Ford Escort
and Mercury Tracer - Car Audio Questions
Thanks
to Quality Car Stereo for the info!
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This page was created
to help the Average Car Audio Enthusiast. I have discovered that a huge
number of people out there are extremely ignorant when it comes to the
proper way of installing electrical components. I hope this area helps
clarify any questions.
1: What makes
a good vented enclosure?
2: Is it possible
to get low, punchy bass from a small enclosure?
3: Can I get
more bass from my enclosure by installing a bigger driver?
4: What about
putting two drivers in the enclosure to increase bass?
5: What should
I do to use two drivers (for more bass)?
6: What does
port or enclosure "tuning" mean?
7: Is it always
necessary to use a port for good bass?
8: Where should
I locate the port(s) with respect to the woofer?
9: What should
the ducts be made of? Is round better than rectangular?
10: What is
the relationship of duct length to port area?
11: How big
should the port be?
12: Can I
use several smaller ports instead of one big one?
13: Is there
a simple mathematical way of designing proper enclosures?
14: Should
the enclosure's baffle be removable?
15: Is there
a preferred shape for loudspeaker enclosures?
16: What is
the best material to use for building enclosures?
17: Is bracing
necessary? How much should be used?
18: How should
I mount drivers on the baffle?
19: Do I need
fiberglass inside the enclosure?
20: Does Fiberglass
significantly affect enclosure tuning?
21: What is
the purpose of adding a Large capacitor to my system?
22: Advanced
Questions about Speaker Enclosure Design
1.
What makes a good vented enclosure?
Basically, an enclosure
serves to partition the front and rear of the driver's cone, preventing
the opposing air pressure changes produced by cone motion from canceling,
and allowing the radiation of sound from the front of the driver only.
In addition, vented enclosures allow the compressibility of the air inside
the enclosure to work as a more active part of the "system" consisting
of driver and enclosure. Beyond these two basic functions, a low frequency
loudspeaker enclosure should do absolutely not hing, that is, it should
add no effects of its own--no vibration, no tonality, no motion-- nothing
to interfere with or absorb acoustic energy produced by the driver.
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2.
Is it possible to get low, punchy bass from a small enclosure?
Yes, if the driver in the
enclosure is designed for low bass operation in a small enclosure. Unfortunately,
it's usually a small driver that can work properly in a small enclosure,
and that dictates that lower sound levels will result from the small amount
of air such a small driver can move. Larger boxes (with larger bass drivers)
produce more bass, smaller boxes produce less bass. It's a fact of life,
like the fact that it takes a bass viol, a tuba, longer piano strings,
or very large organ pipes to produce bass energy in the air. Low bass requires
that more air move, and bigger boxes contain more air that can be put to
work making low bass.
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3.
Can I get more bass from my enclosure by installing a bigger driver?
A given enclosure will not
automatically produce more bass when a larger driver is installed, in fact
the opposite is often the result.
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4.
What about putting two drivers in the enclosure to increase bass?
Placing two bass drivers
in an enclosure designed for one will usually produce less bass and more
midrange output, and will upset the operation of the driver-enclosure system
because each driver will behave as though it is installed in an enclosure
which has only half the internal volume of the original enclosure (with
one driver).
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5.
What should I do to use two drivers (for more bass)?
There are two alternative
possibilities. When using two identical drivers, you can build an enclosure
with twice the internal volume of the original enclosure that contained
one driver, or you can duplicate the original enclosure and stack the two.
As the latter alternative suggests, when building the double enclosure,
it's necessary to treat the enclosure as if it were two enclosures--you
must double the porting used on the single smaller enclosure--although
it is not necessary to divide the volume of the double enclosure unless
two different driver models are used and their interaction would be undesirable.
A usable example of this might be a 227 liter (8 cubic foot) enclosure
divided into two chambers so that one type of driver occupies 57 liters
(2 cubic feet) and another occupies 170 liters (6 cubic feet). In this
case, the ports tuning either chamber to the same desired frequency will
be quite different.
