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Archive-name: car-audio/part1
Rec-audio-car-archive-name: FAQ/part1
Version: 2.5
Last-modified: 10 Aug 94


Special editorial note - this is the last time that I will be posting
the rec.audio.car FAQ. I have turned over my duties after a year and a
half to Ian Bjorhovde. To everyone who ever submitted material,
pointed out mistakes, or just wrote to express gratitude for my
efforts, thank you. I truly enjoyed the time I spent doing this work.

*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*
+ 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,
maintained by Jeffrey S. Curtis ([email protected]), 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.

This document is posted once per month to the Usenet newsgroups
rec.audio.car, rec.answers, and news.answers, and is available
via anonymous FTP from rtfm.mit.edu in the directory
"/pub/usenet/rec.audio.car/FAQ". It is also available on the
World Wide Web (WWW) and can be accessed using the URL

http://copsg2.pharm.arizona.edu/~ianbjor/rac.faq/

If you do not have access to FTP or a WWW client (such as NCSA
Mosaic) and cannot find the FAQ on any of the aforementioned
newsgroups, you can obtain the FAQ via email by sending email
to [email protected] with the following three lines in
the message body:

send usenet/rec.audio.car/FAQ/part1
send usenet/rec.audio.car/FAQ/part2
send usenet/rec.audio.car/FAQ/part3

Note that the rtfm.mit.edu server is a very popular machine,
and accordingly, responses to requests may be slow, especially
via the mail-server. Please be patient when using this
service.


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 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
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: [email protected]
Argonne National Laboratory | Voice: 708/252-1789
9700 South Cass Avenue, ECT-221 | Fax: 708/252-9689
Argonne, IL 60439 |
Archive-name: car-audio/part2
Rec-audio-car-archive-name: FAQ/part2
Version: 2.5
Last-modified: 10 Aug 94


3 Components

This section describes various components that you can have in
a car audio system, along with common specifications, desirable
features, some of the best and worst brands, and so on.

Be aware that there is no standardized testing mechanism in
place for rating car audio products. As such, manufacturers
are open to exaggerating, "fudging", or just plain lying when
it comes to rating their own products.

3.1 What do all of those specifications on speakers mean? [JSC,CD]

"Input sensitivity" is the SPL the driver will produce given
one watt of power as measured from one meter away given some
input frequency (usually 1kHz unless otherwise noted on the
speaker). Typical sensitivities for car audio speakers are
around 90dB/Wm. Some subwoofers and piezo horns claim over
100dB/Wm. However, some manufacturers do not use true 1W
tests, especially on low impedance subwoofers. Rather, they
use a constant voltage test which produces more impressive
sensitivity ratings.

"Frequency response" in a speaker refers to the range of
frequencies which the speaker can reproduce within a certain
power range, usually +/-3dB.

"Impedance" is the impedance of the driver (see 1.1), typically
4 ohms, although some subwoofers are 8 ohms, some stock Delco
speakers are 10 ohms, and some stock Japanese imports are 6
ohms.

"Nominal power handling" is the continuous power handling of
the driver. This figure tells you how much power you can put
into the driver for very long periods of time without having to
worry about breaking the suspension, overheating the voice
coil, or other nasty things.

"Peak power handling" is the maximum power handling of the
driver. This figure tells you how much power you can put into
the driver for very brief periods of time without having to
worry about destroying it.

3.2 Are component/separates any better than fullrange or coaxials? [JSC]

Usually, yes. Using separates allows you to position the
drivers independently and more carefully, which will give you
greater control over your imaging. For rear fill applications,
however, coaxial speakers will perform fine, as imaging is not
a primary concern.

3.3 What are some good (and bad) brands of speakers? [JSC]

People will emotionally defend their particular brand of
speakers, so asking what the "best" is is not a good idea.
Besides, the best speaker is the one which suits the
application the best. In general, however, various people have
claimed excellent experiences with such brands as Boston
Acoustics, MB Quart, a/d/s/, and Polk. Also, most people agree
that you should avoid brands like Sparkomatic and Kraco at all
costs.

3.4 What do all of those specifications on amplifiers mean? [JSC,BG]

"Frequency response" refers to the range of frequencies which
the amplifier can reproduce within a certain power range,
usually +/-3dB.

"Continuous power output" is the power output of the amplifier
into one channel into a certain load (usually four ohms) below
a certain distortion level (usually at most 1%THD) at a certain
frequency (usually 1kHz). A complete power specification
should include all of this information, e.g. "20W/ch into 4
ohms at < 0.03%THD at 1kHz" although this can also be stated as
(and be assumed equivalent to) "20W/ch at < 0.03%THD". The
amplifier should also be able to sustain this power level for
long periods of time without difficulties such as overheating.

"Peak power output" is the power output of the amplifier into
one channel into a certain load (usually four ohms) below a
certain distortion level (usually much higher than the
continuous rating level) at a certain frequency (usually
1kHz). A complete power specification should include all of
this information, e.g. "35W/ch into 4 ohms at < 10.0%THD at
1kHz" although this can also be stated as (and be assumed
equivalent to) "35Wch at < 10.0%THD". Consumer warning: some
manufacturers will state the "peak power output" rating by
including the amount of power which can be drawn from
"headroom", which means power supply capacitors. They usually
will not tell you this in the specification, however; indeed,
they tend to prominently display the figure in big, bold
letters on the front of the box, such as "MAXIMUM 200W PER
CHANNEL!!!" when the continuous rating is 15W/ch and the unit
has a 5A fuse.

