Archive-name: car-audio/part1
Rec-audio-car-archive-name: FAQ/part1
Version: 2.3
Last-modified: 5 Jul 94


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+ Frequently Asked Questions with answers for rec.audio.car +
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      0 Introduction and Table of Contents

 This is the FAQ list for the Usenet newsgroup rec.audio.car,
 maintained by Jeffrey S. Curtis (curtis@anl.gov), with
 contributions from many other people (see the credits
 section).  The contents of this document are based on the
 contributors' opinions; neither the contributors nor the FAQ
 maintainer accept any responsibility or liability for any
 damages brought about by the information contained herein.

 This document may be freely distributed and reproduced as long
 as it remains wholly unaltered and includes this notice.  If
 you do redistribute this document, especially on a commercial
 basis, please contact the FAQ maintainer before doing so.

 If you have suggestions for improvements to this document, or
 if you fail to understand any part of it, please feel free to
 send a note to the FAQ maintainer or to the author of the
 relevant section.  The initials of the author(s) of each
 section can be found in brackets following each question.

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 via anonymous FTP from rtfm.mit.edu in the directory
 "/pub/usenet/rec.audio.car/FAQ".  It is also available on the
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 Table of Contents
 ===== == ========

 1 Definitions
    1.1 What do all of those acronyms mean (A, V, DC, AC, W, Hz,
        dB, SPL, THD, ohm)?
    1.2 What is meant by "frequency response"?
    1.3 What is a "sound stage"? What is an "image"?
    1.4 What is meant by "anechoic"?
 2 Electrical
    2.1 My speakers make this high-pitched whine which matches the
        engine's RPMs.  What is it, and how can I get rid of it?
    2.2 What is the best power wire to use?
    2.3 What is the best speaker wire to use?
    2.4 I heard that I should run my power wire directly to my
        car's battery.  Why should I bother, and how do I do it?
    2.5 Should I do the same thing with my ground wire, then?
    2.6 Sometimes when I step out of my car, I get a really bad
        shock.  What is wrong with my system?
    2.7 When my car is running and I have the music turned up
        loud, my headlights dim with the music.  Do I need a new
        battery or a new alternator?
    2.8 What is a "stiffening capacitor", and how does it work?
    2.9 Should I install one in my car? If so, how big should it
        be, and where do I get one?
 3 Components
    3.1 What do all of those specifications on speakers mean?
    3.2 Are component/separates any better than fullrange or
        coaxials?
    3.3 What are some good (and bad) brands of speakers?
    3.4 What do all of those specifications on amplifiers mean?
    3.5 What is "bridging"? Can my amp do it?
    3.6 What is "mixed-mono"? Can my amp do it?
    3.7 What does "two ohm stable" mean? What is a "high-current"
        amplifier?
    3.8 Should I buy a two or four (or more) channel amplifier?
    3.9 What are some good (and bad) brands of amplifiers?
   3.10 What is a crossover? Why would I need one?
   3.11 Should I get an active or a passive crossover?
   3.12 How do I build my own passive crossovers?
   3.13 Should I buy an equalizer?
   3.14 What are some good (and bad) brands of equalizers?
   3.15 What do all of those specifications on tape deck head
        units mean?
   3.16 What are features to look for in a tape deck?
   3.17 What are some good (and bad) brands of tape decks?
   3.18 What are features to look for in a CD head unit?
   3.19 Should I buy a detachable faceplate or pullout CD player?
   3.20 What are some good (and bad) brands of CD head units?
   3.21 Can I use my portable CD player in my car? Won't it skip
        a lot?
   3.22 What's that weird motor noise I get with my portable CD
        player?
   3.23 What are some good (and bad) brands of portable CD
        players?
   3.24 What's in store for car audio with respect to MD, DAT and
        DCC?
   3.25 Are those FM modulator CD changers any good? What are my
        other options?
   3.26 What are some good (and bad) brands of CD changers?
   3.27 Why do I need a center channel in my car, and how do I do
        it?
   3.28 Should I buy a sound field processor?
   3.29 What are some good (and bad) brands of signal processors?
 4 Subwoofers
    4.1 What are "Thiele/Small parameters"?
    4.2 What are the enclosure types available, and which one is
        right for me?
    4.3 How do I build an enclosure?
    4.4 What driver should I use?
    4.5 Is there any computer software available to help me choose
        an enclosure and a driver?
    4.6 What is an "aperiodic membrane"?
 5 Installation
    5.1 Where should I buy the components I want?
    5.2 What mail-order companies are out there?
    5.3 What tools should I have in order to do a good
        installation?
    5.4 Where should I mount my speakers?
    5.5 What is "rear fill", and how do I effectively use it?
    5.6 How do I set the gains on my amp?
    5.7 How do I select proper crossover points and slopes?
    5.8 How do I flatten my system's frequency response curve?
 6 Competition
    6.1 What is IASCA, and how do I get involved?
    6.2 What is USCA, and how do I get involved?
    6.3 What are the competitions like?
    6.4 Should I compete?
    6.5 What class am I in?
 7 Literature
    7.1 What magazines are good for car audio enthusiasts?
    7.2 Are there any newsletters I can read?
    7.3 What books can I read?
 8 Credits


