Most people who read a lot, maybe
even, read a little too much, begin to design their home theater
by trying to figure out what their room ratios should be.
Room ratios are magic numbers that are supposed to proportion
the room to make it sound great. But there’s a problem
with room ratios. They were originally developed for engineers
who had to design reverb test chambers. By itself, that doesn’t
seem like much of a problem. After all, reverb chambers have
always been used for testing of acoustical materials. To get
the job done, they were designed to create the most uniform
sound field possible. That way, it wouldn’t matter which
part of which room a test was done, the results would always
be the same.
The acoustic engineering that
went into reverb chamber design is about 80 years old, just
about as old as audio. This reverb test chamber theory may
or may not be a good way to design your home theater, but
it certainly is the reason behind the quest for the original
mode-perfect room ratio. By the way, the room ratio that has
the top “acoustic quality” is 1.0 to 1.9 to 1.4.
if you ever get to step inside one of those reverb chambers,
or even look at some pictures of it, you will notice three
things. The marble walled room has a reverb time of about
15 seconds. The speaker position is always in one of the tri
corners of the room. And the “listening position”
is where the microphone is located, always in another tri
corner of the room. Only when the speaker is in a tri corner
can all the room modes be generated and only when the listening
position, the microphone, is in a tri corner will all the
modes be heard.
The perfect reverb chamber may
sport the most uniform distribution of modes possible but
it doesn’t sound musical inside, it doesn’t even
sound good. It’s horrible to be inside of one. It’s
an acoustic torture chamber, not a listening room. But, why
then is so much attention being paid to room modes these days?
Most every home theater magazine runs an article in most every
issue on room modes. Can everybody be wrong? Well, yes and
do play a part in room acoustics. Problem is that mode acoustics
is not as simple as the magazines make it out to be. For example:
If you play a very short tone in a room, just a toot, you
hear just a toot. You won’t hear “room modes”.
The toot started and stopped before the room mode ever got
built up. It
takes time to build modes in room. A mode is not built instantly.
Before the mode is built up, the sound from the speaker sounds
just fine. Many sounds in movies and music are so short lived
that they never create a standing wave, otherwise known as
takes time to build up and create a mode, typically about
1 second. This means that the tone has to be held steady for
at least one second in order to create a mode. Actually the
amount of time it takes to create a mode is exactly equal
to the reverb time, the RT-60 for the decay of the mode. If
you like listening to a lot of slow paced organ music, you
probably will care a lot about room modes. But if you listen
to most music, tones aren’t held long enough to even
and real rooms
If you design a room based on
the ideal “mode distribution ” formula, then you
will have a room that has its resonant modes uniformly spaces
along the frequency scale. And what’s more wonderful
is that you can actually hear all this wonder only when your
loudspeaker is stuffed into one tri corner of the room and
you stand on a ladder with your head is stuffed into another
tri corner of the room.
happens if you pull the speaker out of the corner and set
it up out on the floor, away from the walls, as if it was
a normal speaker and then you go back to the corner and listen?
You hear only some of the modes, a subset of all the modes.
The speaker can’t stimulate all the modes when it is
not in the tri corners of the room. It can only stimulate
modes that happen to have pressure zones located where the
speaker is located.
Next push the speaker back into
the corner of the room. Instead of standing on the ladder
in the corner, go sit in your favorite listening chair out
in the middle of the room. Now you’ll hear a fraction
of all the modes that have been generated, a subset of the
modes. The only way you can hear all the modes is to have
your ear placed in the tri corner of the room, where all modes
have pressure zones. This is why corner loaded bass traps
(TubeTraps) work so well, they’re always in the pressure
zones for every mode.
What happens if you also pull
the speaker out into the room? Now the speaker generated a
subset of all the modes, depending on where it is located,
and you will hear a subset of that subset, depending on where
your chair is located. Normal speaker locations and normal
listening positions end up generating just a fraction of all
the possible modes for the listener. And the million-dollar
question is: Which modes do you hear and how are well spaced
are those modes?
The over simplistic, “ideal”
room mode calculations are pretty easy to set up on a computer
and print out. But move the speaker out into the room and
it generates a subset of the room modes. But what subset?
Move your chair into a practical position and you can only
hear a subset of that subset. But which subset of what subset?