"Damping factor" represents the ratio of the load being driven
(that is, the speaker - usually four ohms) to the output
impedance of the amplifier (that is, the output impedance of
the transistors which drive the speakers). The lower the
output impedance, the higher the damping factor. Higher
damping factors indicate a greater ability to help control the
motion of the cone of the speaker which is being driven. When
this motion is tightly controlled, a greater transient response
is evident in the system, which most people refer to as a
"tight" or "crisp" sound. Damping factors above 100 are
generally regarded as good.

"Signal to Noise" or "S/N" is the ratio, usually expressed in
decibels, of the amount of true amplified output of the
amplifier to the amount of extraneous noise injected into the
signal. S/N ratios above 90 to 95dB are generally regarded as
good.

3.5 What is "bridging"? Can my amp do it? [JSC]

Bridging refers to taking two channels of an amplifier and
combining them to turn the amplifier into a one channel
amplifier. In normal operation, one wire which goes to a
speaker from the amplifier is "neutral", that is, the potential
never changes (with respect to another fixed point, like
ground). The other wire is "hot", that is, it carries the
fluctuating AC speaker signal. The speaker "sees" a potential
between these two leads, and so there is a voltage applied to
the speaker. When an amplifier is bridged, both leads are
"hot". However, one signal must be inverted, or else the
speaker will never see a potential, as both wires are carrying
roughly the same signal. With one signal inverted, the speaker
will see a signal that is twice as great as one signal alone.
Thus, if your amplifier does not have a switch or button of
some sort which inverts one channel, you cannot bridge your
amplifier (unless you build an external inverter). With
respect to power, the commonly accepted definition is that when
you bridge an amplifier, you add all of the characteristics of
the bridged channels together. Thus, if you bridge an
amplifier that is 50W/ch into 4 ohms at < 0.05%THD, your
bridged channel is 100W/ch into 8 ohms at < 0.10%THD.
Therefore, an amplifier which is 2 ohm stable in stereo mode is
only 4 ohm stable in bridged mono mode, and an amp which is 4
ohm stable in stereo is only 8 ohm stable in bridged mono.

3.6 What is "mixed-mono"? Can my amp do it? [JSC]

Some amplifiers which are both bridgeable and able to drive low
impedance loads also allow you to use "mixed-mono" mode. This
involves driving a pair of speakers in stereo mode as well as
simultaneously driving a single speaker in bridged mono mode.
What happens is that you put your amp in bridged mode, which
inverts one output signal. You then connect the mono speaker
as you normally would in bridged mode. To the channel which is
not inverted, you connect your stereo speaker as you normally
would. To the channel which is inverted, you connect the other
stereo speaker with its leads reversed (+ to - and - to +)
since the signal is inverted.

However, for this to work, the amplifier must actually use both
input channels in bridged mode. Many amplifiers, when placed
in bridged mode, will simply "copy" and invert either the left
or the right channel. This practice ensures high output to the
mono speaker, but eliminates the possibility of mixed mono
since you lose one channel.

3.7 What does "two ohm stable" mean? What is a "high-current"
amplifier? [JSC]

An x ohm stable amplifier is an amp which is able to
continuously power loads of x ohms per channel without
encountering difficulties such as overheating. Almost all car
amplifiers are at least four ohm stable. Some are two ohm
stable, which means that you could run a pair of four ohm
speakers in parallel on each channel of the amplifier, and each
channel of the amp would "see" two ohms. Some amps are
referred to as "high-current", which is a buzzword which
indicates that the amp is able to deliver very large
(relatively) amounts of current, which usually means that it is
stable at very low load impedances, such as 1/4 or 1/2 of an
ohm. Note that the minimum load rating (such as "two ohm
stable") is a stereo (per channel) rating. In bridged mode,
the total stability is the sum of the individual channels'
stability (see 3.5).

3.8 Should I buy a two or four (or more) channel amplifier? [JSC]

If you only have one line-level set of outputs available, and
wish to power two sets of speakers from a single amplifier, you
may be able to save money by purchasing a two channel amplifier
which is stable to two ohms rather than spending the extra
money for a four channel amp. If you do this, however, you
will be unable to fade between the two sets of speakers
(without additional hardware), and the damping factor of the
amplifier will effectively be cut in half. Also, the amp may
run hot and require fans to prevent overheating. If you have
the money, a four channel amp would be a better choice. You
would need to add a dual-amp balancer in order to maintain
fader capability, however, but it is more efficient than
building a fader for a two channel amp. If you wish to power a
subwoofer or additional speakers as well, you may want to
purchase a five or six channel amp.

3.9 What are some good (and bad) brands of amplifiers? [JSC]

As with speakers, people emotionally defend their amplifier, so
choosing the best is difficult. However, some brands stand out
as being consistently good while others are consistently bad.
Among the good are HiFonics, Phoenix Gold, a/d/s/, and
Precision Power.

