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Archive-name: car-audio/part3
Rec-audio-car-archive-name: FAQ/part3 Version: 4.55 Last-modified: 08 August 2005 See reader questions & answers on this topic! - Help others by sharing your knowledge
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, viscosity,
etc.); dimensionless
`n0'
The reference efficiency of the system (eta sub 0) dimensionless,
usually expressed as a percentage
`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'
Density of air at STP 1.18 kg/m^3 (rho)
4.2 How does speaker sensitivity affect real world SPL? Will
a higher sensitivity give me a larger SPL? [MS]
===================================================================
When it comes to mids and highs, efficiency (sensitivity) is a fairly
good indicator of output differences at the same power level. When it
comes to subwoofer performance, the driver's sensitivity is irrelevant
unless you are also specifying a box volume.
An efficient sub requires a larger box to achieve equivalent extension
to a less efficient sub. In a small box, the less efficient sub will
actually be LOUDER at low frequencies at the SAME POWER as the more
efficient sub.
Linear excursion is a very good indicator of ultimate output capability
(given sufficient power to drive the speaker to that point.) To make
sound you must move air; therefore, the more air you move, the more
sound you make. When comparing two speakers of equal surface area, the
one with greater excursion capability will play louder given sufficient
power.
4.3 What are the enclosure types available? [JLD, JG]
============================================================
Only the order of the enclosure itself is shown here. The addition 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.
First Order
Infinite-Baffle or Free-Air
|
|
/
/
||
||
\
\
|
|
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.
Quasi-Eighth Order
Series-Tuned Dual-Reflex Eighth Order
Bandpass Triple-Reflex Bandpass
_ _______________ _ ____________ _____________
| | | | | | | | | | |
| | | / | | | | | | |
| / | | |
| || | | |
| || | | |
| \ | |____ _____________ ____|
| \ | | | | | | | |
| ____| | | | | / | | |
| | | / |
| ____ | | || |
|_____________|___________| | || |
| \ |
| \ |
|______________|______________|
* 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. |__|
4.4 Which enclosure type is right for me? [IDB, DK]
==========================================================
This answer is not designed to tell you exactly what kind of enclosure
to build, but rather to give an idea of the advantages and
disadvantages to the simple configurations (Infinite baffle [1st
order], Sealed [2nd order], Ported [4th order] and basic bandpass).
Building and designing more complicated systems (order > 4) is not for
the light at heart.
4.4.1 Infinite Baffle ("free-air")
-----------------------------------------
* Advantages...
- No box necessary!
- This means it's usually cheaper to design and implement
in your system
* Disadvantages...
- Requires that a good seal be obtained between front and
rear of driver. In a car, this can be quite difficult and
may require the installer to remove trim panels to plug any
holes that would let energy "bleed through".
- The responsibility for damping cone motion rests solely
on the driver's suspension. As fatigue sets in, this becomes
a critical issue in infinite baffle set-ups.
- Less efficient in the sub-bass region than above
mentioned enclosures.
- Potentially more expensive drivers than good boxable
woofer - The suspension must be extremely hearty and
long-lasting to withstand high power applications.
4.4.2 Sealed Box
-----------------------
* Advantages...
- Small enclosure volumes
- Shallow (12 dB/Octave) roll off on low end
- Excellent power handling at extremely low frequencies
- Excellent transient response/ group delay
characteristics
- Easy to build and design
- Forgiving of design and construction errors
* Disadvantages...
- Not particularly efficient
- Marginal power handling in upper bass frequencies
- Increased distortion in upper bass over ported design
- When using high power and small box, magnet structure
is not in an ideal cooling environment
4.4.3 Ported Box
-----------------------
* Advantages...
- 3-4 dB more efficient overall than sealed design
- Handles upper bass frequencies better with less
distortion
- Magnet is in good cooling environment
- When properly designed, a ported box will slaughter a
sealed in terms of low frequency extension
* Disadvantages...
- Size (not so critical outside the mobile environment)
- Woofer unloads below Fb
- More difficult to design/ can result in boomy, nasty
sounding bass if misaligned
4.4.4 Bandpass Box
-------------------------
* Advantages...
- When properly designed and implemented, can provide
superior LF extension and efficiency.
- Cone motion is controlled more and therefore mechanical
power handling is increased.
