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.

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                I                L                 I
                I                                  I
                I       S                 S        I
                I                                  I
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        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.

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--
Bob Neidorff; Texas Instruments     |  Internet: neido...@ti.com
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