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Subject: FAQ: rec.audio.* Rooms 2/99 (part 6 of 13)
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Organization: Texas Instruments Corp.
Summary: Answers to common questions about audio equipment, selecting,
  buying, set-up, tuning, use, repair, developments, and philosophy.
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Archive-name: AudioFAQ/part6
Last-modified: 2002/08/29
Version: 2.15

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!

|    Here are some generally-accepted-as-good guidelines for good
|    sound.  If you use these as a starting point, you will be far
|    ahead in terms of getting good sound from your speakers and
|    room.  But these are just a guide.  Each room and each speaker
|    is a little different.  Experiment to see if a change will
|    help.  Also, if the manufacturer recommends something
|    different, give that a try, too.  Then use what sounds best to
|    you.

    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.

    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.

COPYRIGHT NOTICE
The information contained here is collectively copyrighted by the
authors. The right to reproduce this is hereby given, provided it is
copied intact, with the text of sections 1 through 8, inclusive.
However, the authors explicitly prohibit selling this document, any
of its parts, or any document which contains parts of this document.

--
Bob Neidorff; Texas Instruments     |  Internet: [e-mail]
50 Phillippe Cote St.               |  Voice   : (US) 603-222-8541
Manchester, NH  03101 USA

Note: Texas Instruments has openings for Analog and Mixed
Signal Design Engineers in Manchester, New Hampshire.  If
interested, please send resume in confidence to address above.