- Cones are physically protected from contents of trunk
flying around.
- Output is easily channeled directly into the interior
of sedans.
* Disadvantages...
- Difficult to build (not recommended for newbies), and
very sensitive to misalignment due to calculation or
construction errors.
- Their characteristic filtering often masks any
distortion that occurs as a result of amplifier clipping or
overexcursion and thus will give the user no warning that the
driver is over-stressed and about to fail.
- Need substantial mid-bass reinforcement to make up for
narrow bandwidths in efficient alignments.
- Transient response is largely dependent upon the
alignment chosen....wider bandwidths will result in sloppier
performance, narrower bandwidths (and thus higher effiencies)
result in better transient performance.
- They can oft times be quite large.
4.5 How do I build an enclosure? [AO]
============================================
These instructions are for building a first order (sealed) subwoofer
enclosure. Building ported or bandpass boxes is more difficult, and
those designs are less forgiving of mistakes. These instructions apply
for all box designs, but be sure of the measurements before you make
your cut. Building your own enclosure can save you a lot of money, but
only if you don't need to buy all of your materials twice because of
mistakes!
You will need:
*Wood*
I only use MDF (see Section 4.6), but others have reported
success using other hardwoods like birch and oak. Do not use
plywood - it's far to flexible and porous. Use a minimum of 3/4"
wood - flexing sub enclosures lose precious energy!
*Screws*
For one inch wood use #8 2 inch wood screws. For 3/4 inch wood use
#8 1 3/4 inch screws. Double grip Drywall screws also work well.
*Adhesive*
I use "Liquid Nails" which comes in a caulk tube or a bucket, but
any paste type of adhesive will work. Spray adhesives will not
work.
*Silicone sealant*
White, brown, clear, caulk tube, or squeeze bottle, it doesn't
matter. Make sure you don't get silicone lubricant (which comes
in a spray can)!
*Terminals/Terminal Cup*
To allow easy connections from your amp.
Besides these materials you will need several tools:
*Table saw or radial arm saw*
I use a radial arm saw, just because it's a little easier and
accurate, but a table saw will work also. You can use a circular
saw, but be very careful to make your cuts straight.
*Jig saw*
For cutting your speaker hole.
*Drill*
You will also need a 1/8" drill bit, a screwdriver bit, and a
countersinking bit.
*Pencil*
To mark your cuts, make notes, etc.
*Measuring tape*
*Safety Goggles*
*Face Mask*
Breathing MDF dust has not been proven to cause health-related
problems, but hang around with with a few installers at your local
shop for an afternoon and you'll see why you need a face mask. :-)
Start by marking the cuts you need to make on your wood. Double check
your math, and your measurements.
Use the table or radial arm saw to cut your wood. When you're done you
should have six pieces of wood which fit together tightly to form a box.
At this point you will need to trace the cut out for your subwoofer onto
the front of the box. Remember that if you have a 10 inch subwoofer you
do NOT want a 10 inch cutout. The 10 inch measurement is from the
outside of the mounting ring. The actual cutout diameter should be with
your instructions. Transfer the proper sized circle onto the sub box
and cut it out with the jig saw. If you have trouble starting cuts with
a jig saw, drill a 1/2" hole in the wood inside the circle. You can
drop your blade into the hole and then cut out to the edge of the circle
and around.
After you have cut out your mounting hole you will need to cut out a
square on one of the sides for your terminal cup. Transfer the proper
size rectangle onto the wood and cut it out with the jig saw.
Now you are ready to start assembling the enclosure. Choose one of the
ends, and one of the sides. Apply a bead of adhesive along the edge of
the end piece. Affix it to the edge of the bottom piece. Flip it over
(have a friend hold the other end and hold the end in place,) and screw
the edge to the end. Use one screw at each corner and then one more
screw about every 8 inches. Drill a pilot hole with your 1/8" drill
bit, then drill a countersink with your countersinking bit. Finally,
drive the screw in. Make sure that you don't strip the hole.
Repeat the above procedure with the other end. You should now have the
two ends connected to one side. Affix the other three sides the same
way.
Finally, you'll want to seal the insides of the box with silicone. Apply
a bead of silicone across all the inside edges and around the terminal
strip.
