Usenet 1993 July/InfoMagic USENET CD-ROM July 1993.ISO/answers/AudioFAQ/part1

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Newsgroups: rec.audio,news.answers,rec.answers
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From: [e-mail]
Subject: rec.audio FAQ (part 1 of 4)
Message-ID: <[e-mail]>
Followup-To: poster
Summary: Answers to common questions about audio equipment, selecting,
  buying, set-up, tuning, use, repair, developments, and philosophy.
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Organization: Unitrode Integrated Circuits Corp.
Date: Mon, 7 Jun 1993 21:54:28 GMT
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Xref: senator-bedfellow.mit.edu rec.audio:66812 news.answers:9143 rec.answers:1122

Archive-name: AudioFAQ/part1
Last-modified: 1993/6/4
Version: 1.5

1.0 Contents:

    Part 1:
    1.0    Contents
    2.0    Organization
    3.0    Purpose
    4.0    Omissions
    5.0    Credits
    6.0    Errors and Corrections
    7.0    Disclaimer
    8.0    Copyright Notice
    9.0    Speakers
    10.0    Amplifiers
    Part 2:
    10.13    Amplifiers (continued)
    11.0    CD Players, CDs, Turntables, and LPs
    Part 3:
    12.0    High Fidelity Systems
    13.0    Listening Rooms and Houses
    14.0    Recording
    15.0    Mail Order
    Part 4:
    16.0    Wire
    17.0    The Press
    18.0    Retail
    19.0    Miscellaneous
    20.0    Net Protocol

2.0 Organization:

This FAQ is divided into a preamble and a list of subjects.  Each
subject is described by a list of questions and answers.  Some
questions have no answers yet.  That's life.

This FAQ is split into four individual postings.  This FAQ also
references a frequent posting of sources for audio via Mail Order.
That list is maintained by [e-mail] (William R. Nau).  Contact
Mr. Nau directly for information or corrections to that posting.
The list is also available via FTP in the pub/rec.audio directory
of SSESCO.com.

Lines beginning with "|" are new since the last version of the FAQ.

3.0 Purpose:

The purpose of this FAQ is to address frequently-asked questions as
a whole, so that rec.audio volume can be reduced.  Towards this end,
we assembled a list of common questions, and some general answers to
these questions.  Audio is part science and part art, so some of the
answers are objectively correct, while others try to open-mindedly
present both sides of a subject.

4.0 Omissions:

Many valuable things have been left out of the FAQ.  In part this is
because there is just too much to say about audio.  In part, this is
because the general reader doesn't need that much detail.  Also, some
things were omitted because they are too controversial or inflammatory.

5.0 Credits:

This FAQ is the work of many people.  Allow me to thank everyone who
helped now.  We appreciate all of your contributions.  However, all
blame and criticism should go to Bob Neidorff at [e-mail].
Some of the contributors to this FAQ are listed below.  Others have
made great contributions, but are no less appreciated.

    [e-mail] (Andrew Hume)
    [e-mail] (jj)
    [e-mail] (Bob Neidorff)
    Rick Oakley (no internet access)
    [e-mail] (Richard d Pierce)
    [e-mail] (Paul Smee)
    [e-mail] (Gabe M. Wiener)

6.0 Errors and Corrections:

Some errors are intentionally included in this FAQ.  (It's easier to say
that and later admit we were wrong, than to claim that this is a best
effort).  If you have a correction to the FAQ, additional information,
or a new topic for the FAQ, please send e-mail to [e-mail]
describing your thoughts in detail.  Please include your e-mail address
in your submission, so that we can stay in touch.  Every submission will
be considered for inclusion in the next release of the FAQ.  This FAQ
will be reposted every month.

7.0 Disclaimer:

Everyone's human.  Nothing is perfect.  The people who wrote the
information here put varying amounts of research into their work.
To the best of my knowledge, no one made any contribution or
comment because of a vested interest.

Audio is a very lucrative and competitive industry, filled with
honest companies, aggressive marketing people, people who stretch the
truth very thin, excellent products, and lousy products.  We tried
hard to screen the hype from valuable data.  If we insulted, omitted,
or otherwise disturbed you, your company, your product, or something
you feel strongly about, please let me know.

The information here attempts to paraphrase a large portion of the
information exchanged on rec.audio.  At times, people will make
recommendations or suggestions to others on rec.audio.  Some of these
statements are included here because we have been asked to include
statements of that sort.  These statements, and everything in this
document is the opinions of various people.  Nothing here is intended
as recommendation or suggestion.

Further, no matter how it is worded, nothing here should be taken as
fact.  The authors take no responsibility for any use of this
information.

8.0 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.

9.0 Speakers:

9.1 What should I listen to when evaluating speakers?
    The most important thing is to listen to recordings that
    you *know*.  Any good salesman will play you recordings
    that highlight that particular speaker.  Do not be embarrassed
    about bringing a stack of CDs with you to the hi-fi shop.

    Do not spend your valuable listening time switching between a
    dozen pairs every 3 seconds.  If you are shopping at a quality
    store, the dealer will, from the description of your room, your
    size requirements, your musical tastes, and your budget, be able
    to show you a couple of pairs that will be close to what you
    want.  Spend several minutes listening to each.  When you think
    you're close, don't be embarrassed about spending half an hour
    or more listening to the speakers.  You're going to have them in
    your home for a lot longer, and many speakers will cause
    "listening fatigue" after a short time.  Make sure you really
    like them before you hand over money.

