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FAQ:
rec.audio.* Rooms 2/99 (part 6 of 13)
13.0 Listening Rooms and Houses
13.1 How should I place speakers in my
room? What size room is best?
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.
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.
Some speakers want to be aimed right at the
listener (toed in) while others work best pointed straight ahead.
Experiment.
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. |
