A box ranges in complexity from the "plain
vanilla box" (sealed) to bandpass and even more exotic enclosures.
Each enclosure has advantages and disadvantages and should be designed
accordingly to the individual speaker parameters (the "one size fits all"
rule DOES NOT apply to subwoofers and boxes).
Subwoofers need more amplifier power than
everything else in the system. This is because human ears are less
sensitive at lower frequencies, so a higher bass level is needed for
everything to sound even. A low-pass crossover is required to block
off high frequencies.
What type of subwoofer is better? A
bigger subwoofer gives more bass, but needs a bigger box. Since most
people like to have a trunk, 10 and 12-inch woofers are most common.
When buying a subwoofer always keep in mind that bigger size is not
necessarily better. A good quality 8-inch sub will outperform a cheap
12-incher. Big subs (12", 15") have slower responses, yielding to
boomier bass. Small subs (8", 10") have a tight and more controlled
Types of boxes
|Free Air - subwoofers are either
mounted under the rear deck or behind the rear seat of a car. This
configuration will not work very well for hatchbacks. Holes have to be
cut where the woofers are to be mounted. Since the woofers use the
whole trunk as a box, the trunk has to be as sealed as possible from
the cabin. Trunk can be isolated usually by putting particle board
under the deck and behind the seat.
The drawback of free air
subwoofers is that bass will not be very accurate (especially at lower
frequencies), and more amplifier power will be required than with a
regular box, but then again, you still have a full
|Sealed - is the most common box
and easiest to build. These boxes will give the flattest frequency
response, and best overall sound quality (especially at lower
frequencies). The box internal volume should be as close as possible
to the recommended by the manufacturer. If a box is smaller than what
it is supposed to be, the sound will be tighter, but more amplifier
power will be required. If the box is too big, then the sound will be
|Ported - boxes are usually
bigger in size than sealed and have a "tube" (port) that lets some air
out of the box. The idea of a ported box is that the speaker port
pushes (or pulls) air at the same time as the woofer, reinforcing
bass. The box itself acts as an amplifier, yielding to more bass than
a sealed enclosure (3 to 4 dB). Ported boxes do not have a linear
frequency response. If the box is not built according to
specifications, it will not sound good. The box design acts as a
filter, cutting off lower frequencies.
|Isobaric - configuration is a
good way to get bass in a smaller box. This is done by building a box
about half the volume of a sealed box, and placing two woofers facing
each other. Note that everything must be sealed, including space
between woofers. A spacer between both woofers must be used in most
cases to avoid subs hitting each other. When wiring, make sure that
woofers are out of phase: Wire one of them backwards (negative to
positive, and positive to negative), so that both pull or push at the
same time. An isobaric configuration will NOT put out much more power
than a box using a single woofer. Its main purpose is to reduce
box size. Another drawback is that since one of the subs is
exposed, it is more prone to damage.
|Band Pass - enclosures
consist of a woofer between a sealed and ported box. Bandpass
boxes will yield more bass than sealed and ported boxes
(especially at lower frequencies), but over a narrower frequency
range. Since the box acts as a filter, mechanically blocking lower and
upper frequencies, a crossover is not needed in most cases.
These enclosures are usually big, and very unforgiving when precise
volumes and port sizes are not followed. Bandpass boxes also
tend to mask distortion. If you can't hear distortion and turn
your stereo down in time, you could damage your subs.
|Aperiodic - Very small boxes
that "breathe" through a moving membrane. Both the membrane and
cone can not be in the same exterior space. Either the membrane
part has to be isolated by cutting a hole in the car so that it is
outside, or the subwoofer has to be isolated from the rest of the
trunk in a similar fashion to free air woofers. The "box" has to
be as small as possible (ideally the membrane should be right up
against the sub), since it is used only for coupling the sub and
membrane. Aperiodic membrane configurations are very hard to
design and tune, but give good frequency response and respond faster
to transients, giving accurate and tight bass as opposed to boomy
sound. They are not ruled by Thiele-Small parameters like other
designs, so any woofer would work with the membrane.
Amplified Bass Boxes
A good choice for small cars and (ideal) for
hatchbacks and pickup trucks. They usually take up very little room,
putting out to fairly good bass. The most known manufacturer is Bazooka® for
it's Bass Tubes®. Their design is a ported box. The woofer has to be
close to a wall or, better yet, to a corner. To fine-tune, the bass
tube is moved either closer, or farther form the wall or
It is convenient to get an amplified tube,
since amplifier, crossover and subwoofer are all integrated in a small
package. If you buy the components separately, you will end up spending more
money. Another good feature of tubes is the fact that they can be easily and
quickly installed and removed.
If you decide to get one, keep in mind that
even though they all look the same, cheaper brands will not sound good. A
decent tube will run in the $300's (amplified), and in the $100's for a
Custom Bass Boxes
Many manufacturers such as JL and MTX are
making custom boxes (with subs included) to fit in center consoles, under
seats, or in other small spaces. Although these boxes do cost a lot of
money, most give superb performance and integrate easily in a car without
taking up too much room.