    One thing to try is well recorded "Spoken Word" records; most
    people have a very good ability to tell when a speaking voice
    sounds unnatural, even if they've never heard the person
    speaking live.  If you play an acoustic instrument, find
    something that features that instrument solo, or in a small
    group; make sure it really sounds like it should.  Almost
    everyone has heard a live piano.  Piano can be very revealing.

    Blues, jazz, folk, or 'easy listening' music with simple
    instruments and a female vocalist is also revealing.  Well done
    female singing voices provide a very good test of a system's
    response.  Try something simple and soft, which will let you
    hear any noises coming from the system; and something complex,
    with lots of instruments all happening at once, to make sure the
    system doesn't go muddy when things get complicated.  And, of
    course, try a few of your favorites, and see if you like what
    happens with them.

    If a sales person suggests some music to listen to, the odds are
    that it isn't the most revealing.  Sales people tend to suggest
    things which sound great.  Anything you own and like is good,
    because you know it and are happy to listen to it carefully.  No
    matter how good the recording, if you don't like Opera, you
    won't listen to it as carefully as your favorite, scratchy,
    1940's rhythm and blues.

    Most important is to listen to something you are familiar with.
    Even if a recording is flawed (and what ones aren't?), how is it
    different from your normal setup?  Some of the most important
    differences are "Gee, I never heard that instrument before!"

9.2 What should I listen for when evaluating speakers?
    When comparing two speakers side-by-side, doing an AB
    comparison, be extremely careful to match the levels before
    evaluating.  A slight level difference can make one speaker
    sound better, even though the difference may not be perceived
    as a level difference.  Some claim that you will be influenced
    by a difference of less than 1/2 dB!

    First and foremost, the sound should be natural.  If you listen
    to vocals, close your eyes and try to picture someone singing in
    the same room with you.  Does it sound realistic?  Likewise with
    instruments.  You selected recordings of instruments that you
    like and have heard live.  Do they sound like what you remember
    them sounding like live?

    Your very first impression should be something like "what nice
    sound".  If your initial gut reaction is "gosh, what a lot of
    detail", the system is likely to be heavy in the treble (often
    interpreted by beginners as "more detailed") and you'll probably
    find that annoying after a while.  If your first reaction is
    "hey, what powerful bass", then the system is probably
    bass-heavy, rather than ideal.  The most common mistake for
    beginners is to buy a system with REALLY powerful bass, because
    it sounds "impressive" at first.  After a while, though, you'll
    get tired of being thumped on the head by your music.

    Not to say that good bass and treble aren't important.  But your
    first realization should be that the music is all there, and
    that it comes together as good music, without one particular
    part trying to dominate it.  Sit back and listen to it for a
    bit.  You should be able to pick out the individual instruments
    if you want.  They shouldn't force themselves on you, and you
    should also be able to hear the music as a single piece, the sum
    of its parts, without feeling like each of the instruments is
    trying to grab your attention away from the others.

    You should check how things sound with the amp turned up, and
    also with it turned down to a fairly low volume level.  Some
    speakers which sound very nice at low levels begin to sound
    confused, like they can't cope, when turned up.  On the other
    hand, some sound nice loud, but sound thin and bodiless when you
    turn them down a bit.  With the spoken word or female vocalist,
    listen for "sibilance", a pronounced 'hiss' at the end of 's'
    and 'z' sounds.  It shouldn't be there.  Most planar speakers
    just can't play very loud.  Whatever you hear, do some
    auditioning at the maximum volume you anticipate ever wanting.

    It is acceptable and sometimes desirable to switch the stereo to
    mono to evaluate naturalness.  Mono is a good test of both the
    room and the speakers.  The image should be rock-solid dead
    center, and not move with signal or level.  If it isn't perfect
    mono, it will be nearly impossible to create a good stereo.

    A speaker in a large box is capable of producing low frequencies
    at higher volumes with more efficiency than a small box, but
    that doesn't mean that a small box can't have great bass, it
    just won't be as efficient and can't play as loud.

    Good speakers can "recreate a natural stereo sound stage",
    placing some instruments to the left of the left speaker, some
    sounds in the middle, and some to the right of the right
    speaker.  Poorer speakers make it harder to localize voices.

9.3 Why use a subwoofer?  Will it help?  One or two?
    One reason to get a subwoofer is to add bass to a feeble system.
    A second reason is to move the lowest frequencies to a separate
    driver, and thereby reduce a particular kind of distortion
    caused by the nonlinear mixing of different sounds, called
    "intermodulation distortion". A third is to increase the power
    handling ability of the system and the overall reliability.  All
    are valid reasons, but it isn't so simple.

    To improve the sound of a good speaker system, a subwoofer must
    "integrate smoothly" into the system, extending the bass without
    causing peaks or dips.  Many subwoofers have a crossover that
    goes between your amp and your main speaker which sends the lows
    to the subwoofer and sends the higher frequency signals to the
    main speakers.  This may damage the perfect sound of a good
    system, it may sound similar, or it may sound better.

    Most good small speaker systems have a bass peak at resonance,
    which attempts to compensate for the absence of lower bass.
    Like it or not, this is the only way to make a small system
    sound realistic. If the small system is done well, the
    improvement you will get from a subwoofer will be small, but
    still real and, to many, significant.

    Correctly done, a good subwoofer will enhance the sound of a
    good small-box system.  Done wrong or haphazardly, anything is
    possible.  Even a fine large speaker system might benefit from
    careful addition of a subwoofer.  However, the better the
    original system, the more likely it will be that a modest
    subwoofer will do more harm than good.