Allow the box to dry over night and then place your speaker into the
hole. Screw it down and you're done!
4.6 MDF for Dummies [IDB]
================================
Since MDF (Medium Density Fiberboard) is used so frequently in building
subwoofer enclosures and other projects, this section provides some more
detailed information.
4.6.1 What is MDF? [PS]
------------------------------
MDF (Medium Density Fiberboard) is a hardboard product comprised of hot
compressed wood fibers glued together. The color of this material can
vary from light to dark brown depending on the brand of manufacturer.
Particle Board or Flakeboard consist mostly of recycled wood chips. The
size of the wood chips used varies which means that the coarser the wood
chips reduces the structural density of the material.
4.6.2 Where can I get MDF? [PS]
--------------------------------------
Wholesale distributors of lumber products. Home Improvement Centers,
such as Home Depot or Home Base, and Lumber Yards may or may not stock
MDF products, but they will be able to order it for you. Contact
woodworking or cabinet making shops in your area, ask them where to get
it or if it may be possible to buy the material from their business.
4.6.3 What type of saw blade works best when cutting MDF? [PS]
---------------------------------------------------------------------
If you're using a circular saw, a good 40 tooth carbide tipped blade
works best. If you're using a table saw, a 50/60 tooth carbide tipped
combination or ripping blade will provide smooth cuts.
4.6.4 What type of router bits work well with MDF? [PS]
--------------------------------------------------------------
As far as router bits go, use only two flute carbide bits and make
several passes as opposed to one single pass. If possible, trace the
outline of what you intend to router and remove the majority of the
material with a jigsaw which will reduce the amount of material being
removed and will put less strain on the router as well, not to mention,
this will all reduce the amount of MDF dust.
4.7 What driver should I use?
====================================
4.8 Is there any computer software available to help me choose
an enclosure and a driver? [JSC, MH, DK]
=====================================================================
Various enclosure design software is available via ftp from
`ftp://ftp.uu.net/usenet/rec.audio.high-end/Software/'.
The most popular program there is Perfect Box, which is in the file
`perf.uu' (or `perf.zip').
Note that NO program can tell you what enclosure is best for YOUR car!
The program does not take into consideration your space limitations,
the type of car you drive, the type and number of mid-bass drivers you
use, your musical preferences and the goals you have for your system.
Many people follow (blindly) what a computer program says is "optimal,"
and end up unhappy with the results. Therefore, it is always a good
idea to discuss a design you think looks good with a qualified
installer or (even better) with the manufacturer.
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.
4.9 What is an "aperiodic membrane?" [CD, DK]
====================================================
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 well below 0.7. In contrast, the most commonly
used sealed enclosures have Qtc's in the range of 0.8 to 1.1 which are
considered, by definition, to be underdamped. When improperly used, a
high-Q system may have poor transient response, nasty peaks in
frequency response, and high rates of roll-off. Aperiodic systems will
feature excellent Aperiodic systems are characterized by better
transient response, flatter frequency response and somewhat extended
low frequency response.
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.
4.10 Can I use my subs in the winter? [MS]
=================================================
The following applies to all speakers in extremely cold conditions, but
the question most often occurs in reference to subwoofers.
The suspension of the speakers will stiffen considerably at very cold
temperatures (lower than 30 degrees F). So will certain cone materials
which may become more brittle.
If a very cold speaker is played very hard there is a small potential
for damage because more stress is placed on the cone's neck. The
likelihood of damage is minimal for well-constructed and well-designed
automotive speakers, however.
Thermally, the danger is minimal because the ambient temperature and
the coil temperature are so low that it is highly unlikely that a coil
will overheat and burn, despite limited movement and ventilation.
At temperatures between +20 degrees F and 0 degrees F, it is a good idea
to play the system at a moderate level until the car's heater has warmed
the vehicle interior. As the speakers warm up, they will play louder
and lower signifying that their suspensions are warming up and returning
to nominal compliance.
If the temperature is extremely cold (less than 0 degrees F), you should
avoid playing the system at all until the vehicle interior is warm.
This is to avoid stress fractures in the surround material (especially
with rubber surrounds).
4.11 How can I carpet my enclosure? [AO]
===============================================
What you will need:
* Adhesive (3M Super 77 or Super 90 is excellent.)
* Carpet.