      1 Definitions

 This section contains background information which defines some
 of the acronyms and terminology commonly used in the car audio
 world.  Understanding these definitions is important in order
 to understand the other sections of this document.

    1.1 What do all of those acronyms mean (A, V, DC, AC, W, Hz, dB,
 SPL, THD, ohm)? [JSC]

 "A" is for "amperes", which is a measurement of current equal
 to one coulomb of charge per second.  You usually speak of
 positive current - current which flows from the more positive
 potential to the more negative potential, with respect to some
 reference point (usually ground, which is designated as zero
 potential).  The electrons in a circuit flow in the opposite
 direction as the current itself.  Ampere is commonly
 abbreviated as "amp", not to be confused with amplifiers, of
 course, which are also commonly abbreviated "amp".  In
 computation, the abbreviation for amps is commonly "I".

 "V" is for "volts", which is a measurement of electric
 potential.  Voltages don't "go" or "move", they simply exist as
 a measurement (like saying that there is one mile between you
 and some other point).

 "DC" is for "direct current", which is a type of circuit.  In a
 DC circuit, all of the current always flows in one direction,
 and so it is important to understand which points are at a high
 potential and which points are at a low potential.  For
 example, cars are typically 12VDC (twelve volts direct current)
 systems, and it is important to keep track of which wires in a
 circuit are attached to the +12V (positive twelve volts) lead
 of the battery, and which wires are attached to the ground (or
 "negative") lead of the battery.  In reality, car batteries
 tend to be about 13-14VDC.

 "AC" is for "alternating current", which is a type of circuit
 in which the voltage potential fluctuates so that current can
 flow in either direction through the circuit.  In an AC
 circuit, it is typically not as important to keep track of
 which lead is which, which is why you can plug household
 appliances into an outlet the "wrong way" and still have a
 functioning device.  The speaker portions of an audio system
 comprise an AC circuit.  In certain situations, it is indeed
 important to understand which lead is "positive" and which lead
 is "negative" (although these are just reference terms and not
 technically correct).  See below for examples.  The voltage of
 an AC circuit is usually given as the RMS (root mean square)
 voltage, which, for sinusoidal waves, is simply the peak
 voltage divided by the square root of two.

 "W" is for "watts", a measurement of electrical power.  One
 watt is equal to one volt times one amp, or one joule of energy
 per second.  In a DC circuit, the power is calculated as the
 voltage times the current (P=V x I).  In an AC circuit, the RMS
 power is calculated as the RMS voltage times the RMS current
 (Prms=Vrms x Irms).

 "Hz" is for "hertz", a measurement of frequency.  One hertz is
 equal to one inverse second (1/s); that is, one cycle per
 second, where a cycle is the duration between similar portions
 of a wave (between two peaks, for instance).  Frequency can
 describe both electrical circuits and sound waves, and
 sometimes both.  For example, if an electrical signal in a
 speaker circuit is going through one thousand cycles per second
 (1000Hz, or 1kHz), the speaker will resonate at 1kHz, producing
 a 1kHz sound wave.  The standard range of human hearing is
 "twenty to twenty", or 20Hz-20kHz, which is three decades
 (three tenfold changes in frequency) or a little under ten
 octaves (ten twofold changes in frequency).