    Low frequencies travel less directionally than high frequencies,
    so many people say that only one subwoofer is required for good
    sound.  This is true to some extent, but not completely true.

    There are a few reasons for getting two subwoofers.  Some feel
    that you need two subwoofers to accurately reproduce the stereo
    image, no matter how little low-frequency stereo information
    there is.  Others feel that two subwoofers are much easier to
    set up in a room, less likely to excite standing waves in the
    room, and give smoother sound.

    A third reason is that two subwoofers can produce twice the
    sound of one.  Finally, even though subwoofers produce very low
    frequency sound and very low frequency sound is non-directional,
    subwoofers also have output at 100 Hz, and sound at 100 Hz is
    directional, so two subwoofers will give a slightly better
    stereo image than one.  Assuming, of course, that the two are
    separated by at least two feet.

    Finally, even though original source signals rarely contain any
    music with stereo components below 50Hz, there may be some noise
    component with low-frequency out-of-phase noise.  This unusual
    noise might add a sense of space to a recording if it is
    reproduced by a system in which the woofers are very far apart.

    It is still true that a single good subwoofer, correctly added
    to a system will help the sound but two will probably help more.

9.4 How do you connect a subwoofer to a stereo?
    Many subwoofers contain their own amplifier and crossover.
    For these, take the preamp output and feed it into the subwoofer
    amp input and also into the main amplifier.

    For other subwoofers, just run them in parallel with your main
    speakers, or combine them into your system with your own bass
    amplifier and crossover.

    Some A/V receivers contain a splitter specifically for use with
    subwoofers.  If you have one of these, you will either want a
    separate amplifier for your subwoofer or an amplified subwoofer.

    Consult the manual which comes with the subwoofer.

9.5 What do I need for surround sound?
    "Surround Sound" has referred to a number of different products
    over the years.  Many mass-fi receivers have "Surround Sound"
    buttons that do little more than muck up the imaging.

    In recent years the term "Surround Sound" has become synonymous
    with the surround systems produced by Dolby Laboratories.  Dolby
    Surround comes in several flavors, such as passive surround
    (which simply decodes the phase information and sends it to the
    rear speakers) and the more advanced system called Pro Logic.
    Pro Logic system uses computer circuitry to route directional
    information to the appropriate speakers.

    Generally, one needs at least two more speakers beyond the main
    stereo pair.  Advanced Pro Logic systems such as the Lexicon and
    Fosgate can accommodate several more speakers beyond the two
    additional ones (usually placed in the rear).  Often one can
    find Pro Logic systems with two front, two rear, two side, as
    well as a center channel speaker for dialogue.

9.6 I was just approached (accosted?) by a couple of kids driving a van
    that said they had some GREAT speakers to sell. They are
    overstocks, used by major recording studios and DJs or even
    hot, and they normally sell for $1000/pr, but they'll let me
    have them for just $399. Am I getting ripped off?

    Yes, you most certainly are. The speakers these people sell are
    none of what they describe. They are never used in studios.
    There might be one or two DJs out there that use them because
    they can't afford anything else.  They are not overstocks, and
    in all likelihood, they are NOT HOT!.

    Are they good speakers? No, they're, at best, no better than the
    big boom boxes you find in $400 rack systems in department
    stores.  They are worth no more than what the kids paid for them
    ($100/pr).

    The speakers go under names like "Acoustic Monitor DB IV",
    "Acoustic Linear," "Pro-Poly," "Audio Reference 4350" and so on.
    They all "feature" things like "liquid cooled 3" tweeter",
    poly-cone 12" woofer, fantastic (but impossible) frequency
    response, 98 db/watt sensitivity, and so on.

    These speaker are made by a couple of manufacturers with the
    intent of being sold exactly this way.  They cost the kids in
    the van about $100 a pair, and the kids are given minimal
    training about what kinds of stories to use, what parking lots
    are the most likely to generate sales (department store parking
    lots near colleges in September is a great time for these guys).
    Anything over and above the $100 dollars the kids paid is pure
    profit (except for gas money).

    Stay away, you're getting ripped off.

9.7 What speakers should I consider in the $XXX/pair price range?
    This is probably the most commonly asked question on rec.audio,
    and also the most impossible to answer.  The market keeps
    changing, everyone has different tastes, and no one has the time
    to listen to even 10% of the products available in any country.
    Also, many good products are only available in specific regions
    or countries.

    If you really want recommendations and are willing to listen to
    the opinions of others, check the past few issues of Stereophile
    Magazine.  Although they are strongly biased towards very
    expensive gear and have their own particular other biases, they
    do steer you to some very good equipment in their
    frequently-updated list of "RECOMMENDED COMPONENTS".

9.8 Can you build better speakers than you can buy?
    Some people can build better than you can buy.  These people are
    either experts, golden ears, extremely well equipped, inspired,
    or a combination of the above.

    Some companies have plans available to entice you into buying
    their drivers: Audio Concepts, Audax, Dynaudio, Focal, KEF, and
    Scanspeak.  Your success rate with these plans will probably be
    very good IF your cabinetry skills are very good and IF you
    follow the plans precisely.  If you deviate (as everyone does),
    anything is possible.

    Stereophile has published three different plans designed by Dick
    Olsher which are similar two-way ported systems. A recent one of
    these was in Stereophile Nov '90, pages 94-127. Audio Magazine
    published a plan called "The Pitts" by Ken Kantor, in Audio, Nov
    '88 pages 65-71 continued in Dec '88 pages 73-77.  This plan is
    a two-way sealed box.