3.10 What is a crossover? Why would I need one? [JSC]

A crossover is a device which filters signals based on
frequency. A "high pass" crossover is a filter which allows
frequencies above a certain point to pass unfiltered; those
below that same point still get through, but are attenuated
according to the crossover slope. A "low pass" crossover is
just the opposite: the lows pass through, but the highs are
attenuated. A "band pass" crossover is a filter that allows a
certain range of frequencies to pass through while attenuating
those above and below that range. There are passive
crossovers, which are collections of purely passive (unpowered)
devices - mainly capacitors and inductors and sometimes
resistors. There are also active crossovers which are powered
electrical devices. Passive crossovers are typically placed
between the amplifier and the speakers, while active crossovers
are typically placed between the head unit and the amplifier.
There are a few passive crossovers on the market which are
intended for pre-amp use (between the head unit and the
amplifier), but the cutoff frequencies (also known as the
"crossover point", defined below) of these devices are not
typically well-defined since they depend on the input impedance
of the amplifier, which varies from amplifier to amplifier.

There are many reasons for using crossovers. One is to filter
out deep bass from relatively small drivers. Another is to
split the signal in a multi-driver speaker so that the woofer
gets the bass, the midrange gets the mids, and the tweeter gets
the highs.

Crossovers are categorized by their "order" and their
"crossover point". The order of the crossover indicates how
steep the attenuation slope is. A first order crossover "rolls
off" the signal at -6dB/octave (that is, quarter power per
doubling or halving in frequency). A second order crossover
has a slope of -12dB/octave; third order is -18dB/octave; etc.
The crossover point is generally the frequency at which the
-3dB point of the attenuation slope occurs. Thus, a first
order high pass crossover at 200Hz is -3dB down at 200Hz, -9dB
down at 100Hz, -15dB down at 50Hz, etc.

It should be noted that the slope (rolloff) of a crossover, as
defined above, is only an approximation. This issue will be
clarified in future revisions of this document.

The expected impedance of a crossover is important as well. A
crossover which is designed as -6dB/octave at 200Hz high pass
with a 4 ohm driver will not have the same crossover frequency
with a driver which is not 4 ohms. With crossovers of order
higher than one, using the wrong impedance driver will wreak
havoc with the frequency response. Don't do it.

3.11 Should I get an active or a passive crossover? [JSC]

Active crossovers are more efficient than passive crossovers.
A typical "insertion loss" (power loss due to use) of a passive
crossover is around 0.5dB. Active crossovers have much lower
insertion losses, if they have any loss at all, since the
losses can effectively be negated by adjusting the amplifier
gain. Also, with some active crossovers, you can continuously
vary not only the crossover point, but also the slope. Thus,
if you wanted to, with some active crossovers you could create
a high pass filter at 112.3Hz at -18dB/octave, or other such
things.

However, active crossovers have their disadvantages as well.
An active crossover may very well cost more than an equivalent
number of passive crossovers. Also, since the active crossover
has separate outputs for each frequency band that you desire,
you will need to have separate amplifiers for each frequency
range. Furthermore, since an active crossover is by definition
a powered device, the use of one will raise a system's noise
floor, while passive crossovers do not insert any additional
noise into a system.

Thus, if you have extra money to spend on an active crossover
and separate amplifiers, and are willing to deal with the
slightly more complex installation and possible noise problems,
an active crossover is probably the way to go. However, if you
are on a budget and can find a passive crossover with the
characteristics you desire, go with a passive.

3.12 How do I build my own passive crossovers? [JSC]

A first order high pass crossover is simply a capacitor placed
inline with the driver. A first order low pass crossover is an
inductor inline with the driver. These roles can be reversed
under certain circumstances: a capacitor in parallel with a
driver will act as a low pass filter, while an inductor in
parallel with a driver will act as a high pass filter.
However, a parallel device should not be the first element in a
set; for example, using only a capacitor in parallel to a
driver will cause the amplifier to see a short circuit above
the cutoff frequency. Thus, a series device should always be
the first element in a crossover.

When like combinations are used, the order increases: a
crossover in series followed by an inductor in parallel is a
second order high pass crossover. An inductor in series
followed by a capacitor in parallel is a second order low pass
crossover.

To calculate the correct values of capacitors and inductors to
use, you need to know the nominal impedance (Z) of the circuit
in ohms and the desired crossover point (f) in hertz. The
needed capacitance in farads is then 1/(2 x pi x f x Z). The
needed inductance in henries is Z/(2 x pi x f). For example,
if the desired crossover point is 200Hz for a 4 ohm driver, you
need a 198.9 x 10^-6 F (or 199uF) capacitor for a high pass
first order filter, or a 3.18 x 10^-3 H (or 3.18mH) inductor
for a low pass first order filter.

To obtain low insertion losses, the inductors should have very
low resistance, perhaps as low as 0.1 to 0.2 ohms.

Also, be sure to select capacitors with proper voltage
ratings. The maximum voltage in the circuit will be less than
the square root of the product of the maximum power in the
circuit and the nominal impedance of the driver. For example,
a 4 ohm woofer being given 100W peak will see a maximum voltage
of sqrt(100*4) = sqrt(400) = 20V. Make sure that the
capacitors are bipolar, too, since speaker signals are AC
signals. If you cannot find bipolar capacitors, you can use
two polar capacitors in parallel and in opposite polarity (+ to
- and - to +). However, there are some possible problems with
this approach: the forward voltage rating will probably not be
equal to the reverse voltage rating, and there could be a
reverse capacitance as well. Both problems could adversely
affect your circuit if you decide to use opposite polarity
capacitors in parallel.