 "dB" is for "decibel", and is a measurement for power ratios.
 To measure dB, you must always measure with respect to
 something else.  The formula for determining these ratios is
 P=10^(dB/10), which can be rewritten as dB=10log(P).  For
 example, to gain 3dB of output compared to your current output,
 you must change your current power by a factor of 10^(3/10) =
 10^0.3 = 2.00 (that is, double your power).  The other way
 around, if you triple your power (say, from 20W to 60W) and
 want to know the corresponding change in dB, it is
 dB=10log(60/20)=4.77 (that is, an increase of 4.77dB).  If you
 know your logarithms, you know that a negative number simply
 inverts your answer, so that 3dB corresponding to double power
 is the same as -3dB corresponding to half power.  There are
 several other dB formulas; for instance, the voltage
 measurement is dB=20log(V).  For example, a doubling of voltage
 produces 20log2 = 6.0dB more output, which makes sense since
 power is proportional to the square of voltage, so a doubling
 in voltage produces a quadrupling in power.

 "SPL" is for "sound pressure level" and is similar to dB.  SPL
 measurements are also ratios, but are always measured relative
 to a constant.  This constant is 0dB which is defined as the
 smallest level of sound pressure that the human ear can
 detect.  0dB is equal to 10^-12 (ten to the negative twelfth
 power) W/m^2 (watts per square meter).  As such, when a speaker
 is rated to produce 92dB at 1m when given 1W (92dB/Wm), you
 know that they mean that it is 92dB louder than 10^-12W/m^2.
 You also know than if you double the power (from 1W to 2W), you
 add 3dB, so it will produce 95dB at 1m with 2W, 98dB at 1m with
 4W, 101dB at 1m with 8W, etc.

 "THD" is for "total harmonic distortion", and is a measure of
 the how much a certain device may distort a signal.  These
 figures are usually given as percentages.  It is believed that
 THD figures below approximately 0.1% are inaudible.  However,
 it should be realized that distortion adds, so that if a head
 unit, equalizer, signal processor, crossover, amplifier and
 speaker are all rated at "no greater than 0.1%THD", together,
 they could produce 0.6%THD, which could be noticeable in the
 output.

 "ohm" is a measure of resistance and impedance, which tells you
 how much a device will resist the flow of current in a
 circuit.  For example, if the same signal at the same voltage
 is sent into two speakers - one of which is nominally rated at
 4 ohms of impedance, the other at 8 ohms impedance - twice as
 much current will flow through the 4 ohm speaker as the 8 ohm
 speaker, which requires twice as much power, since power is
 proportional to current.

    1.2 What is meant by "frequency response"? [JSC]

 The frequency response of a device is the range of frequencies
 over which that device can perform in some fashion.  The action
 is specific to the device in question.  For example, the
 frequency response of the human ear is around 20Hz-20kHz, which
 is the range of frequencies which can be resolved by the
 eardrum.  The frequency response of an amplifier may be
 50Hz-40kHz, and that of a certain speaker may be 120Hz-17kHz.
 In the car audio world, frequency responses should usually be
 given with a power ratio range as well, such as (in the case of
 the speaker) 120Hz-17kHz +/-3dB.  What this means is that given
 an input signal anywhere from 120Hz to 17kHz, the output signal
 is guaranteed to be within an "envelope" that is 6dB tall.
 Typically the extreme ends of the frequency range are the
 hardest to reproduce, so in this example, the 120Hz and 17kHz
 points may be referred to as the "-3dB points" of the
 amplifier.  When no dB range is given with a frequency response
 specification, it can sometimes be assumed to be +/-3dB.

    1.3 What is a "soundstage"? What is an "image"? [CD]

 The "soundstage" is the position (front/back and high/low)
 that the music appears to be coming from, as well as the depth
 of the stage.  A car with speakers only in the front will
 likely have a forward soundstage, but may not have enough
 rear fill to make the music seem live.  A car with both front
 and rear speakers may have anything from a forward to a rear
 soundstage, with an accompanying fill from the softer drivers
 depending on the relative power levels and the frequencies
 reproduced.  The high/low position of the soundstage is
 generally only obvious in a car with a forward soundstage.  The
 music may seem to be originating in the footwells, the dash, or
 out on the hood, depending on how the drivers interact with the
 environment.

 The "stereo image" is the width and definition of the
 "soundstage".  Instruments should appear to be coming from
 their correct positions, relative to the recording.  The
 position of the instruments should be solid and easily
 identifiable, not changing with varying frequencies.  A car can
 image perfectly with only a center-mounted mono speaker, but
 the stereo placement of the music will be absent.

    1.4 What is meant by "anechoic"? [JSC]

 Anechoic means "not echoing".  It usually refers to a style of
 measuring a speaker's output which attempts to eliminate echoes
 (or "reflections") of the speaker's output back to the
 measurement area, which could alter the measurement (positively
 or negatively).