    I have built one published design and one manufacturer's design.
    I believe that both met my expectations.  They took me a long
    time to build, taught me a lot, were fun projects, and sounded
    good when finished.

    I also believe that a commercial system which cost what my parts
    cost will never sound anywhere near as good as the one I build.
    If you consider $2/hour for my time, however, building is
    financial suicide.

    Designing your own system is even more a can-of-worms, and
    should be left to those with either a strong stomach, a very
    forgiving ear, infinite resources, or excellent guidance.

9.9 Where can I read more about speaker building?
    Speaker Builder Magazine
        Audio Amateur Publications
        PO Box 494
        Peterborough NH  03458 USA
        603-924-9464
    The Loudspeaker Design Cookbook, Fourth Edition
        by Vance Dickason (C) 1991
        ISBN 0-9624-191-7-6
        $29.95 + $2.00 S&H from
        Old Colony Sound Lab
        PO Box 243
        Peterborough NH  03458-0243 USA
        603-924-9464
    High Performance Loudspeakers by Martin Colloms
    Synergetic Audio Concepts Classes and Newsletters
        Syn-Aud-Con teaches classes on Audio and Acoustics
        12370 W. Co. Rd. 100 N.
        Norman IN  47264 USA
        812-995-8212
9.10 Where can I buy speaker drivers?
    A & S Speakers (Broad line)
        3170 23rd Street
        San Francisco CA  94110 USA
        415-641-4573
    Audio Concepts (Their own kits plus drivers)
        901 South 4th Street
        LaCrosse WI  54602 USA
        800-346-9183
    Phil Baker (Surplus cabinets only)
        546 Boston Avenue
        Medford MA  02155
    Carbonneau
        609 Myrtle NW
        Grand Rapids MI  49504 USA
        616-456-9528
    Drexler Audio Systems (Bandor Speaker Distributor)
        14 Rose Lane
        Rosemont PA  19010
    Gold Sound (Broad line including pro speakers)
        PO Box 141
        Englewood CO  80151 USA
        303-789-5310
    Madisound (Broad line)
        8608 University Green
        Box 4283
        Madison WI  53711 USA
        608-831-3433
    McGee Electronics (Polydax, Pyle, Motorola, EV, MTX)
        1901 McGee Street
        Kansas City MO  64108-1891 USA
        913-491-5287 or 800-876-2433
    Meniscus
        2442 28th Street SW Ste D
        Wyoming MI  49509 USA
        616-534-9121
    Parts Express
        340 East First Street
        Dayton OH  45402-1257 USA
        513-222-0173
    Solen Electronique (Airborne, Audax, Ceratech, Dynaudio, Eton,
        Lpg, Morel, Peerless, Scan-Speak, Seas, Solen, Vifa)
        4470 Avenue Thibault
        St.-Hubert, QC  J3Y 7T9 Canada
        Voice 514-656-2759
        FAX [phone]
    Speakers Etc.
        1828 West Peoria Avenue
        Phoneix AZ  85029 USA
        602-944-1878
    SRS Enterprises (Pyle, Pioneer)
        318 South Wahsatch Avenue
        Colorado Springs CO  80903 USA
        719-475-2545
    Transducer Technology (Cabasse, Accuton, Vieta,
            Audiom by Focal, Stage, Tekton)
        4320 Spring Valley Road
        Dallas TX  75244
        214-991-6994
    Zalytron (Broad line including kits)
        469 Jericho Turnpike
        Mineola NY  11501 USA
        516-747-3515

9.11 Where can I buy loudspeaker kits?
    Audio Concepts, Inc. (Wide range of kits.  Catalog available)
        901 South Fourth Street
        La Crosse WI  54601 USA
        608-784-4570
    Fried Products (Parts kits starting $550.  Catalog available)
        (Emphasizes high-end transmission line speakers)
        (Parts kits have plan, crossover, and driver)
        Conshocken Road
        Conshocken PA  19428 USA
        215-277-9004
    Mahogany Sound (Parts kits and Woodstyle kits)
        (Parts kits have plan, crossover, and driver)
        (Woodstyle kits also have 3/4" MDF veneered boxes)
        (Prices $150/pair to $500/pair.  Catalog available)
        (Two way, three way & subwoofer kits)
        2610 Schillingers Rd #488
        Mobile AL  36695 USA
        205-633-2054
    Also see above, under suppliers for speaker drivers.

9.12 How can I improve the sound of my speakers?
    The best way to change the sound of your speakers is to change
    where you put them.  Ideally, the speakers should be located at
    ear level, in front of you, squared off between you.  It's then
    a matter of fiddling with a) the angles, b) the distance apart,
    c) the distance from you, and d) the distance from the wall.
    Just moving the speakers around in the room or putting them onto
    stands can make a major difference. For more on speaker
    placement, see 13.1 below.

    Other than that, speaker modifications can be a can of worms, or
    can produce very subtle changes, which you might prefer.  For
    example, you might improve a speaker by adding some cross braces
    of 1"x1" wood from left to right and from front to back.  This
    will stiffen the cabinet and reduce speaker cabinet wall
    vibrations, which probably hurt sound quality.  Alas, this will
    be most effective with lower-cost and poorly built speakers.

    Along similar lines, some claim success putting lead wire or
    epoxy putty on thin parts of the speaker to damp out resonances.
    You can try doing this to the thinner parts of the speaker
    "basket" or frame, or to the front "baffle" or supporting panel.

