AOH :: CARAUD-1.FAQ|
Frequently Asked Questions about Car Audio, Part 1
Last-modified: 5 September 95
+ Frequently Asked Questions with answers for rec.audio.car +
0 Introduction and Table of Contents
This is the FAQ list for the Usenet newsgroup rec.audio.car,
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Changes to the FAQ
======= == === ===
Additions since the last posting (12/6/95)
Changes since the last posting (12/6/95)
Table of Contents
===== == ========
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"?
2.1 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?
3.1 What do all of those specifications on speakers mean?
3.2 Are component/separates any better than fullrange or
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"
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 design 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
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
3.22 What's that weird motor noise I get with my portable CD
3.23 What are some good (and bad) brands of portable CD
3.24 What's in store for car audio with respect to MD, DAT and
3.25 Are those FM modulator CD changers any good? What are my
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
3.28 Should I buy a sound field processor?
3.29 What are some good (and bad) brands of signal processors?
3.30 How do I build my own passive crossovers?
4.1 What are "Thiele/Small parameters"?
4.2 What are the enclosure types available?
4.3 Which enclosure type is right for me?
4.4 How do I build an enclosure?
4.5 What driver should I use?
4.6 Is there any computer software available to help me choose
an enclosure and a driver?
4.7 What is an "aperiodic membrane"?
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
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?
5.9 How do I wire speakers "in series" and "in parallel"?
5.10 Are there any alternatives for Dynamat? It's too expensive!
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?
6.6 Where can I find out when these Sound-Offs are?
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?
7.4 Can I contact any manufacturers on-line?
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 have a potential difference of slightly higher than
12V, and the charging system can produce upwards of 14.5V when
the engine is running.
"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
"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
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
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? [IDB]
The answer to this section was generously provided by David
Navone of Autosound 2000. The material in these instructions was
adapted from the Autosound 2000 Troubleshooting Flow Chart by Ian
Bjorhovde with the permission of Autosound 2000. For more
information about Autosound 2000, see Section 7.
This is a set of instructions to debug a stereo installation if
there is any noise present after it is completed. Follow each step
carefully! If you have more than one amplifier, repeat level one
for each amp to be sure that none of them are responsible for the
LEVEL 1: Check out the Amplifier(s)
After you have determined that there is noise in the system,
determine if the amplifier is causing the noise. To do this, mute
the signal at the inputs to the amp by using shorting plugs. If
there is no noise, then the amp is fine, and you can proceed to
level 2. However, if there is noise, then use a test speaker at
the amp's output. If this stops the noise, then the problem is
originating in the speaker wiring, or the passive crossovers.
Check to make sure that none of these are shorting with the body of
the car, and start again at level 1.
If noise is still present when using the test speaker, then
there may be a problem with the pwoer supply on the amp. Try
connecting an isolated power supply - if this gets rid of the noise,
then there is something seriously wrong with the amp, and it should
be replaced. However, if the noise is still present, then there may
be a problem with power supply filtering or isolation. This can be
fixed by changing the amp's gound point or b adding external supply
LEVEL 2: Reduce the System
The amps have been determined to be noise free. If you have
any processors between the head unit and the amps, disconnect them
and connect the head unit directly to the amp. If this gets rid of
the noise, then one (or more) of the processors must be at fault,
so proceed to level 5. Otherwise, try running the signal cables over
a number of different routes. If you are able to find one that does
not produce any noise, permanently route the cables in the same
manner, and proceed to level 5. If not, then you must isolate the
head unit from the car's chassis (except for its ground!) -- don't
forget to disconnect the antenna, since it is also grounded to the
If isolating the head unit does not solve the problem, the move
the grounding point of the head unit. Hopefully the noise will be
gone, and you can install the head unit with a quiet ground and
proceed to level 5, otherwise go on to level 3.
LEVEL 3: Move the Head Unit
The amplifiers are fine, but moving both the ground for the
head unit and the signal cables does not solve the noise problem.
Take the unit completely out of the dash, and put it on either the
seat or carpet, and run new signal cables to the input of the amp.
If this solves the problem, re-install the head unit, one step at a
time and skip to level 5. But if the noise persists, then move the
head unit as close to the amp as possible and use the shortest
possible signal cables. This will verify that the original signal
cables are not causing the problem -- assuming the noise is gone,
reinstall the head unit one step at a time and go to level 5.
Otherwise, there may be a problem with the power filtering for the
head unit. As with the amps, power the head unit with an isolated
power supply (again making sure that the head unit isn't touching
the car's chassis at all). If the noise goes away, you can add
power supply filtering or an isolated power supply; go to level 2.
But if the isolated power supply does not solve the problem, then
you can either replace the head unit and go to level 2, or check the
car's electrical system in level 4.
LEVEL 4: Testing the Car
There does not seem to be a problem with either the head unit
or the amplifier, and the car's charging system is suspect. To see
if this is the case, we can use a system in a car that is already
known to be "quiet." Bring both cars together as if you were going
to jump one, and use jumper cables to connect the two batteries.
Start the engine of the car with the noise problem, and listen to
the "quiet" car's system. If the noise does not go away, there is a
SERIOUS problem with your car's electrical system (possibly a bad
alternator). Have a qualified mechanic check the charging system
out. If there is no noise in the "quiet" car, then the "noisy"
car's charging system is definately quiet, so continue with level 5.
LEVEL 5: Adding Signal Processors
We have proven that the amplifiers are good, the head unit is
good, and the car's electrical system is good. Now we need to
reconnect each signal processor. Repeat this level for each signal
processor used in your system; if you have added all of your signal
processors, and there is no longer any noise, CONGRATULATIONS!
You've removed the noise from your system!
Connect the signal processor. If there isn't any noise, then
go on to the next signal processor. Otherwise, try re-routing the
signal cables. If this cures the problem, the route them permanently
over the quiet path, and install the next processor. If not,
then isolate the processor from the car's chassis except for a
single grounding point. If this works, then permanently isolate the
processor, and move on to the next processor. If isolation does not
help, then advance to level 6.
LEVEL 6: Processor Isolation Tests
Now, noise enters the system when one particular processor is
installed, but regrounding it does not help. Move the processor
very close to the amp, and check for noise again. If there isn't
any, then re-install the processor, carefully routing the cables to
ensure no noise, and continue at level 5 with the next processor.
Otherwise, use an isolated power supply to power the processor,
making sure that no part of the processor is touching the car's
chassis. If this solves the problem, the consider permanently
installing an isolated power supply or possibly a 1:1 transformer,
and go to level 5 with the next processor. Otherwise, separate the
processor and isolated power supply from the car by many feet and re-
test. If there is still noise, then there is a serious problem with
the processor's design. Get a different processor, and continue at
level 5 with it. If separating the power supply and processor from
the car does solve the noise problem, then either the processor is
damaged, or your tests were inaccurate. Repeat level 5.
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, IDB]
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.
Some cars (Audi, Porsche) have galvanized bodies, and in these
cars, you must find one of the manufacturers' grounding points
or else some noise can result.
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
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 (25 ohm, 1/2 watt) 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.
Ian D. Bjorhovde UofA Div. of Neural Systems, Memory & Aging
ianbjor@NSMA.Arizona.EDU UNIX Systems Administrator
Pager/VoxMail: 520/571-4347 WWW: http://www.nsma.arizona.edu/~ianbjor
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