To build a second order passive crossover, calculate the same
initial values for the capacitance and inductance, and then
decide whether you want a Linkwitz-Riley, Butterworth, or
Bessel filter. An L-R filter matches the attenuation slopes so
that both -3dB points are at the same frequency, so that the
system response is flat at the crossover frequency. A
Butterworth filter matches the slopes so that there is a peak
at the crossover frequency, and a Bessel filter is in between
the two. For an L-R filter, halve the capacitance and double
the inductance. For a Butterworth filter, multiply the
capacitance by 1/sqrt(2) and the inductance by sqrt(2). For a
Bessel filter, multiply the capacitance by 1/sqrt(3) and the
inductance by sqrt(3).

You should realize, too, that crossovers induce a phase shift
in the signal of 90 degrees per order. In a second order
filter, then, this can be corrected by simply reversing the
polarity of one of the drivers, since they would otherwise be
180 degrees out of phase with respect to each other. In any
case with any crossover, though, you should always experiment
with the polarity of the drivers to achieve the best total
system response.

As with the definition of crossover slopes, the above
definition of the phase shift associated with a crossover is
also an approximation. This will be addressed in future
revisions of this document.

3.13 Should I buy an equalizer? [JSC]

Equalizers are normally used to fine-tune a system, and should
be treated as such. Equalizers should not be purchased to
boost one band 12dB and to cut another band 12dB and so on -
excessive equalization is indicative of more serious system
problems that should not simply be masked with an EQ. However,
if you need to do some minor tweaking, an EQ can be a valuable
tool. Additionally, some EQs have spectrum analyzers built in,
which makes for some extra flash in a system. There are two
main kinds of EQs available today: dash and trunk. Dash EQs
are designed to be installed in the passenger compartment of a
car, near the head unit. They typically have the adjustments
for anywhere from five to eleven (sometimes more) bands on the
front panel. Trunk EQs are designed to be adjusted once and
then stashed away. These types of EQs usually have many bands
(sometimes as many as thirty). Both types sometimes also have
crossovers built in.

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? [JSC]

You can use any portable CD player in a car provided that you
have either an amplifier with line level inputs (preferred) or
a tape deck. If you have the former, you can simply buy a 1/8"
headphone jack to RCA jack adapter and plug your CD player
directly into your amplifier. If you have the latter, you can
purchase a 1/8" headphone jack to cassette adapter and play CDs
through your tape deck. The cassette adapters tend to be far
more convenient; however, there is a significant tradeoff: by
using cassette adapters, you limit your sound to the frequency
response of the tape head, which is sometimes as much as an
entire order of magnitude worse than the raw digital material
encoded onto the CD itself.

Portable CD players which were not designed for automotive use
will tend to skip frequently when used in a car (relatively).
CD players that are specially designed for automotive use, such
as the Sony Car Discman, tend to include extra dampening to
allow the laser to "float" across the bumps and jolts of a
road. Some people have indicated success with using regular
portable CD players in a car when they place the CD player on a
cushion, such as a thick shirt or even on their thighs.

3.22 What's that weird motor noise I get with my portable CD player? [JSC]

Many people report problems while playing CDs from a portable
CD player into their car audio systems. The problem, stated
very simply, has to do with the stepping of the motor requiring
a varying amount of current and non-isolated power and audio
signal grounds. Using a liberal application of capacitors and
inductors, this voltage variance can be restricted to a window
of 8.990 to 9.005V for a 9V CD player, yet even the swing
between these two levels is enough to cause annoyingly loud
noise on the outputs. It has been reported that this entire
problem can be solved by using a true DC-DC inverter at the
power input to the 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? [HK]

MiniDisc (MD) seems to have a better future than Digital Audio
Tape (DAT) or Digital Compact Cassette (DCC) which don't seem
to have appeal to the public. Ease of use seems to be an
important factor and the CD formats allows direct access to
musical tracks at an instant. Although MD doesn't match the
sound quality of the standard CDs it will probably be popular
since the players have a buffer to eliminate skipping. DAT
will remain as a media for ProAudio for recording purposes
before pressing CDs.

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? [HK,
JSC]

If a proper center image isn't achievable via a two channel
configuration, installation of a center channel can help.
Since the majority of recordings are done in two channel, a two
channel system designed correctly should be able to reproduce a
center image which was captured during recording. A center
channel is not simply a summation of the left and right
channels, like bridging an amplifier; rather, it is an
extraction of common signals from the left and right channels.
This usually means the lead vocals, and perhaps one or two
instruments. These signals will then be localized to the
center of the stage, instead of perhaps drifting between the
left center and right center of the stage. A signal processor
is usually required in order to properly create a center
channel image. The image should then be sent to a driver in
the physical center of the front of the car, at an
amplification level somewhat lower than the rest of the
speakers. The correct frequency range and power levels will
depend on the particular installation, though a good starting
point is perhaps a pass band of 250-3000Hz at an amplification
level of half the power of the main speakers (3dB down).