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6.
What does port or enclosure "tuning" mean?
In exactly the same way
the resonant note from a bottle can be raised and lowered by adding or
pouring out liquid to change the bottle's air volume, enclosure tuning
is affected by the ratio of air volumes in the port (the bottleneck) with
its attendant flow resistance, and the enclosure interior volume. Tuning
of loudspeaker enclosures is a result of manipulating the differences in
effective air mass between the enclosure interior and the air in the port.
The bottle-like nature of a vented enclosure is known as a "Helmholtz resonator."
The ports or ducts in a vented enclosure work only over a narrow band of
frequencies near the chosen tuned frequency, producing the same effect
noted when blowing across a bottleneck--a single distinct pitch.
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7.
Is it always necessary to use a port for good bass?
Vented designs produce lower
distortion at the lowest operating frequencies, afford the driver protection
against mechanically destructive large cone excursion, and better enable
the driver to absorb and utilize its full power rating from an amplifier
when operating at low frequencies. It is important to keep in mind that
porting and tuning an enclosure provides air loading for the bass driver
down to frequencies just below the Helmholtz frequency, but does not provide
any loading for the driver at frequencies below that, such as subsonic
turntable rumble, record warp or SUPER low frequencies sometimes used in
'bass music." If you intend to operate a sound system at high power
levels, we highly recommend an electronic high-pass filter to eliminate
subsonic
input to the power amplifier(s). This will substantially increase the
available useful power from the amplifier which will then only operate
in the audible frequency range.
Note: It is a generally
held belief [by manufacurers] that SEALED enclosures are safer for the
do-it-yourselfers because there is more tolerence for error, thus there
is a lower chance in crushed drivers do to incorrectly vented enclosures.
Because of this, you will see more suggestions from manufactures to use
SEALED enclosures.
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8.
Where should I locate the port(s) with respect to the woofer?
Bass reflex enclosures are
usually designed to tune from about 100 hertz and down. The length of sound
waves at these low frequencies is over 11 feet, so port placement is not
critical. Ports may be located anywhere on the baffle with no change in
bass performance; some designs even locate ports on the back of the enclosure
which works well as long as the enclosure is not close to a wall (a couple
of port diameters away) and there is an unobstructed air path between the
woofer and the port. Overall, it's safest to locate the port somewhere
on the baffle with the woofer(s) far enough away from side walls to avoid
interaction between port and enclosure wall or the fiberglass insulation
on the wall.
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9.
What should the ducts be made of? Is round better than rectangular?
Port ducts may be made of
anything rigid, such as paper cardboard with about a 1.5 mm (1/16") or
larger wall thickness. They can be any shape, square or rectangular (such
that port area remains constant) and made of wood or other suitable material.
It is not necessary to use PVC pipe for port tubing, particularly when
most carpet stores throw away large amounts of heavy carboard tubing of
between 3 and 4-1/2 inches inside diameter.
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10.
What is the relationship of duct length to port area?
When port area is increased,
independently of other factors, enclosure tuning is raised. If duct length
is increased, independently of other factors, enclosure tuning is lowered.
To keep the same tuning (Helmholtz frequency) you will need to increase
duct length as you increase port area.
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11.How
big should the port be?
The bigger, the better.
Any port causes some resistance to air movement, and so introduces unavoidable
losses in output to the system as a whole. The ratios of port area and
length and enclosure volume determine the Helmholtz frequency tuning. Mechanical
reactance elements, stiffness and air mass, control the effective air mass
ratios. At very low operating levels, where air in the port does not move
very fast, a small short port will behave the same as a large longer port
as far as enclosure tuning is concerned. At high power levels however,
the restricted air flow of the smaller port will produce output level losses,
some de-tuning and at high enough levels a small port will cause the enclosure
to behave like a sealed enclosure with little or no contribution from the
port. To minimize resistive losses, the largest practical port should be
used. Computer listings of port choices calculated to limit air velocity
inside the port duct will list duct sizes which are normally impractical.
