A U D I O... P A P E R
ROOM TREATMENTS - WHERE DO I START?
by Steve Deckert
Dimensionally there are good rooms and there
are bad rooms. Few rooms are dimensionally perfect unless built
or modified to comply with the following formulas:
If your room doesn't come close to one of
these three ideal formulas, welcome to the real world.
I need to stop right here and mention that IF
it is possible to move a wall or change a ceiling height so that your
room hits one of these formulas, do it. Many rooms can be
made smaller to hit one of these formulas.
There are two forms of room treatments.
A) absorption and B) diffusion. You will need the
correct balance of both to treat any room.
Assuming you have hi-end gear you will know
if you need room treatment by how long it takes you to get tired of
listening to it. If you get tired of listening at all, you
probably need room treatments.
Two things are effected by the acoustics of
any room. A) frequency balance and B) sound stage resolution.
It is important to work on A) frequency balance, FIRST. It
is common for any room to create holes and peaks in the frequency
balance for any given place in the room. The most easy to notice
area is low frequency response.
Since low frequencies are larger more evenly
spaced wave fronts they tend to wash the room evenly and fold back on
themselves evenly. When the waves fold back on themselves after
hitting the walls, they usually cancel each other out. This
phenomenon creates a zone though the center of the room with high
cancellation, or no bass. In the corners of the room and along
the walls the bass seems considerably stronger because there is less
High frequencies are smaller wave fronts
packed close together and they do not wash the room evenly. In
fact, they radiate from your speaker like beams of light that bounce
all over your room endlessly. Every time their paths cross, the
angle of incident and timing determines one of two results. A)
Cancellation or B) Addition. In other words, standing in one spot
a 2 kHz tone may be 75dB, move 6 inches to another spot it may be 85 dB
and in another spot be only 65 dB. If you could see sound, the
combination of A and B would make your room look like a block of Swiss
The Sound stage resolution or imaging quality
of a system is effected by wall, floor and ceiling reflections the hit
your front wall behind the speakers and then pass back by your ear a
second time. Your brain can't tell the direct from the reflected
energy because they are too close together in both time and volume.
To correct this problem these reflections must be BOTH delayed in
time, and lowered in volume. Once done, your brain will interpret
the reflections as ambience and the result will be a crystal clear
perfectly focused sound stage.
To correct frequency balance problems you
typically use ABSORPTION.
To correct sound stage resolution you
typically use DIFFUSION.
Absorption means simply absorbing sound
across a specific band of frequencies. Diffusion means simply diffusing
sound energy into a wider pattern with less energy.
THE PLACE TO START in any room is a
simple test of where your at. How good, or how bad IS the room.
I always focus on frequency balance first. Walk around in
your room and clap your hand once from different locations.
Listen to the sound - in particular, the decay. The hand
clap should sound dry and tight. You should hear the sound come
from everywhere at the same time. The sound should stop almost
immediately after each clap. If you hear the sound come from your hands
and nothing from the room, the room is too dead. If you hear the
sound come from the room as being louder then the sound from your hands
the room is too live.
50 milliseconds is considered a good
decay. In a hard room it's not uncommon for the decay to last
over 1 second. When this happens, there is a ringing associated
with each clap. You can repeat this test over as you add treatment to
Assuming your room needs some treatment
(most do) The most obvious place to start is with the bass. When
bass is properly treated two things happen. A) it tightens up and
becomes articulate and B) reduces the difference between the hole in
the middle of the room with no bass and the area around the perimeter
of the room where there is usually too much bass. The most common
surprise once someone has corrected the bass response is how much bass
effects the midrange and overall focus of the sound stage.
The common 60 to 80 cycle region that
is typically way too loud in most moderate size rooms is called "room
boom". Room boom is bad. Bass traps are the most common way
to correct low frequency issues. The problem is, many bass traps
only work across a narrow band of frequencies, most of them too high to
correct the whole problem. Bass traps to be effective always end
up being large. It's easy when your building a room to
incorporate bass traps into the structure in a way that can not be
seen. In finished rooms you have limited options. The best
product I've seen so far that works for everyone is the CWAL unit by
Acoustic Control Co. It is a large corner unit that absorbs
across a wide band of frequencies and is always noticeable. Tube
traps are less effective but easier to purchase since you don't have to
build them yourself.
A separate paper would be required to
deal with all the different types of traps and how to use them best so
I won't cover it here. If you're not a handyman, the best thing
is to buy what you can find, and settle for something is better than
The next thing you should do is deal
with midrange and high frequencies. Absorbing these is far easier
than absorbing bass frequencies. A sponge on the wall can absorb
a wavelength equal to around 4 times it's thickness. That means a
2 inch thick sponge can easily handle high frequencies, but it would
take a sponge about 15 feet thick to handle low bass notes.
That's why bass traps were invented, an attempt to reduce the
size needed to get the job done through more efficient means of
The ringing associated with the hand
clap starts at around 2 kHz and can easily be treated with thick rugs,
2 inch foam, or heavy draperies. Carpet, and the type of padding
under it will have more effect on the decay of your room than anything
you will put on the walls. Unless of course you plan to create
layered absorbers for your walls, by placing an air space or padding
behind thick materials.
Your room should be a combination of
reflective and absorptive surfaces. For example, a soft floor
(carpet with padding) and a hard ceiling. An ideal to shoot for
is NO TWO PARALLEL SURFACES should be the same. If an area of one
wall is treated with absorption, the opposing surface should be
reflective. It is possible to make two whole walls soft, and
leave two walls hard and get a fair result. A better result comes
from mixing it up a bit more than that.
Diffusers can be considered to replace
the reflective surfaces of your room and should be especially if your
room is small. The smaller your room the more important diffusers
The illustration above deals with
diffusion. It shows how the reflected sound always finds its way
back to the front wall. From the point where that reflection
leaves the front wall on is the enemy. Having a hard reflective
surface on this wall ensures that you will never experience a high
resolution sound stage or genuine sound stage depth. This is where the
big screen TV dilemma comes in. This is why we always suggest NOT
trying to blend the home theater with high quality stereo playback.
If you diffuse the sound that reflects
off the front wall you will have depth in your sound stage that appears
to go right through the wall. Once you have diffusers placed on
your front wall, some carefully placed diffusers on each side wall will
enhance the performance of those already placed on the front wall.
In addition they will allow your sound stage width to expand in
the same manor as the depth did when you treated the front wall.
Adding diffusers to the rear wall will
enhance the performance of any other diffusers you have in your room.
An ideal that is not too realistic would be to replace every flat
surface in the room with some sort of diffusion.
If you listen to a system without
diffusion vs. one with diffusion you will notice something else begins
to happen that is closely related to frequency balance. In the
untreated room the louder you play the stereo, the worse it typically
sounds. This is the point where the room takes over. In a
treated (diffused) room this point where the room overcomes the system
is pushed to a point beyond your worries. In fact it almost seems
the louder you play it, the better and bigger it sounds.
In conclusion, you need to create
absorptive surfaces with either foam panels or wall rugs in places
around your room so that you eliminate as many parallel surfaces of the
same hardness as possible. You need to install some sort of bass
traps in the corners to absorb the room boom. You need to install
diffusers in the correct locations to enhance the focus and size of
your sound stage. Doing so will add consistency to the sound of
your system. You will have less moments where it sounds bad, less
distractions to keep you from the music. Only under these
conditions is your system capable of fully showing you the differences
between one type of cable or another.
Decware is a trademark of High Fidelity
Copyright © 1996 1997
1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 by Steve