4. Subwoofers
4.1 What are "Thiele/Small parameters?"
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
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?
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.
4.4 Which enclosure type is right for me?
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.
4.5 How do I build an enclosure?
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!
4.6 MDF for Dummies
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?
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?
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?
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 is an "aperiodic membrane?"
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.
4.8 Can I use my subs in the winter?
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.
5 Installation
5.1 Where should I buy the components I want?
Most of the time, you will either buy from a local dealer, or from a
mail-order house. Buying from a local dealer can be good because you
get to deal directly with a person: you can show them your car, ask
specific questions, haggle prices, get quick service when there are
problems, get deals on installation, etc. But there can also be
advantages to buying mail-order: generally cheaper prices, sometimes
better service, etc. In either case, you should always check prices
before you buy, inquire about warranty service, and ask about trial
periods.
5.2 What is "rear fill", and how do I effectively use it?
"Rear fill" refers to the presence of depth and ambiance in music. A
properly designed system using two channels will reproduce original
rear fill on the source without rear high frequency drivers. Since
recordings are made in two channels, that is all you will need to
reproduce it. What is captured at the recording session (coincident
pair mics, Blumlein mic patterns, etc.) by a two channel mic array will
capture the so called rear fill or ambiance. Many of the winning IASCA
vehicles have no rear high frequency drivers. Also a lot of this has
to do with system tuning. If rear high frequency drivers are added,
however, the power level of the rear fill speakers should be lower than
that of the front speakers, or else you will lose your front-primary
staging, which is not what you want (when was the last time you went to
a concert and stood backwards?). The proper amount of amplification
for rear fill speakers is the point where you can just barely detect
their presence while sitting in the front seat. Separates are not a
requirement for rear fill; in fact, you may be better of with a pair of
coaxial speakers, as separates may throw off your staging.
5.3 How many devices can I attach to my remote turn-on lead?
The remote turn-on lead that most head units will not provide very much
current (usually 250-300mA), so there is a limit to the number of
components you can activate with it. Generally, it is safe to hook up
two devices to the lead without having to worry about problems.
However, if you'll be activating more components, then you should
probably use a relay.
6. Competition
6.1 What is IASCA, and how do I get involved?
IASCA is the International Auto Sound Challenge Association, a
sanctioning body for car audio competitions held throughout the world.
Competitors earn points at each competition, and those that perform the
best each year can advance to the finals. Prizes (trophies, ribbons,
and sometimes cash) are usually given out to the top competitors in
each class at every competition.
6.2 What is USAC, and how do I get involved?
USAC is another sanctioning body, similar to IASCA. However, USAC
places greater emphasis on SPL measurements than IASCA.
7. Literature
7.1 Can I contact any manufacturers on-line?
Yes, there are a number of manufacturers who have started to use the
Internet and can offer advice for component selection, installation and
advice. A list of these manufacturers is available on the WWW, at the
URL:
www.mobileaudio.com/intdir/
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