* A good sharp pair of scissors.
* A razor sharp utility knife. Buy a BOX of blades, they go dull
fast.
* Solvent to clean up excess adhesive.
Before you start, find a large, clean, flat surface on which to set the
box as you carpet it. Start by unrolling the carpet onto the surface,
smoothing it out so that its flat, and setting the box on top of it
edgewise. Also, make sure that you remove the speaker, any ports and
terminal cups from the box.
The instructions on how to carpet the box are as follows:
1. Place the box such that it is centered on the carpet lengthwise,
and one edge of the box is about one inch from the edge of the
carpet.
2. Roll the box back so that the side of the box that was previously
done faces forward, and the carpet beneath it is exposed. Coat
both the box and carpet with adhesive, but do NOT apply the carpet
to the box - the adhesive needs a few minutes to set up (follow
the instructions from your adhesive to find out how long you
should wait).
3. After the adhesive has set up, roll the box back into position.
Wait a few more minutes for the adhesive to bond.
4. Now coat the side of the box adjacent to the remaining carpet (the
side facing backwards) and the carpet next to it with adhesive,
let the adhesive set up, and roll the side you just coated onto
the coated carpet. Repeat this until three sides of the box are
carpeted.
5. Before carpeting the next side, the 1 inch of carpet sticking over
the edge must be removed. To do this, rotate the box so that the
first side that was carpeted is up. Pull the carpet sticking over
the edge down towards the uncarpeted edge and cut it off with the
knife, flush with the uncarpeted side of the box. You will have to
run the knife nearly parallel to the uncarpeted side to get a
perfect cut.
6. This done, spray the remaining side and carpet, and roll the box
onto it. Shear off the remaining carpet sticking out from all
edges with the scissors leaving a 1 inch border everywhere.
7. Clean up the ends of the box so that the carpet is flush with the
sides of the box as in step 4.
8. Next cut off the remaining 1 inch flap of carpet (located at the
point where you began carpeting). This is the tricky part, as you
don't want to be able to see this seam. Again, pull the flap down
over the edge of the box, but this time cut it at roughly a 45
degree angle. If you are successful you shouldn't be able to see
the wood under the seam, but will probably see the white of the
adhesive and the back of the carpet.
9. Soak some of the solvent onto a rag and use this to scrub the edge
you just cut off. It should dissolve the adhesive and the carpet
backing somewhat, causing the carpet on the edge to become fuzzy.
Keep scrubbing the edge until you can no longer see the seam.
10. Now carpet the ends of the box. Cut two pieces of carpet slightly
larger than the ends of the box and lay one of them flat on the
surface. Spray the carpet and one end of the box with adhesive,
and set the end of the box on the carpet, so the box stands on end.
11. After the adhesive has dried sufficiently cut off the remaining
border of the carpet as in 7 and 8.
12. Repeat step 9 and 10 for the other end of the box.
Congratulations! You've just carpeted your box!
4.12 Are large magnets always better than small magnets? [ST]
====================================================================
Magnet *size* is meaningless!
Every speaker will have an optimal BL ((see Section 4.1),) product,
the field strength in the air gap multiplied by the length of the voice
coil wire in the field.
If the BL product is too low, the speaker is electrically not very well
damped (which will result in a woofer with a high Qts). A bump in
frequency response and a level drop in midband efficiency may be the
result. If the BL product is too high, the speaker is electrically
overdamped (Low Qts woofer). A very high midband efficiency, but the
driver starts to roll of early.
An high BL product can be achieved in a number of ways: increase field
strength; or increase wire length in magnetic gap.
The increase in field strength is limited; so some manufacturers use
very thin wire for the voice coil, as such they can achieve a high BL
product with a low field strength (cheap magnet). Or they use an 8 layer
voice coil... needless to say that electrical power handling will
decrease enormously.
Long stroke woofers, having only a part of the voice coil in the air
gap, need a very high field strength to achieve a high BL product. Often
this means a big magnet as well...
Use magnet size as an indication, but as nothing more than that.
4.13 I know the box volume required for my subwoofer, but what
are the best dimensions for my enclosure? [IDB]
=====================================================================
The specific dimensions of a subwoofer enclosure aren't really
important. Once you know the appropriate volume of the box, and you
know where in your car you want to install it, you will have some idea
of the restrictions in the dimensions. For example, if the distance
between the floor of your trunk and the bottom of the rear deck is 16",
then you probably shouldn't make your box any taller than 16".