    Still another "tweak" is to add sound deadening felt pads to the
    inside walls of the speaker.  Instead of felt pads some advocate
    sand-filled latex coatings on the inside walls of speakers.
    Others advocate ceramic tiles held in place with "thinset".
    Still others rave about commercial products like AC Glop,
    Acoustic Magic, and Bostik Sheet.  However, the people who rave
    about these products tend to be the same people who sell them.

    Any change along the lines of adding felt, cross-bracing, or
    putty will have subtle effects on the sound.

    For the brave at heart, you can replace old or cheap drivers
    with better ones, but the results of this one change can be very
    dissatisfying if you happen to get the wrong type of driver for
    that application, and may never sound right, even if you use a
    similar driver. Speaker system design is still somewhat of a
    science and somewhat of an art.  Throwing paint on a canvas
    often makes a mess.

    Whatever change you try, don't "burn your bridge" home.  Be sure
    that you can undo whatever change you did, just in case.  Many
    tweaks to good speakers, no matter how well thought through,
    will correct for one flaw, but create others, or correct a flaw
    that the designer had cleverly used to his advantage.

9.13 How can I replace/re-cone my old speakers?
    The best chance of success is to buy an identical replacement
    speaker driver from the manufacturer of the system.

    Second choice is to buy the exact same driver from a
    distributor.  This is sometimes difficult because it is hard to
    learn exactly what driver the manufacturer used.  In addition,
    EVEN IF the manufacturer used stock speakers, they might have
    used matched pairs or selected speakers by hand for an exact set
    of specific characteristics.

    There are companies that rebuild drivers, but they charge quite
    a bit.  I have heard $75 per driver. This is rarely done for
    anything but very expensive commercial drivers.  Speaker
    manufacturers will often sell owners the materials that they
    need to repair a speaker.  If you are handy with delicate
    things, it is worth a try.

    In addition to speaker manufacturers, there are companies which
    sells rebuild kits for approximately $30 per pair, containing
    new foam, a special glue, and instructions.  If you have a blown
    or distorted voice coil, this still won't help. A few netters
    have used rebuild kits from this company successfully. Contact:
        Stepp Audio Technologies
        PO Box 1088
        Flat Rock NC  38731 USA
        800-747-3692

    Another vendor of speaker repair parts is:
        Simply Speakers
        P. O. Box 22673
        St. Petersburg FL  33742 USA
        800-767-4041 or 813-571-1245

    Some speaker manufacturers have very good warranties.
    Electro-Voice warranties all professional products for life.
    KEF has a similarly broad warranty on their speakers.  Contact
    the manufacturer first.

9.14 What computer programs can I use to design speakers?
    There are many useful programs available, but none are complete
    without a good knowledge of speaker design. Further, you will
    NEED to supplement any program with hand tweaking for the best
    sound.  Finally, no simulation program is ever useful without
    good model parameters, and the parameters which manufacturers
    give you are often imperfect, so many good designers strongly
    recommend your own lab measurements.  The Loudspeaker Design
    Cookbook (see 9.9) tells you how to measure a speaker, and also
    enough theory to feel confident with a good program.

    You can get a lot done with a simple spreadsheet and the
    equations in a book like The Loudspeaker Design Cookbook.

    For more information on speaker programs for speaker design
    and speaker design hardware, such as measurement systems, get
    the archive "audio.speaker" in directory "dist" on node
    "research.att.com" (also known as 192.20.225.2).

9.15 Can I magnetically shield my speakers for use near a TV?
    You probably will need to buy speakers that are made with an
    integral magnetic shield.  Magnetic shielding is usually done
    by either shielding the speaker magnet or by cancellation of the
    magnetic field very close to the magnet, or by both.  Shielded
    speakers are NOT built by lining the enclosure with metal.
    While it sounds like a good idea, it doesn't work.

    A common magnet shield is a mild steel cup around the magnet.
    This is the cheapest shield, and is usually fairly ineffective.
    It also will interfere with the speaker's critical magnet gap,
    so this type of shield can hurt speaker performance by shorting
    the magnetic field and reducing the magnetic flux density in the
    gap, which can reduce efficiency and affect the speaker's low
    frequency performance.

    Cancellation is done using a reverse-polarized magnet glued to
    the back of the main magnet.  If done right, it can almost
    completely cancel the rear stray field.  In some cases it can
    also increase the magnetic flux density in the gap, which may
    or may not be desirable.

9.16 What are all of these abbreviations people use for speakers?
    Most of these parameters are well documented in the Loudspeaker
    Design Cookbook. (see 9.9)  In summary:

    Fs    Driver free air resonance, in Hz.  This is the point at
        which driver impedance is maximum.
    Fc    System resonance (usually for sealed box systems), in Hz
    Fb    Enclosure resonance (usually for reflex systems), in Hz
    F3    -3 dB cutoff frequency, in Hz

    Vas   "Equivalent volume of compliance", this is a volume of
        air whose compliance is the same as a driver's
        acoustical compliance Cms (q.v.), in cubic meters

    D     Effective diameter of driver, in meters
    Sd    Effective piston radiating area of driver in square meters
    Xmax  Maximum peak linear excursion of driver, in meters
    Vd    Maximum linear volume of displacement of the driver
        (product of Sd times Xmax), in cubic meters.