3.28 Should I buy a sound field processor?

3.29 What are some good (and bad) brands of signal processors?

--
Jeffrey S. Curtis | Email: [email protected]
Argonne National Laboratory | Voice: 708/252-1789
9700 South Cass Avenue, ECT-221 | Fax: 708/252-9689
Argonne, IL 60439 |
Archive-name: car-audio/part3
Rec-audio-car-archive-name: FAQ/part3
Version: 2.5
Last-modified: 10 Aug 94


4 Subwoofers

This section describes some elements necessary for
understanding subwoofers - how they operate, how to build
proper enclosures, how to pick the right driver for you, and
how to have a computer do some of the work for you.

4.1 What are "Thiele/Small parameters"? [CD,RDP]

These are a group of parameters outlined by A.N. Thiele, and
later R.H. Small, which can completely describe the electrical
and mechanical characteristics of a mid and low frequency
driver operating in its pistonic region. These parameters are
crucial for designing a quality subwoofer enclosure, be it for
reference quality reproduction or for booming.

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
Ql The system's Q at Fb, due to leakage losses;
dimensionless
Qa The system's Q at Fb, due to absorption losses;
dimensionless
Qp The system's Q at Fb, due to port losses (turbulence,
viscousity, etc.); 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 resistive 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 propagation velocity of sound at STP, approx. 342 m/s
p (rho) density of air at STP 1.18 kg/m^3

4.2 What are the enclosure types available, and which one is right
for me? [JLD]

Only the order of the enclosure First Order
itself is shown here. The addition Infinite-Baffle or Free-Air
of a crossover network increases
the order of the system by the |
order of the crossover. |
Example: If a First-Order, 6dB/Oct. /
crossover (single inductor in series /
with the speaker) is used with a ||
Fourth Order enclosure, the total ||
system is a fifth order. \
Note: Air volumes and ratios shown \
here may not be to scale. This is |
designed to provide order information |
only.


Second Order Second Order
Acoustic- or Air-Suspension Isobaric* Acoustic-Suspension
or Sealed (Compound Loaded)
_______________________ _______________________
| | | _____|
| / | / /
| / | / /
| || | || ||
| || | || ||
| \ | \ \
| \ | \____\
|_______________________| |_______________________|


Fourth Order Fourth Order Fourth Order
Bass-Reflex or Passive Radiator Isobaric*
Vented or Ported Bass-Reflex Bass-Reflex
_______________ _______________ _______________
| | | | | ____ |
| / | / | / /
| / | / | / /
| || | || | || ||
| || | || | || ||
| \ | \ | \ \
| \ | \ | \____\
| | | | | |
| | | / | |
| | | / | |
| ____| | | | ____|
| | | |
| ____ | \ | ____
| | | \ | |
|_______________| |_______________| |_______________|


Fourth Order Fourth Order
Single-Reflex Bandpass Isobaric* Single-Reflex Bandpass
_________________ ____ _______________________ ____
| | | | | | | | | |
| / | | | | / \ | | |
| / | | / \ |
| || | | || || |
| || | | || || |
| \ | | \ / |
| \ | | \ / |
|_________|_______________| |_______________|_______________|


Fourth Order Fourth Order
Three Chamber Three Chamber Isobaric*
Single-Reflex Bandpass Single-Reflex Bandpass
____________ ____________ ______________ ______________
| | | | | | | | | | | |
| / | | \ | | / \ | | / \ |
| / \ | | / \ / \ |
| || || | | || || || || |
| || || | | || || || || |
| \ / | | \ / \ / |
| \ / | | \ / \ / |
|______|_____________|______| |_______|_______________|_______|


Fifth Order = Fourth Order Enclosure + First Order Crossover
= Third Order Enclosure + Second Order Crossover, etc.


Sixth Order Sixth Order
Dual-Reflex Bandpass Isobaric* Dual-Reflex Bandpass
____ _____________ ____ ____ ____________ ____
| | | | | | | | | | | | | |
| | | / | | | | | | / \ | | |
| | | / | | | | / \ |
| || | | || || |
| || | | || || |
| \ | | \ / |
| \ | | \ / |
|_______________|_____________| |______________|_____________|

Sixth Order
Three Chamber Quasi-Sixth Order
Dual-Reflex Bandpass Series-Tuned Bandpass
_ _________ _________ _ _________________ ____
| | | | | | | | | | | | | | |
| | | / | | \ | | | | / | | |
| / \ | | / |
| || || | | || |
| || || | | || |
| \ / | | \ |
| \ / | | \ |
|________|_____________|________| | ____| |
| |
| ____ |
| | |
|___________|_____________|


Seventh Order = Sixth Order Enclosure + First Order Crossover, etc.


* Isobaric or Coupled Pair (Iso-group) Variations:

A variety of configurations may be used in the isobaric loading
of any order enclosure. Physical and acoustic restrictions may
make one loading configuration preferable to another in a
particular enclosure.