A 380 mm (15 in) diameter port is not an unreasonable choice for a 380
mm bass driver, however the necessary length would dictate that such a
port might itself have a volume of many cubic feet, sometimes equal to
or larger than the original enclosure. A good rule of thumb would be to
avoid ports whose circular area is smaller than at least 1/3 the diameter
of the driver such as a 127 mm (5 in) diameter port for a 380 mm (15 in)
driver. This will usually provide sufficient port area so that the port
will not "whistle" when the system is operated at high power levels near
the helmholtz frequency--a sure indication of severe system losses and
potential power compression and low-frequency output limiting.
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12.
Can I use several smaller ports instead of one big one?
Yes, however there is a
phenomenon associated with air resistance resulting from air drag on the
internal surfaces of port ducts and turbulence at the ends of the ports
that requires a duct length correction when several ports are used. For
example, when using four 100 mm (4 in) tubes instead of one 200 mm (8 in)
tube (which has the same port area but one-quarter the internal surface
area), the length needed will be slightly less than that needed for the
single 200 mm tube, perhaps 5% to 10% less, depending on overall enclosure
volume. These effects exhibited by port ducts is exaggerated by proximity
of the duct to enclosure interior surfaces or any other type of boundary
that may cause air turbulence near the end of the duct, therefore it's
important to keep duct ends away from the rear of the cabinet or other
obstructions by an amount at least equivalent to or larger than the dimension
across the port. If you are using a rectangular port that has as one of
its sides, an enclosure wall, you might have to use some correction.
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13.
Is there a simple mathematical way of designing proper enclosures?
Yes, a JBL scientist, D.B.
Keele Jr., simplified the work of A. Neville Thiele and Dr. Richard
Small so that anyone with a pocket calculator and a ruler or straight edge
can design the right enclosure volume and choose the right port or duct
for a given loudspeaker driver. We should have more information regarding
these technics posted here soon.
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14.
Should the enclosure's baffle be removable?
This is a question of mechanical
strength and rigidity. All enclosures, particularly those intended for
rough portable use, should be constructed with all sides permanently fixed
by glue and screws, and sealed air-tight by virtue of well cut and glued
joints. It is preferable to mount loudspeakers from the front of the baffle
board to eliminate the possiblity of reflections from the inside of the
loudspeaker mounting hole, thus it becomes unnecessary to provide for removing
the baffle. Woofer openings are usually large enough to reach through in
order to work inside the box, for example, to install other components.
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15.
Is there a preferred shape for loudspeaker enclosures?
There are a number of shapes
that improve performance and some that cause distinct degradation in performance.
For single, full-range drivers a sphere is the ideal shape for an enclosure
because the curved surfaces avoid the diffraction effects of cabinet edges,
which bend sound waves in a manner dependent on frequency. For multi-way
loudspeaker systems, spheres are usually impractical because of the large
size needed and because of the precise orientation required for optimal
listening. Conventional enclosures work best mounted flush into a wall
where diffraction is controlled by virtue of the wall surface, and for
free-standing enclosures, tilting, angled and curving surfaces may be employed
to help reduce or control edge diffraction. As for subwoofers specifically;
slightly angled enclusures ensure that there are minimal standing waves
inside the enclosure; you want to stay away from having too many parallel
sides. It is best to avoid enclosure dimensions that are multiples of each
other, such as 1 X 2 X 4 ratios, and strive to use dimensions that have
somewhat unrelated ratios such as 1 X 1.23 X 1.41.
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16.
What is the best material to use for building enclosures?
For car and home use, high
density particle wood (or MDF) is the most cost-effective material for
general enclosure construction. The best wood to use for portable enclosure
construction is 14 to 20 ply per inch Finland birch type. Birch plywood
is very expensive however, and a carefully braced enclosure made of high
grade void-free fir plywood can do the job just as well in most cases.
The thicker you can make the cabinet walls, the better the results will
be because of reduced wall vibration and resonance, but the tradeoff is
cost and weight. Enclosure walls should be cut so that edges form an air-tight
seal when glued together. Cleats and caulking can also be used if needed
to insure a good fit and tight air seal.