    Re    Driver DC resistance (voice coil, mainly), in ohms
    Rg    Amplifier source resistance (includes leads, crossover,
        etc.), in ohms

    Qms   The driver's Q at resonance (Fs), due to mechanical
        losses; dimensionless
    Qes   The driver's Q at resonance (Fs), due to electrical
        losses; dimensionless
    Qts   The driver's Q at resonance (Fs), due to all losses;
         dimensionless
    Qmc   The system's Q at resonance (Fc), due to mechanical
        losses; dimensionless
    Qec   The system's Q at resonance (Fc), due to electrical
        losses; dimensionless
    Qtc   The system's Q at resonance (Fc), due to all losses;
        dimensionless

    n0    The reference efficiency of the system (eta sub 0)
        dimensionless, usually expressed as %

    Cms   The driver's mechanical compliance (reciprocal of
        stiffness), in m/N
    Mms   The driver's effective mechanical mass (including air
        load), in kg
    Rms   The driver's mechanical losses, in kg/s

    Cas   Acoustical equivalent of Cms
    Mas   Acoustical equivalent of Mms
    Ras   Acoustical equivalent of Rms

    Cmes  The electrical capacitive equivalent of Mms, in farads
    Lces  The electrical inductive equivalent of Cms, in henries
    Res   The electrical resistave equivalent of Rms, in ohms

    B     Magnetic flux density in gap, in Tesla
    l     length of wire immersed in magnetic field, in meters
    Bl    Electro-magnetic force factor, can be expressed in
        Tesla-meters or, preferably, in meters/Newton

    Pa    Acoustical power
    Pe    Electrical power

    c     propogation velocity of sound at STP, approx. 342 m/s
    p     (rho) density of air at STP 1.18 kg/m^3

9.17 What are fluid-filled (fluid-cooled, ferro-fluid) tweeters?
    These tweeters are built almost exactly the same as other
    tweeters.  They look and act almost exactly the same, too.
    The only difference is that they have a small, controlled
    amount of a special fluid inserted into the gap between the
    magnet and the voice coil.

    One big effect of adding this fluid to a tweeter (or to any
    speaker) is that it makes the voice coil capable of dissipating
    more heat.  This means that the speaker can have a lighter voice
    coil, for better performance, or a higher power rating for the
    same voice coil.  The other big effect of this fluid is to add
    mechanical damping.  The frequency response and transient
    response of the driver will change, possibly for the better.

    In addition, this fluid may help center the voice coil, may
    lubricate the voice coil, and may help keep dirt out of the gap.
    This fluid will not increase the magnetic field, concentrate the
    magnetic field or otherwise change the magnetic circuit.  Nor
    will it cushion impact if the voice coil bottoms.

    The fluid used for this purpose is often called "ferrofluid".
    It consists of sub-microscopic particles of magnetic material
    suspended in special oil.  This fluid stays in the gap because
    of the strong magnetic pull of the magnet.  There is some debate
    over whether these fluids can dry out with time.  Manufacturers
    claim that the oil used is non-volatile.

    It is possible to use ferrofluids in mid-range drivers and
    woofers.  However, as tweeters tend to have the most fragile
    voice coils, tweeters have the most to gain from ferrofluid.
    There are various different fluids on the market, some of which
    have characteristics tailored to tweeters, some to woofers, etc.

    It is very risky to blindly add fluid to a driver.  It may not
    be compatible with the adhesives used in the driver, may not be
    practical with the particular driver layout, and is impossible
    to remove.  Permanent driver damage is possible.

10.0 Amplifiers
    Note: A receiver contains an amplifier, so the following
    questions apply to both receivers and amplifiers.  In the
    following text, "amp" and "amplifier" are used synonymously.

10.1 What is Biamping? Biwiring?
    Most speakers are connected to an amplifier by one pair
    of terminals on each speaker. Within these speakers, a
    crossover distributes the signal (modified appropriately)
    to each of the drivers in the speaker.

    Some speakers are set up to be either biwired or biamped. A
    much smaller number allows triwiring and triamping.  The same
    principles apply but use three sets of wires or three amplifiers
    instead of two.  Most speakers that support biamping/biwiring
    have two pairs of terminals and some mechanism for shorting
    the two pairs together when used in the normal way. This
    mechanism is most likely a switch or a bus bar. To help
    the descriptions below, I will refer to these two pairs as
    LO and HI  (because normally one pair connects to the woofer
    and the other pair connects to the tweeter/midrange).

    Biwiring means that a speaker is driven by two pairs of wires
    from the same amplifier output. One cable pair connects HI to
    the amp, and the other cable pair connects LO to the same amp
    output that you connected the HI cable to.  Biwiring is
    controversial; some folks hear a difference, some do not. The
    most plausible explanation involves magnetic induction of noise
    in the relatively low current HI cable from the relatively high
    current signal in the LO cable.  Accordingly, Vandersteen
    recommends the two cable pairs for a channel be separated by at
    least a few inches. In any case, the effect appears to be small.

    Biamping means that the two pairs of terminals on a speaker are
    connected to distinct amplifier outputs. Assuming you have two
    stereo amplifiers, you have two choices: either an amp per
    channel, or an amp per driver. For the amp per channel, you
    connect each terminal pair to a different channel on the amp
    (for example, the left output connects to HI and the right side
    to LO). In the other configuration, one amp connects to the LO
    terminals, and the other amp is connected to the HI terminals.

    The point of biamping is that most of the power required to
    drive the speakers is used for low frequencies. Biamping allows
    you to use amps specialized for each of these uses, such
    as a big solid-state amplifier for the LO drivers and higher
    quality (but lower power) amp for the higher frequencies.
    When you have two identical stereo amps, some folks
    recommend distributing the low-frequency load by using an amp
    per channel.  In any case, whenever you use two different
    amplifiers, be careful to match levels between them.