Composite or Push-Pull Compound or Piggy-Back
or Face-to-Face Loading or Tunnel Loading
_________________ ___________________________
| | | ____|
| / \ | / /
| / \ | / /
| >>> || || >>> | >>> || || >>>
| >>> || || >>> | >>> || || >>>
| \ / | \ \
| \ / | \___\
|_________________| |___________________________|

Back-to-Back Loading Planar Loading
_________________________ ___________________________
| _________| | | |
| \ / | / |
| \ / | / |
| >>> || || >>> | || >>> |
| >>> || || >>> | || >>> |
| / \ | \ |
| /_______\ | \ |
|_________________________| |________________________| |
| |
/ |
/ |
|| <<< |
|| <<< |
\ |
>>> indicates direction of \ |
>>> simultaneous cone movement. |__|

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? [JSC,MH]

Various enclosure design software is available via ftp from
ftp.uu.net in the /usenet/rec.audio.high-end/Software
directory. The most popular program there is Perfect Box,
which is in the file "perf.uu" (or "perf.zip").

For an overview of many programs and devices available for
enclosure design, obtain the file "sahfsd01.doc" at the
ftp.uu.net archive. The filename stands for "Software and
Hardware for Speaker Design", and was added to the archive
in June 1994 by an anonymous contributor.

4.6 What is an "aperiodic membrane"? [CD]

An aperiodic membrane is one part of a type of subwoofer
enclosure. It is an air-permeable sheet which has
frequency-dependent acoustical resistance properties. The
original design goes back to Naim, for use in home systems, but
has been applied by several individuals and companies in car
audio.

The completed system will be aperiodic, which means it will
prove to be over-damped with a Q below 0.7. In contrast, most
car audio systems range from sort of to grossly underdamped,
with Q's > 0.8 and higher. These high-Q systems have poor
transient response, nasty peaks in frequency response, and high
rates of roll-off. Aperiodic systems will feature excellent
transient response, smooth frequency response, and extended
very-low frequency reproduction.

Another benefit of the system is that you can pretty much
choose whichever driver you'd like to use, as long as they are
big. The Thiele/Small parameters (which would normally
determine what kind of box would be used) are taken into
consideration by the membrane designers so that the response is
extended and overdamped, regardless of the characteristics of
the driver.

Physically, the aperiodic membrane isn't for every car. It
requires sealing the trunk from the passenger compartment in an
air-tight manner, as well as sealing the trunk from the outside
for best results. The drivers are then mounted into the baffle
between the passenger compartment and the trunk, as would be
standard in an infinite-baffle/free-air set-up. The aperiodic
membrane is then placed either in front of the driver or behind
the driver, depending on the type. When mounting behind the
driver, the membrane is used as the rear-wall of a very small
box which the driver sits in (as in Richard Clark's infamous
Buick Grand National). So, in short, it's not suitable for
trucks, jeeps, R/V's, or hatchbacks.

You should probably only get an aperiodic membrane if you've
got money to burn, lots of amplifier power, some big subs, a
sedan, a desire for trunk space, and no wish to boom. If your
tastes lean towards bass-heavy booming, as opposed to
well-recorded acoustic instruments, you're not going to be
pleased with the result.


5 Installation

This section describes how to do what you want once you know
what it is you want to do.

5.1 Where should I buy the components I want? [JSC]

Most of the time, you will either buy from a local dealer, or
from a mail-order house. Buying from a local dealer can be
good because you get to deal directly with a person: you can
show them your car, ask specific questions, haggle prices, get
quick service when there are problems, get deals on
installation, etc. But there can also be advantages to buying
mail-order: generally cheaper prices, sometimes better service,
etc. In either case, you should always check prices before you
buy, inquire about warranty service, and ask about trial
periods.

5.2 What mail-order companies are out there? [JSC,JM,MM]

Crutchfield 800/955-3000
1 Crutchfield Park
Charlottesville, VA 22906 USA

Advantages: great customer service; generally knowledgeable
sales and tech support personnel; custom mounting
kits, wiring harnesses, etc. free of charge.

Disadvantages: limited product line; generally higher prices
than local shops.


J.C. Whitney 312/431-6102
1917-19 Archer Avenue
P.O. Box 8410
Chicago, IL 60680 USA

Advantages: lots of "miscellaneous" items; 10kW amps for $19.99

Disadvantages: 10kW amps that really only put out 1mW and break
after first 10 minutes of use.


Parts Express 800/338-0531
340 E. First St.
Dayton, OH 45402 USA

Advantages: large selection of electronics supplies at
respectable prices. Showroom prices said to be
better than catalog prices.

Disadvantages: also carries some of the same quality-level
components as J.C. Whitney.


Classic Research/Z-Box 602/571-0171
5070 E. 22nd St.
Tucson, AZ 85711

Advantages: creates custom door panels with car audio in mind.

Disadvantages: only services expensive sports and luxury cars.


MCM Electronics 800/543-4330
650 Congress Park Drive
Centerville, OH 45459-4072

Advantages: sells lots of decently priced trinkets (fuses, fuse
holders, wire, etc.) and has excellent service and
available technical support.

Disadvantages: ?

5.3 What tools should I have in order to do a good installation? [JSC]

Electrical tape - lots of it. Make sure you get some that can
withstand extreme temperature ranges.

Wire cutters/strippers and crimpers. Get a big pair with
stripper holes precut for individual wire sizes.

Angled screwdrivers. Makes taking dash and rear deck speakers
out a lot easier.

Multiple size screwdrivers, both flathead and Phillips.
Magnetic screwdrivers can be a big help when trying to get
screws into (or out of) tight spaces.

