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1993-12-07
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Path: senator-bedfellow.mit.edu!bloom-beacon.mit.edu!nic.hookup.net!europa.eng.gtefsd.com!howland.reston.ans.net!vixen.cso.uiuc.edu!uxa.cso.uiuc.edu!jsc52962
From: stealth@uiuc.edu (Jeffrey S. Curtis)
Newsgroups: rec.audio.car,rec.answers,news.answers
Subject: rec.audio.car FAQ (part 2/3)
Supersedes: <rac-faq-p2-063122Nov61993@uxa.cso.uiuc.edu>
Followup-To: rec.audio.car
Date: 6 Dec 1993 06:34:36 GMT
Organization: University of Illinois at Urbana
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Approved: news-answers-request@mit.edu
Distribution: world
Message-ID: <rac-faq-p2-063448Dec61993@uxa.cso.uiuc.edu>
Reply-To: stealth@uiuc.edu (Jeffrey S. Curtis)
NNTP-Posting-Host: uxa.cso.uiuc.edu
Summary: This article describes the answers to the frequently
asked questions on the rec.audio.car newsgroup. This
article is posted once per month.
Originator: jsc52962@uxa.cso.uiuc.edu
Xref: senator-bedfellow.mit.edu rec.audio.car:16634 rec.answers:3260 news.answers:15529
Archive-name: car-audio/part2
Version: 1.9
Last-modified: 05 Nov 93
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 and 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.
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 generally collections of capacitors and
inductors and sometimes resistors, which are placed in between
the amplifier and the speakers. There are also active
crossovers which are powered electrical devices which are
placed between the head unit and the amplifiers.
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.
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. 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 -37.2dB/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.
Thus, if you have extra money to spend on an active crossover
and separate amplifiers, 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 can also be reversed: a
capacitor in parallel with the driver is a low pass filter; an
inductor in parallel with the driver is a high pass filter.
When like combinations are used, the order increases: a
crossover in series (inline) 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 reversing the polarity
of one of the drivers. In any case with any crossover, though,
you should always experiment with the polarity of the drivers
to achieve the best total system response.
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 have the adjustments for
anywhere from five to eleven (and 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]
MD seems to have a better future than DAT or 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 CD's 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 CD's.
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?
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? [MH]
Various enclosure design software is available via ftp from
csd4.csd.uwm.edu in the directory "/pub/high-audio/Software".
The most popular program there is Perfect Box, which is in the
file "perf.uu" (or "perf.zip").
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.
--
Jeffrey S. Curtis - stealth@uiuc.edu <> "Resplendent in full regalia, they
Network Technician - UoI Housing Div <> revel in their role as self-appointed
Proton - Phase Linear - Polk - Sanyo <> critics of the establishment..."
StreetWires - Jensen - Sony - Sennet <> -- INXS _Welcome to Wherever You Are_