    Biamping also allows you to use high-quality electronic
    crossovers and drive the speaker's drivers (the voice coils)
    directly, without the series resistance and non-linear
    inductance of a passive crossover.  Biamping which uses the
    speaker's crossover is therefore much less desirable.  Replacing
    a good speaker's crossover with an electronic crossover has
    advantages, but involves some very critical tradeoffs and tuning
    which is best left to those well-equipped or experienced.

    See also section 16.0 below, on wire and connectors in general.

10.2 Can amplifier X drive 2 ohm or 4 ohm speakers?
    Almost any amplifier can drive almost any load if you don't turn
    the volume up too high.  Tube amplifiers are one exception.
    Some amps clip if you play them too loud.  This is bad and
    damages speakers.  Other amplifiers shutdown if they are asked
    to play too loud.  Many will overheat, with bad consequences.
    However, in almost all cases, it takes seriously loud sound or
    low speaker resistance (less than 4 ohms) to do damage.  Running
    two sets of 8 ohm speakers at once with common amplifiers
    represents a 4 ohm load.  Four sets of 8 ohm speakers makes a 2
    ohm load.  Two sets of 4 ohm speakers also makes a 2 ohm load.
    If you stay sober and don't turn it up past the point where it
    distorts, you are PROBABLY safe with most amplifiers and most
    loads.  See 10.3 for more information.

10.3 How do I drive more than two speakers with one stereo amplifier?
    One amp can drive many speakers.  However, there are two limits
    to this practice.  The first is that you can overheat or damage
    an amplifier if you drive too low of an impedance to loud
    listening levels.  Avoid loading any amplifier with a lower
    impedance than recommended.  Adding two speakers to one amp
    output loads that output with half the impedance of one speaker.
    (See also 10.2 above)

    The second is that with tube amplifiers, which are uncommon in
    today's common system, it is important that the speaker
    impedance and the amplifier output impedance be well matched.

    When driving two or more speakers from one amp output, always
    wire them in parallel, rather than series.  Series connection,
    while safe in terms of impedance levels, can hurt sound quality
    by raising the impedance that the speakers themselves see.

    Many amplifiers have connectors for two pairs of speakers.  In
    general, these amplifiers also have a speaker selector switch.
    These amplifiers almost exclusively connect the speakers in
    parallel when both are selected.  It is common for these
    amplifiers to specify 8 ohm speakers only.  The reason that they
    specify 8 ohm speakers is because the amplifier is built to
    drive either 4 or 8 ohms, and two sets of 8 ohm speakers loads
    the amplifier as if it were one set of 4 ohm speakers.  It is
    probably perfectly safe to connect one set of 4 ohm speakers to
    an amplifier with two sets of outputs, provided that you NEVER
    use the second set for any set of speakers.

10.4 How big an amplifier do I need?
    Unfortunately, amplifier power ratings and speaker power ratings
    are almost always misleading.  Sometimes, they are factually
    wrong.  Speaker ratings are almost useless in evaluating needs.

    More importantly, human hearing is sensitive to the log of the
    power coming out of the speaker.  This means that the perceived
    difference between an 80 watt amplifier and a 100 watt
    amplifier, all else equal, is very small. (under 1 dB!)

    There is a wide variation in the "efficiency" and "sensitivity"
    of the various speakers available.  I have seen good speakers
    with under 80 dB per watt efficiency and have also seen good
    speakers with over 96 dB per watt efficiency, measured one meter
    from the speaker.  This difference of 16 dB represents a factor
    of 40 difference in power requirement!

    So the first step in determining amplifier requirements is to
    estimate relative speaker efficiency.  Other factors include how
    loud you will want to listen, how large your room is, and how
    many speakers you will drive with one amplifier.  This
    information will give you a rough starting point.  For an
    example, a typical home speaker will produce 88 dB at 1 watt.
    In an average room, a person with average tastes will be happy
    with this speaker and a good 20 watt per channel amplifier.
    Someone who listens to loud music or wants very clean
    reproduction of the dynamics of music will want more power.
    Someone with less efficient speakers or a large room will also
    want more power.

    Past that point, you will have to use your ears.  As with all
    other decisions, your best bet is to get some candidates, borrow
    them from a friendly dealer, take them home, and listen to them
    at your normal and loudest listening level.  See if they play
    cleanly when cranked up as loud as you will ever go, into your
    speakers in your room.  Of course, it is also important to be
    sure that the amp sounds clean at lower listening levels.

10.5 Do all amplifiers with the same specifications sound alike?
    Some say that they do.  Some say that they don't.  Some
    demonstrated that many amplifier differences can be traced to
    very slight frequency response difference.  Let your own ears
    guide you.  If you want to compare amplifiers, you can do it
    best in a controlled environment, such as your home, with your
    music and your speakers.  Also be very careful to match levels
    precisely.  All you need to match levels of amplifiers is a high
    input-impedance digital voltmeter set to AC volts and a test
    recording or signal generator.

10.6 Is this amplifier too big for that set of speakers?
    There is no such thing as an amplifier that is too big.  Small
    amplifiers are more likely to damage speakers than large ones,
    because small amplifiers are more likely to clip than larger
    ones, at the same listening level.  I have never heard of
    speakers being damaged by an overly large amplifier.  I have
    heard of 100 watt speakers being damaged by a 20 watt
    amplifier, however, in really abusive hands.