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17.
Is bracing necessary? How much should be used?
Bracing should be added
to the enclosure interior to minimize enclosure wall vibration. Enclosure
walls simply cannot be stiff enough since wall vibration indicates that
energy is being wasted to move enclosure panels rather than moving air.
25 X 76 mm (1 X 3 in) pine bracing fixed on edge with glue and screws to
the enclosure walls will help provide the minimum necessary stiffening
without affecting the internal volume significantly. If you are building
large subwoofer enclosures, bracing with two-by-fours works better, though
you should take the bracing volume into account since a 3 m (10-foot) length
takes up 12.9 liters (0.36 cubic foot) of enclosure volume.
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18.
How should I mount drivers on the baffle?
Mount drivers on the front
of the baffle whenever possible to avoid the reflections from inside the
mounting hole. Heavy drivers should normally be front-mounted using Tee-nuts
and machine screws. If Tee-nuts are used, apply a bit of Bostic or
Pliobond type rubber glue to the inside of the nut flange to help avoid
losing the Tee-nut inside the enclosure when installing the driver. Baffle
board construction is much easier if all baffle parts are assembled prior
to final box assembly.
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19.
Do I need fiberglass inside the enclosure?
Some manufacturers use a
25 mm (1 in) padding of 1/2-pound density fiberglass stapled to the enclosure
interior on all surfaces except the baffle. You should use 100 mm (4 in)
thick dacron or 25 mm (1 in) fiberglass on at least three of the surfaces
of parallel interior walls; this is assuming that you are running your
woofer in the 200 hz range or above. Keep sound absorbing materials
away from the port(s) as the air velocity inside the port can be sufficient
to tear off bits of the material and squirt them out of the enclosure.
It is not necessary to cover the inside of the baffle, but doing so will
rarely degrade system performance. The enclosure exterior may be covered
with your choice of any suitable finish or decoration; this will not affect
bass performance and in some cases (as with Formica) may help stiffen the
enclosure walls.
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20.
Does Fiberglass significantly affect enclosure tuning?
No, not unless the enclosure
is stuffed full of fiberglass, in which case the apparent volume of the
enclosure increases by 12% to 20% as seen from the point of view of the
bass driver. Stuffing the enclosure full with fiberglass is not recommended
because it introduces system losses, is expensive and interferes with port
operation. The exception to this would be a sealed "air suspension" type
system enclosure where more virtual volume is needed and actual volume
is not available, and/or where box dimensions which are multiples of each
other can't be avoided and the fiberglass stuffing will help absorb the
internal sound reflections.
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21:
What is the purpose of adding a large capacitor to my system?
Caps do work if there used
properly. A Capacitor must be installed using the same guage wire as the
power wire used in the system.
It makes your system much
more responsive to dynamic peaks, and can really make an audible difference,
especially with bass. A cap releases vast amounts of current much
faster than your car's electrical system ever could, and it eases the load
on the rest of your vehicles electrical system. Think of a cap as a small
car battery that rechargers in a millisecond when discharged, as aposed
to a regular car battery that take several minutes to recharge. A capacitor
is rated using units called Farads. 1 Farad = 1,000,000 MicroFarads.
Advanced
Questions about speaker Enclosure design
1. Power Handling
The power rating of a driver
is usually (but not always) specified in
watts RMS by the manufacturer.
This is the continuous thermal power
rating of the driver. Exceeding
this rating for more than a moment will
cause voice coil overheating,
which can result in warping or burn-out.
Speaker systems also have
a displacement-limited power rating (Per).
This is the amount of power
the system can take without exceeding the
absolute maximum voice coil
displacement. Per is a function of frequency
and depends on the design
of the enclosure in addition to the peak
displacement limit of the
driver, xmax. Thus, it is meaningless for
manufacturers to specify
peak power handling without also specifying the
enclosure and the frequency
range.