Just try to make that calculation. If you do, it won’t
be pretty and it’s guaranteed, you won’t find
it in any of those room mode magazine articles.
room modes but mode free speaker positions
you can’t use modes for your room design criteria, what
can you do? If you are still hooked on modes, there’s
hope. What we want a loudspeaker to do in a hifi listening
environment is to not stimulate any modes. We don’t
want to listen to modes being built and decaying, we want
to listen to sounds and music. If at all possible, we want
to locate speakers in positions where they cannot stimulate
any modes. When a speaker is played outside, it does not stimulate
any modes. When the frequency response for a speaker is tested,
it is in a mode free environment, either outside or in an
anechoic (without echo) chamber. When we want to hear the
speaker, it would be nice if we could hear it in a mode free
room. But the bad news is: If it’s a room, in that it
has a floor, walls and a ceiling, no matter what the shape,
it has modes. The good news is that we can locate a speaker
in a fairly mode free location in a room.
Generally speaking, we need
to locate a speaker at the 29% position in a room. This means
29% of the ceiling height off the floor, 29% of the width
off the sidewalls and 29% of the length off the end wall.
The position of 29% is very close to the minimum room mode
coupling location between any set of parallel surfaces. So,
how we design a room is to figure out where we want the speakers
first and then position the walls so that the woofer is located
29% off each of the three room dimensions. For example, if
the left and right mains are 6.5 feet apart, this corresponds
to 42% of the room’s width. The width of the room should
be 15.5’ wide. If the room is 19’ long, the speaker
fronts should be 29% of 19’ or 5.5’ off the front
control the room layout
of the biggest problems for home theater is that the surround
speakers are always too close to the listeners. The major
reason for this is that people are always trying to make rectangular
listening rooms and they place the long dimension front to
back and the narrow dimension side to side. They may be sitting
8 to 10’ back from the mains but will be lucky if they
are even as much as 6’ from the surrounds. Their electronic
buddies sell them adjustable time delays, so the time of arrival
is the same. But that’s not the issue at all. What the
issue is really is how close they are sitting to the speaker.
They are so close they can tell where the speaker is located.
hear the wavefront. It is an expanding sphere for a typical
dynamic speaker. People can tell where a sound is by how curved
the wave front happens to be. That’s one reason dipole
speakers and line source speakers are very interesting to
listen to, because they have a cylinder shaped expanding wavefront,
horizontally curved but vertically flat.
Rotate the room. Play the home
theater room sideways. Width is what these rooms need more
than length. Put your couch a little off the back wall and
your speakers 29% off the front wall. Put your surrounds up
high, in front of and near the two back corners. Now everyone
gets to listen to the surrounds without anyone in particular
being overblown. Some times you’re going to be stuck
in a nearly square room that isn’t really very big.
Play the room along the diagonal. Put your couch in the middle
of the room, slightly back from center. Position the surrounds
out from but near the two side corners.
rest of the story
modes, room orientation, speaker, screen and seating positions
are all important. But so is how the room is built. Home theaters
have so much subwoofer energy that when played in a normal
house, the whole house shakes. A shaking house cracks plaster,
rattles dishes off the plate rail and keeps certain members
of the household up past their bedtime. Soundproofing is helpful
to some degree. However, standard soundproofing was only developed
for voice range and voice power. Architectural soundproofing
is about speech privacy, not subwoofers. Standard soundproofing
methods won’t work well for subwoofers. Soundproofing
that uses structural damping materials works better. Standard
“room within a room” soundproofing leaves the
interior surface of the room completely free to quiver, shake
and thunder. It is like listening to music while sitting inside
an acoustic guitar. That’s not how it’s supposed
Build in structural damping materials
into your soundproofing project and you’ll be very happy.
Add corner bass traps and wall mounted treble absorption/diffusion
and you’ll be sitting high, somewhere in movie heaven.
Copyright 2009, Acoustic Sciences
Corp. This article was written by Arthur Noxon, a licensed Acoustical
Engineer. He is president of Acoustic Sciences Corp and the
inventor of the original corner loaded bass trap, the TubeTrap.
He has over 35 years professional experience in acoustics, with
the last 20 years specializing in blending the art and science
of both acoustics and psychoacoustics into the listening experience.
He may be reached at [email protected] or by phone: 1-800-ASC-TUBE.