10.7 Where can I get a cheap low-power amplifier?
    There are very few available.  One source is to buy a cheap boom
    box and only use the amplifier.  Another source is Radio Shack.
    A third alternative is to buy a car stereo booster and get a 12V
    power supply for it.  Finally, you can build an amp pretty
    easily if you are handy, but it probably won't be that cheap.
    Mark V Electronics, for example, sells 20 watt amp kits for
    under $30 and 80 watt amp kits for under $150.  (See 10.14,
    10.15, and 10.16.)

10.8 Is the stuff sold by Carver really awesome?
    Some people really like it.  Others believe that it doesn't
    perform up to specifications.  Still others believe that it
    isn't all that it is hyped up to be. One of Carver's claims to
    fame is lots of watts per pound of weight.  As with almost
    everything else, the best policy is to listen for yourself and
    see what you think.

10.9 What is a preamplifier?
    A preamplifier is an amplifying electronic circuit which can be
    connected to a low output level device such as a phono cartridge
    or a microphone, and produce a larger electrical voltage at a
    lower impedance, with the correct frequency response.  Phono
    cartridges need both amplification and frequency response
    equalization.  Microphones only need amplification.

    In most audio applications, the term 'preamplifier' is actually
    a misnomer and refers to a device more properly called a
    'control amplifier'.  Its purpose is to provide features such
    as input selection, level control, tape loops, and sometimes,
    a minimal amount of line-stage gain.  These units are not
    preamplifiers in the most technical sense of the word, yet
    everyone calls them that.

10.10 What is a passive preamplifier?
    A passive preamplifier is a control unit without any
    amplification at all. It is a classic oxymoron, because it has
    no capability to increase the gain of the signal.  It is only
    used with line level sources that need no gain beyond unity.

10.11 Do I need a preamp?  Why?
    The tasks of a preamp are to:
        Switch between various input signals,
        Amplify any phono inputs to line level,
        Adjust the volume,
        Adjust the treble and bass if necessary,
        Present the right load impedance for the inputs, and
        Present a low source impedance for the outputs.

    If you have a turntable, you NEED a preamp with a phono input.
    This is because the turntable has an output which is too
    small for driving amplifiers and because the output of the
    turntable requires frequency response equalization.  You
    can't connect any other source to a phono input other than a
    turntable (phono cartridge).  Also, you can't connect a phono
    cartridge or turntable to any input other than a phono input.

    Microphones also require special preamplifiers. Some microphones
    also require "phantom power".  Phantom power is operating power
    for the microphone which comes from the preamp.  Microphone
    preamps are often built into tape decks and microphone mixers.

    If you only have high level inputs, such as the output of a CD
    player and the output of a tape deck, the main value of a preamp
    is selecting between inputs and providing a master volume
    control.  If you only listen to CDs, it is plausible to skip
    the preamp entirely by getting a CD player with variable level
    outputs and connecting them directly to a power amplifier.

    Some caveats apply. One, the variable outputs on a CD player are
    often lower sound quality than fixed outputs.  Two, some sources
    have high or nonlinear output impedances which are not ideal for
    driving an amplifier directly.  Likewise, some amplifiers have
    an unusually low or nonlinear input impedance such that common
    sources can't drive the input cleanly.  A good preamplifier
    allows use of such devices without sacrificing sound quality.

    Unfortunately, the only way to be sure that a preamplifier is
    of value with your sources and your amplifier is to try one.

    Almost all receivers contain a phono preamp, a volume
    control, and input switching.  Therefore, if you have a
    receiver, you may never need a preamp.

10.12 Should I leave equipment on all of the time or turn it on and off?
    Some gear draws significant electricity, so you will waste money
    and fossil fuel if you leave it on all of the time.  As an
    example, a common amplifier consumes 40 watts at idle.  High-end
    gear uses far more electricity, but ignoring that, 40 watts x
    168 hours x 52 weeks x US $0.0001 per watt hour (rough estimate)
    is $35/year.  Now add a CD player, a preamp, and a tuner, and it
    really adds up.

    High-end enthusiasts claim that equipment needs to warm up to
    sound its best.  If you care about the best sound, give your
    equipment at least 20 minutes to warm up before serious
    listening.  Warm up will allow the inside temperature to
    stabilize, minimizing offsets, bring bias currents up to their
    proper values, and bringing gain up to operating level.

    Either way, good gear will last a very long time.  Tubes are
    known to have a finite life, but good tube designs run tubes
    very conservatively, giving them life exceeding 10 years of
    continuous service.  Some amplifiers run tubes harder to get
    more power out, and thereby may be more economical to turn off
    between use.

    Filter capacitors will fail after enough time at temperature
    with voltage applied.  They will last longer if turned off
    between use.  However, like tubes, filter caps can last tens of
    years of continuous use, as can power transformers,
    semiconductors, and the like.

    Filter capacitors have a funny problem that justified a simple
    break-in or reforming when they are restarted after many years
    of rest.  It involves bringing up the power line voltage slowly
    with a variable transformer.  For tips on reforming capacitors,
    consult "The Radio Amateur's Handbook", by the ARRL.

    Semiconductors seem to fail more often because of bad surges and
    abuse than age.  Leaving gear off may be best for semiconductors
    and other surge-sensitive gear if you expect power line surges,
    as come from an electrical storm or operation of large motors.

    Fuses seem to age with temperature and get noisy, but they are
    so inexpensive that it should not bias your decision.  However,
    some are inconvenient to change, and may require opening the
    case and even voiding the warranty.
--
Bob Neidorff; Unitrode I. C. Corp.  |  Internet: [e-mail]
7 Continental Blvd.                 |  Voice   : (US) 603-424-2410
Merrimack, NH  03054-0399 USA       |  FAX     : (US) 603-424-3460