13.0 Listening Rooms and Houses
You are after two important, distinct goals: flat frequency
response and good three-dimensional image. At your disposal is
the room size, the room shape, speaker height, speaker
placement, listening position, and room treatments. Even though
good speakers are essential to good sound, room effects are also
extremely important. In many cases, the differences in room
effects will be more noticeable than spending twice as much on
speakers!
For smoothest bass response, a listening room should be as large
as possible, have dimensions as unrelated as possible, and
should be optimally damped. Although nothing is ever ideal,
there are a few room dimension ratios that are better for
listening rooms:
Height Width Length
1 1.14 1.39
1 1.28 1.54
1 1.6 2.33
If your room isn't shaped like that, don't worry. These
effects are not major.
Also for smooth bass response, woofers should be at distances
from the nearest three room boundaries that are as different as
possible. In some cases, the line dividing the listening room
into left and right halves must be considered a room boundary.
Also, for smooth bass response, the listener's ears should be
at distances from the nearest three room boundaries that are
as different as possible.
All of this is essential because a wall near a speaker boosts
the bass from that speaker at some frequencies. If a speaker
is the same distance from three walls, then some frequencies
will be emphasized much more than others, rather than slightly
more.
For best three-dimensional image, a listening room should have
good symmetry about the plane between the two speakers. This
means that if one speaker is in a corner, the other speaker
must be in a corner. If this symmetry is not right, the first
reflection from the wall behind one speaker will be different
from the first reflection from the wall behind the other speaker
and critical parts of the stereo signal will be damaged.
Also, no large object should block the path from speakers to
listener or from speaker to speaker. Speakers should be
elevated so that tweeters are at listener ear height. The
distance between speakers should be no greater than the distance
from each speaker to the listener. Finally, the tweeters should
be aimed at the listeners.
A normal box-shaped listening room with bare walls will have
"slap echo" which will reduce intelligibility. A good cure is
randomly-placed wall hangings consisting of small rugs spaced
an inch or so away from the wall to increase sound absorption.
Another cure is convex-shaped art objects on the walls to
disperse harmful reflections. If money is available, commercial
room treatments such as "Tube Traps" and "RPG Diffusers" are
also valuable, but many of the benefits of these exotic devices
are available with simpler techniques.
As a general rule, in a good room, speakers and listener can be
close to room boundaries with minimal adverse effects. In a bad
room, a good strategy is to place both speakers and listener as
far away from room boundaries as possible.
An excellent starting point for speaker placement is to measure
the listening room diagonal dimensions. Divide that measurement
by three. Put each speaker that distance from a corner, on the
room diagonals.
I----------------------------------I
I I
I L I
I I
I S S I
I I
I----------------------------------I
Place your listening position midway between the two speakers
and approximately half way from the speakers to the wall. Be
sure that there is nothing in the "triangle" formed by the
listening position and the speakers.
Try this and then move things 12" (30cm) at a time to see if
you can improve the sound. Your ears will be a better guide
than any commonly-available instruments. To keep track of
what you are doing, take notes. To remember exactly where
you put the speaker on the floor, a practical trick is to
mark the floor with a sewing needle and thread.
13.2 How do I wire a house for sound?
A fundamental principle of physics is that the farther a signal
travels, the more the signal will be degraded. Translate this
to mean that the shorter the wire, the better. Understanding
this, the idea of running speaker cable between every room of
the house isn't as attractive as it first seems.
If you still decide to wire your house for sound, you should do
it at the same time you're wiring for telephone and electricity.
It is possible to wire a house after the walls are closed, but
it becomes very difficult.
It is economical to use common house wire (Romex, UF, NM, etc)
for speaker wire in the walls, but this may violate building
codes. Check with an electrician or inspector first. It will
also confuse future electricians, so label the wire clearly, all
along its length.
If you want to make your house like a recording studio, it is
best to use the techniques of recording studios. When studios
run long lengths of sound cable from one room to another, they
drive the cable with 600 ohm line amplifiers. They also use
shielded, twisted-pair cable. They only connect the shield at
one end of the cable. Finally, they use balanced inputs at the
other end of the cable.
13.3 Where can I read more about listening room construction and tuning?
"Building a Recording Studio" by Jeff Cooper
Mix Bookshelf
"Handbook for Sound Engineers"
"The Master Handbook of Acoustics" by F Alton Everest
"Sound Engineering 2nd Edition" by Don and Carolyn Davis;
Howard W. Sams & Co. (C) 1990
"Good Sound" by Laura Dearborn
Introductory, but clear and accurate
"Sound Recording Handbook" by John M. Woram
Howard W. Sams & Co. #22583
Excellent General Reference
"Audio Technology Fundamentals" by Alan A. Cohen
Howard W. Sams & Co. #22678
Overview of Audio Theory
"Introduction to Professional Recording Techniques"
by Bruce Bartlett
Howard W. Sams & Co. #22574
"Modern Recording Techniques" by Hubar and Runstein
Howard W. Sams & Co. #22682
"Sound Studio Production Techniques"
by Dennis N. Nardantonio
Tab Books
"The Uneasy Truce Between Music and the Room"
F. Alton Everest
Audio, February 1993, Pgs. 36-42
"Coloration of Room Sound by Reflections"
F. Alton Everest
Audio, March 1993, pgs. 30-37
13.4 What is white noise? What is pink noise?
"White noise" is characterized by the fact that its value
at any two different moments in time are uncorrelated.
This leads to such noise having a flat power spectral
density (in signal power per hertz of bandwidth), and is
loosely analogous to "white light" which has a flat power
spectral density with respect to wavelength.
Pink noise has flat power spectral density per PERCENTAGE
of bandwidth, which leads to a rolloff of -3 dB/octave
compared with white noise.
There are many reasons for using pink noise in audio testing.
One is that music has an average spectral content much closer
to pink noise than white noise. Another is that pink noise
can be readily measured with constant Q bandpass filters and
naturally leads to flat plots on logarithmic frequency scales
- which correspond to the equally tempered musical scale.
Pink noise is often used with 1/3 octave band filters to
measure room acoustics. This idea has merit since 1/3 octave
is a convenient number near the limit of our ears ability to
detect frequency response irregularities, and because
averaging measurements over 1/3 octave bands smooths out the
numerous very narrow peaks and dips that arise due to
standing waves in rooms.
Another term you'll hear about is Gaussian noise - this is
noise with a Gaussian amplitude probability density.
Gaussian noise has the amazing property that linearly
filtering it preserves its Gaussian amplitude density and
that sums of Gaussian random variables are again Gaussian.
The two terms shouldn't be confused. It is possible to have
Gaussian white or pink noise.
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