Various wrenches, pliers, and socket sets, depending on your
vehicle.

Metal drill and saw. You'll need these if you need to modify
your vehicle for new speaker cutouts or to accommodate a new
head unit.

Hot glue gun. Good for putting carpeting or door panel trim
back in place after modifications.

Razor knife. Helps for detailed modifications of door panels
or carpeting, especially when installing new speakers.

Wire.

Shrink wrap or flex tubing. Good for protecting wire,
especially in the engine compartment.

Multimeter. Helps to diagnose installations.

Extra hardware (screws, nuts, bolts, connectors, etc.).

Fuse puller and extra fuses.

Wire ties. Helps to tuck wire away in otherwise exposed
areas.

Small light source. A flashlight will do - you just want
something that you can poke around the innards of your car
with.

Tape measure.

5.4 Where should I mount my speakers?

5.5 What is "rear fill", and how do I effectively use it? [HK,JSC]

Rear fill refers to the presence of depth and ambience in
music. A properly designed system using two channels will
reproduce original rear fill on the source without rear high
frequency drivers. Since recordings are made in two channels,
that is all you will need to reproduce it. What is captured at
the recording session (coincident pair mics, Blumlein mic
patterns, etc.) by a two channel mic array will capture the so
called "rear fill" or ambience. Many of the winning IASCA
vehicles have no rear high frequency drivers. Also a lot of
this has to do with system tuning. If rear high frequency
drivers are added, however, the power level of the rear fill
speakers should be lower than that of the front speakers, or
else you will lose your front-primary staging, which is not
what you want (when was the last time you went to a concert and
stood backwards?). The proper amount of amplification for rear
fill speakers is the point where you can just barely detect
their presence while sitting in the front seat. Separates are
not a requirement for rear fill; in fact, you may be better of
with a pair of coaxial speakers, as separates may throw off
your staging.

5.6 How do I set the gains on my amp? [JSC]

The best way to do this is with a test tone and an
oscilloscope. Since most people have neither item, the
following will work approximately as well.

1) Disconnect all signal inputs to the amp
2) Turn all sensitivity adjustments as low as possible
3) Turn head unit on to around 90% volume (not 100% or else
you'll have head unit distortion in there - unless you've
got a good head unit) with some music with which you're
familiar, and with EQ controls set to normal listening
positions
4) Plug in one channel's input to the amp
5) Slowly turn that channel's gain up until you just start
to notice distortion on the output
6) Turn it down just a wee little bit
7) Disconnect current input
8) Repeat steps 4-7 with each input on your amp
9) Turn off head unit
10) Plug in all amp inputs, and you're done

If by some chance you do have an oscilloscope (and preferably a
test disc), you do essentially the same thing as above, except
that you stop turning the gains up when you see clipping on the
outputs of the amplifier.

Note that if you are paralleling multiple speakers on a single
amp output, you need to set the gains with all of the speakers
in place, since they will be affecting the power and distortion
characteristics of the channel as a whole.

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

This section describes the competition branch of the car audio
world - what it is, and how to get involved.

6.1 What is IASCA, and how do I get involved? [JSC,HK]

IASCA is the International Auto Sound Challenge Association, a
sanctioning body for car audio competitions held throughout the
world. Competitors earn points at each competition, and those
that perform the best each year can advance to the finals.
Prizes (trophies, ribbons, and sometimes cash) are usually
given out to the top competitors in each class at every
competition.

There are registration forms for IASCA in every issue of Auto
Sound and Security (see 7.1).

6.2 What is USAC, and how do I get involved? [HK]

USAC is another sanctioning body, similar to IASCA. However,
USAC places greater emphasis on SPL measurements than IASCA.

6.3 What are the competitions like? [HK,CD]

[HK writes:]

They are much like loud car shows: a lot of cars parked with
their hoods/doors/trunks open showing their audio systems.
There are two types of judging styles: 1) drive through - where
competitors drive their own vehicles to judging stations to be
judged, and 2) walk-arounds - where the teams of judges will
walk around the event site and judge vehicles that fit within
their judging assignments. Typically SPL is done first with
the mic stand in the driver's seat and the competitor in the
passenger side adjusting only the volume. Hearing protection
must be worn. After SPL measurements are completed, RTA
measurements are performed by playing pink noise. When the
volume level is within the specified "window" around
90db-110db, the RTA judge will signal you out, and at that
point you must exit the vehicle for the actual scoring
measurements. The next area for judging should be sound
quality where two judges will sit in your car and judge the
sound quality based on IASCA's reference CD/tape. The next
area is installation judging where the competitor has 5 minutes
to explain and show the installation of his/her vehicle. It is
very useful to have a picture book/album of photos of the
installation that may not be visible to prove that items not
visible do exist. When that is completed, the competitor can
park the vehicle and show spectators the vehicle. These
procedures may differ from show to show, and at the
regional/final levels they are very strict in what can and
can't be done, e.g. a judge will make sure no adjustments are
made after SPL until after sound quality judging is over, ear
protection, etc.

