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Archive-name: car-audio/part3
Rec-audio-car-archive-name: FAQ/part3
Version: 4.54
Last-modified: 05 March 2004
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
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