Likewise, if width of the trunk (between the wheel wells is 38", then
you've got that much space to work with.
You can also infer some other information about your box, from the
speaker specifications (for a PPI PC10):
Mounting Depth: 4.5625"
Speaker Displacement: .032 ft^3
Here is an ASCII drawing of a subwoofer enclosure to help illustrate a
few things:
|--- L ---|
__________ ___
/ /| |
/ + / | | + = Center of speaker
/ x / | D x = center of port
/_________/ | |
| | | _|_ W = Width of box
| | / / L = Length of box
| | / W D = Depth of box
|_________|/ _/_
Because of the speaker's mounting depth, you know that the box MUST be
at 5.5" deep (it's always good to leave at least 1" of space behind the
speaker, but leave more if you can). If you use a straight port (3"
diameter PVC) then the box will need to be at least 13" deep (leaving 2
inches between the end of the port and the back of other side of the
enclosure), assuming that your port will reside completely within the
enclosure. If you use Flex-Port or choose to have a portion of the
port extending outside of the enclosure, you can make the box less deep.
We also know that with a 10" woofer, the length and width should both
be a minimum of 12" (leave 1" on either side of the woofer). Obviously
both dimensions can't be 12" otherwise we can't put the port on the
same face as the speaker.
With volume, remember a couple of things. Recommended volume is the Net
Internal volume. Both the speaker and the port will displace volume
from the box - this means that you MUST account for these (i.e., make
the interior volume of the box larger than the recommended value). In
this case, you know the following:
Required Volume: 1.25 ft^3
Speaker Displacement: 0.032 ft^3
Port Displacement: ??
3" (interior diameter) PVC will usually have an exterior diameter of
3.5" (0.25" wall thickness). Volume of a cylinder = pi*r^2*h, where r
= 1.75" and h=12.25" (13" port length - 0.75" for the thickness of the
MDF). Therefore the port displacement is: 117.9 in^3 or 0.068 ft^3
(1728 in^3 = 1 ft^3).
Required Volume: 1.25 ft^3
Speaker Displacement: 0.032 ft^3
Port Displacement: 0.068 ft^3
TOTAL INTERIOR VOLUME: 1.35 ft^3 or 2332.8 in^3
Now, to calculate the dimensions of the box: For a rectangular box, L
x W x D = Volume.
If we know that the minimum interior depth is 15" (because the port is
13" and you should leave 2" between the port and the wall of the
enclosure), we can say,
L x W x 15" = 2332.8 in^3 or
L x W = 155.52 in^2
Since we know that L must be at least 12" (since the speaker is 10" in
diameter), we can continue:
12" x W = 155.52 in^2
W = 12.96"
So, we have determined that the interior dimensions of the box must be
12" x 12.96" x 15". Obviously this can't work, since there is nowhere
to put the port! You will either have to have the port extend outside
of the enclosure, or use flex-port. Keep in mind that if you have part
of the port "sticking out" of the enclosure, the Port Displacement will
also change!!.
For simplicity, (in this example) let's use flex-port. This will let
you decrease the thickness of your box to about 6". So,
L x W x 6" = 2332.8 in^3
L x W = 388.8 in^2
12" x W = 388.8 in^2
W = 32.4"
This is a little more reasonable. This will be a flat, wide box.
Again, keep in mind that these are internal dimensions. If you use
3/4" MDF (and you should) to build the enclosure, this will add 1.5" to
each of these figures for the external dimensions. e.g.:
13.5" x 33.9" x 7.5"
Remember, the 6" depth and 12" length are just the minimum values, you
can alter them if, for example, you don't have enough space between your
wheel wells for a 34" box. If you only have, say, 28", then:
maximum external width = 28"
maximum internal width = 26.5"
L x W x D = 2332.8 in^3
L x 26.5" x 6" = 2332.8 in^3
L = 14.7" (internal)
Got it? It can be a pain, but that's part of the fun in building a
speaker enclosure.
--
ian d bjorhovde -> ianbjor@mobileaudio.com
internet mobile audio -> http://mobileaudio.com
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