      2 Electrical

 This section describes various problems and concepts which are
 closely related to electronics.

    2.1 My speakers make this high-pitched whine which matches the
 engine's RPMs.  What is it, and how can I get rid of it?

 This is perhaps the single most frequently asked question on
 rec.audio.car, and is coincidentally enough the most difficult
 to answer.  But there will indeed be a lengthy answer soon -
 stay tuned.

    2.2 What is the best power wire to use? [JSC]

 There is much debate over the benefit of certain wiring schemes
 (oxygen-free, multistranded, braided, twisted, air core, you
 name it).  However, most people do agree that the most
 important factor in selecting power wire is to use the proper
 size.  Wire is generally rated in size by American Wire Gauge,
 abbreviated AWG, or commonly just "gauge".  To determine the
 correct wire size for your application, you should first
 determine the maximum current flow through the cable (looking
 at the amplifier's fuse is a relatively simple and conservative
 way to do this).  Then determine the length of the cable that
 your will use, and consult the following chart, taken from the
 IASCA handbook (see 6.1):

                           Length of run (in feet)
 Current     0-4  4-7 7-10  10-13  13-16  16-19  19-22  22-28

      0-20A     14   12   12     10     10      8      8      8
    20-35A     12   10    8      8      6      6      6      4
    35-50A     10    8    8      6      6      4      4      4
    50-65A      8    8    6      4      4      4      4      2
    65-85A      6    6    4      4      2      2      2      0
   85-105A      6    6    4      2      2      2      2      0
 105-125A      4    4    4      2      2      0      0      0
 125-150A      2    2    2      2      0      0      0     00

 If aluminum wire is used instead of copper wire, the next
 larger size (smaller number) should be used.  You should also
 consider the installation demands: will you need to run the
 wire around corners or through doors or into the engine
 compartment? These sorts of problems in the car audio
 application require some special care in cable selection.  You
 will want to have cable that is flexible; it should have thick
 insulation as well, and not melt at low temperatures.  You
 don't want to install wire that is rigid and prone to cracks
 and cuts, or else the results could literally be explosive.

    2.3 What is the best speaker wire to use? [JSC,JW]

 Again, there is much debate over the benefit of the various
 schemes that are being used by different manufacturers.  In
 general, however, you will probably want to upgrade your
 speaker wire from the factory ~20 gauge to something bigger
 when you upgrade your amplifiers and speakers.  In most cases,
 16 or 18 gauge should be sufficient, with the possible
 exception of high-power subwoofers.  According to an example by
 Jerry Williamson, using 18 gauge instead of 12 gauge would only
 result in a power loss of 0.1dB, which is essentially
 undetectable by humans.  Thus, other factors play more
 important roles in the selection of speaker wire.  One issue is
 that different wires will have different line capacitances,
 which could cause the wire to act as a low pass filter.
 Generally, however, the capacitances involved are so small that
 this is not a significant problem.  Be sure to heed the
 warnings above regarding cable flexibility and insulation,
 especially when running wire into doors and other areas with an
 abundance of sharp metal.

    2.4 I heard that I should run my power wire directly to my car's
 battery.  Why should I bother, and how do I do it? [JSC]

 For some components, like head units and equalizers, it's
 acceptable to use the stock wiring for power.  However,
 amplifiers generally require large amounts of power, and
 accordingly will draw large amounts of current.  The factory
 wiring in most cars is not designed to handle large amounts of
 current, and most wires have 10-20A fuses on them.  Thus, you
 will almost always want to run the power line for your
 amplifier directly to the positive terminal of the battery.
 This could require drilling a hole through the car's firewall,
 or at least spending time hunting for an existing hole (the
 steering column is a good place to start looking).  Always
 remember to place a fuse on your wire as near to the battery as
 possible! For various reasons, such as an accident or simple
 wear and tear, your wire's insulation may eventually crack,
 which could allow the conducting wire to make contact with the
 chassis of the car and short the battery through this wire,
 which could lead to a serious fire.  The closer you place a
 fuse to the battery, the more protected you are.  Also, when
 running wire through areas with sharp metal corners, it is a
 good idea to use rubber grommets to provide extra protection
 against tearing through your wire's insulation.

    2.5 Should I do the same thing with my ground wire, then? [JSC]

 No.  In almost every case, the best thing to do is to ground
 your amplifier to a point that is attached to the chassis of
 the car and is as close to the amplifier as possible.  The
 ground wire should not need to be more than about eighteen
 inches long, and should be at least as large as the power
 wire.  The point to which you make your ground connection
 should be an unpainted piece of bare metal.

