I've seen instances where doing this changed the resonances' Q from 9 to 22. This makes any effects from the
resonances harder to perceive, and tightens up the drywall
to minimize the problems that come from vibrating panels.
Lastly, caulk every gap with an acoustical caulk, and
seal all the wall penetrations (light switches, outlets, the
gap between drywall and the floor that's covered by base
molding, etc.) with either expanding foam or acoustical
caulk. Make sure the door seals tightly, too.
So that's the skinny on resonances, "standing waves",
and all that. Beware of opinions and internet information-a lot of it is going to be misleading or just downright wrong. All the best-BB
Corrections
In our November 2016 issue, we published a review of
the Mojave Audio MA-50 condenser microphone, in
which we listed an erroneous spec. The maximum SPL
handling for this microphone is 140 dB, not 125 dB as
previously reported. Sorry about that.
In our December 2016 Talkback column, we mistakenly
conflated two separate dialogues between Paul Stamler and
readers with questions about his First Steps article on
reverb. The first letter was indeed by Adam Stamper, an
engineer/educator at The Omega Studios School of Applied
Recording Arts and Sciences (omegastudios.com), but the
subsequent letters were actually written by reader Alan
Stewart (another "AS") and should not be attributed to Mr.
Stamper. Really sorry about that.-MM

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