At some frequencies, Per
will exceed the thermal RMS power rating. For
continuous tones, the smaller
of the two ratings applies. For signals
with large crest factors
or low duty cycles, Per applies, providing the
average power does not exceed
the thermal rating. Per is calculated for
sine waves, which have a
3 dB crest factor. The peak power rating at a
given frequency is therefore
2*Per.
2. Sealed Box Enclosures
For the sealed box enclosure,
the optimum volume can be determined. Many
designers like to use a
0.62:1:1.62 ratio for the cabinet dimensions.
This is known as the golden
ratio. A box designed to this ratio will
have smaller resonant peaks
than one whose dimensions are equal. Another
ratio sometimes used is
0.8:1:1.25. You can determine the middle
dimension by taking the
cube root of the enclosure volume. (Keep in mind
this is the inside volume
and doesn't take into account the volume taken
up by bracing materials
and the drivers.)
The box will have a resonant
frequency and a Q. For an optimum sealed
box, the resonant frequency
is equal to the -3 dB point, and the Q is
0.707. The -3 dB frequency
is also known as the half-power point,
because it is the frequency
at which the acoustic output power drops by
half. Below this frequency,
the response will have a second order roll
off, e.g., the output decreases
12 dB for every halving of the frequency
below the -3 dB point.
3. Ported Box Enclosures
The ported enclosure is
a little more complicated. As with the sealed
box, the ported enclosure
has an optimum volume and -3 dB point.
The enclosure also has an
optimum tuning frequency, Fb, which is the
resonant frequency of the
enclosure's duct. The tuning frequency is
determined by the cross
sectional area and length of the duct. For a
tubular duct, the following
equation applies, LENGTH =
2118*DIAMETER^2/(Fb^2*Vb)
- 0.73*DIAMETER, where LENGTH is the length of
the duct in inches, DIAMETER
is the inside diameter of the duct in
inches, Fb is the tuning
frequency in Hz, and Vb is the box volume in
cubic feet.
Ported enclosures have a
steeper roll off than sealed boxes. The roll
off is fourth order, or
24dB for every halving of the frequency below
the -3dB point. Below Fb,
the displacement-limited power rating will be
very low because the driver
is essentially operating in free air. It is
therefore wise to roll off
the signal below the -3dB frequency to avoid
damage. This constraint
does not usually apply to sealed boxes, which
dampen cone movement at
all frequencies.
INDIVIDUAL TWEAKS: CABINETS
(Home and Automotive)
•Installing stratigically
located bracing to reduce cabinet reasonance
makes such a tremendous
improvement in the solidness and focus of your
sound that you would be
amazed.
•Meassuring the woofer's
specs and tuning the cabinet exactly to that
woofer will get you maximum
performance with the flattest response. Most
are found to be more than
20% in error!
•Carefully re-install a
port that is aerodynamic with flared or rolled
edges will remove coloration,
wind noise and improve power handling.
•Installing adjustable spikes
on the base of the cabinet will reduce or
eliminate cabinet rocking
which happens on a microscopic level will
greatly enhance the focus
of high frequencies and the solidity of the
bass.
•Installing sound absorbing
materials such as cork, or felt on the
baffle will reduce rarefaction
making it difficult to localise your
speakers. This enhances
sound stage topology in the stereoscopic array.
Makes your speakers have
a chance at disappearing when the music is on.
•Re-installing drivers to
be exactly flush with the baffle will improve
the flatness of the frequency
response of each driver.
•Re-installing high frequency
drivers with composite isolation (poured
flexible sealant) tremendously
improves the bond between driver and
cabinet while at the same
time dissipates cabinet resonance into heat so
that they do not smear or
color the sound from those drivers.
•Re-configuring the array
when necessary and possible to a line source
array will usually improve
imaging and depth enough to justify the new
baffle.
•Installing the tweeter
on the top of the cabinet at the proper angle
will physically time align
the array, completely flatten its response,
require less crossover components
as a result. Makes your sound stage
presentation acquire dramatic
depth.
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