    2.6 Sometimes when I step out of my car, I get a really bad shock.
 What is wrong with my system? [JSC]

 Probably nothing.  This is usually caused by static buildup by
 rubbing against the seats, floormats, etc., just like walking
 across a carpet in a home.  Shocks which can be felt are
 usually in the kilovolt range, so touching a 12VDC wire isn't
 going to do much to you.

    2.7 When my car is running and I have the music turned up loud, my
 headlights dim with the music.  Do I need a new battery or a
 new alternator? [CD,MO]

 The headlights will dim because of a momentary drop in the
 voltage level that is available to power the vehicle's
 accessories, including the headlights, amplifiers, the engine,
 etc.  This voltage drop can be caused by a very large current
 demand by an accessory, such as an amplifier trying to
 reproduce a loud bass note.

 The first thing to do is to get your battery and alternator
 checked for proper functioning.  A failing battery can place
 undesirable loads on the alternator, leaving less power for
 your system.

 If the power system appears to be working correctly, an
 improved alternator may be required for the large current
 demands of the audio system.  When upgrading an alternator,
 be careful in your purchase, for there are some potential
 problems.  An alternator which advertises a certain output
 level may only achieve that output at very high engine RPM
 ranges, for instance.  Also, the new alternator must be
 adjusted to provide an output voltage within a reasonable
 range in terms of the voltage regulator.

 If you find your car will not start after playing the stereo
 for long periods of time with the engine off, and the present
 battery is in good working order, then another, paralleled
 battery could prevent this embarrassing problem.

    2.8 What is a "stiffening capacitor", and how does it work? [JSC]

 "Stiffening Capacitor" (note capitals) is a trademark of
 Autosound 2000 (see 7.2).  However, "stiffening capacitor"
 (note lowercase), as a generic term, refers to a large
 capacitor (several thousand microfarads or greater) placed in
 parallel with an amplifier.  The purpose of doing so is to
 provide a sort of reserve power source from which the amplifier
 can rapidly draw power when it needs it (such as during a deep
 bass note).  The electrical theory is that when the amplifier
 attempts to draw a large amount of current, not only will the
 battery be relatively slow to respond, but the voltage at the
 amplifier will be a little lower than the voltage at the
 battery itself (this is called "line drop").  A capacitor at
 the amplifier which is charged to the battery voltage will try
 to stabilize the voltage level at the amplifier, dumping
 current into the amplifier.  Another way to think about it is
 that a capacitor in parallel with a load acts as a low pass
 filter (see 3.10), and the voltage level dropping at the
 amplifier will appear as an AC waveform superimposed upon a DC
 "wave".  The capacitor, then, will try to filter out this AC
 wave, leaving the pure DC which the amplifier requires.

    2.9 Should I install one in my car? If so, how big should it be,
 and where do I get one? [JSC]

 If you have a problem with dimming headlights when you have
 your music turned up and the bass starts to hit and the engine
 is running and you don't want to upgrade your alternator, or if
 the transient response of your amplifier is unacceptable to
 you, a stiffening capacitor could help you out.  The commonly
 accepted "formula" for determining the proper size capacitor to
 use is 1F/kW (one farad per kilowatt).  For example, a system
 running at 300W would need a 0.3F (or 300,000uF) capacitor.  To
 install the capacitor, you should not simply attach it to your
 power and ground wires near your amplifier, as it will draw
 very large amounts of current from your battery and could blow
 fuses (or overcharge).  Instead, you should insert a
 small-value power resistor (perhaps one ohm) or a 12VDC test
 lamp in between the power lead and the capacitor, and then
 charge it.  If you use a lamp in series with the cap, when the
 lamp goes out, the capacitor is done charging.  When it is done
 charging, carefully remove the capacitor's leads from the
 charging circuit, being certain not to touch the two leads
 together.  You may then permanently install the capacitor by
 wiring it in parallel with your amplifier (be careful not to
 short the leads!).  Large caps are currently available from
 some audio dealers, such as Phoenix Gold.  You could also try
 electronics shops or mail-order houses.

Jeffrey S. Curtis                      | Email: curtis@anl.gov
Argonne National Laboratory            | Voice: 708/252-1789
9700 South Cass Avenue, 221-ECT        | Fax:   708/252-5983
Argonne, IL 60439                      |