[CD writes:]

Most involve a lot of waiting around. Thus, they are perfect
for meeting other people interested in car audio, and seeing
some installations which may give you some ideas. They're also
perfect for listening to some cars that sound a lot better and
a lot worse than your own. In IASCA competition, the cars are
judged on:

Sound Pressure Level (30pts)
Frequency Response (40pts)
Staging (40pts)
Stereo Image (40pts)
Frequency Separation/Clarity (40pts)
Sound Linearity (20pts)
Absence of Noise (40pts)
Ergonomics (20pts)
Wiring/Electrical System Cosmetics and Integrity (35pts)
Cosmetic Integration Consistency (40pts)
Component Installation Integrity (55pts)
General Creativity (20pts)
Attention to Detail (15pts)

6.4 Should I compete? [CD]

You should compete if:

a) You have an ok sounding stereo
b) You have an ok installation (i.e. no amps/changers sliding
around in the trunk)
c) You'd like some pros to comment on your system
d) Your feelings won't get hurt if you don't get first place
e) You've been to a contest and talked to competitors about it
f) You've read the rulebook
g) You've listened to a test disc in your car, and
understand what the sound quality judges are listening for

You can compete even if you don't do all of the above, but the
recommendations will help you understand and gain the most from
competing.

6.5 What class am I in? [HK,JSC]

This section is mainly geared toward IASCA.

[HK,JSC write:]

There are three classes: novice, amateur, and pro. The novice
class is intended to be an unintimidating level where beginners
can start out; however, a competitor may only be in the novice
class for one year, at which time he is automatically moved to
the amateur class. Most competitors stay in the amateur class
indefinitely, unless they become affiliated with a car audio
shop or manufacturer, at which point they are moved into the
pro class.

[CD writes:]

Are you or were you employed by a car audio manufacturer or
dealer?
Yes: You compete in pro
No: Is this your first year of competing?
Yes: You compete in novice for the first year
No: You compete in amateur

Note that modifying your amplifiers, buying your equipment
below retail, or being sponsored by a manufacturer or dealer
will get you kicked into pro.

Also note that any home built active gear in the signal path
(e.g. custom built equalizers, crossovers, or noise gates) will
get you kicked out of novice.

Once you know what group you are, you next need to know what
power category you are in. Add up the 4-ohm non-bridged rating
of all your amplifiers, including your head unit if your head
unit is powering speakers (rather than exclusively feeding
amplifiers). Then, find the category you fit into:

Novice: 1-50 51-100 101-250 251-500 501+
Amateur: 1-50 51-100 101-250 251-500 501-1000 1000+
Pro: 1-100 101-250 251-500 501-1000 1001+

Thus, if you had a Rockford Punch 40 (20Wx2) and a Punch 60
(30Wx2), with a head unit that put out 6Wx2 (powering, perhaps,
a center channel) you're in the 101-250 class. It does not
matter if your amps are bridged down to .002 ohms; it's only
the 4ohm rating that counts. If you no longer used your head
unit to power speakers, you would be in the 51-100 class (or
the 1-100 class if you were a pro).

Competition is usually most vicious in the 101-250 and 250-500
categories at typical contests.


7 Literature

This section describes various literature which you can read to
brush up on your car audio skills, or to keep current, or to
see other people's installations, or whatever else you'd like.

7.1 What magazines are good for car audio enthusiasts? [JSC,MI,NML]

Car Audio and Electronics $21.95/year
P.O. Box 50267 (12 issues)
Boulder, CO 80321-0267 800/759-9557

Car Stereo Review $17.94/year
P.O. Box 57316 (6 issues)
Boulder, CO 80323-7316 303/447-9330

Auto Sound and Security $28.95/year
P.O. Box 70015 (12 issues)
Anaheim, CA 92825-0015 714/572-2255

Car Hi-Fi #23.60/year
Freepost (8 issues)
TK660 081 943 5943
Leicester
KE87 4AW

7.2 Are there any newsletters I can read? [MO,HK]

Autosound 2000 Tech Briefs $35.00/year
2563 Eric Lane, Ste D (6 issues)
Burlington, NC 27215 800/795-1830

7.3 What books can I read? [JSC,JW,TT]

Loudspeaker Design Cookbook
by Vance Dickason
Published by Audio Amateur Press
ISBN 0-9624191-7-6
$25-$30

Designing Speaker Enclosures
by David Weems
Published by ?
ISBN ?
$??.??

Killer Car Stereo on a Budget
by Daniel L. Ferguson
Published by Audio Amateur Press
ISBN 0-9624191-0-9
$19.95


8 Credits

[JSC] Jeffrey S. Curtis ([email protected])
[JLD] Jason Lee Davis ([email protected])
[MI] Matt Ion ([email protected])
[JW] Jerry Williamson ([email protected])
[CD] Cal Demaine ([email protected])
[MO] Mark Obsniuk ([email protected])
[HK] Harry Kimura ([email protected])
[RDP] Dick Pierce ([email protected])
[BG] Brian Gentry ([email protected])
[JM] Jeff Meyers ([email protected])
[MH] Marvin Herbold ([email protected])
[TT] Trevor Tompkins ([email protected])
[MM] Matthew E. Meiser ([email protected])
[NML] N. M. Lines ([email protected])

--
Jeffrey S. Curtis | Email: [email protected]
Argonne National Laboratory | Voice: 708/252-1789
9700 South Cass Avenue, ECT-221 | Fax: 708/252-9689
Argonne, IL 60439 |


 December 17, 2017  Add comments

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