Custom Car Audio Boxes - Subwoofer Enclosures:
Don't just any old
The simplest enclosure type is an infinite baffle
or free air system. Note: Free-Air™ is a trademark
of Stillwater Designs (Kicker) In this system the
woofer has the front of its cone isolated from the
back of its cone by mounting the woofer to the rear
deck or on a board placed across the back of the
Advantages of this design include simplicity,
minimal occupation of space and lower cost due to
not having to have a box type enclosure.
Disadvantages of this design are a larger power
requirement, difficulty (sometimes extreme) in
completely isolating the two sides of the cone from
each other, and slightly less accurate sound
The simplest box type system is a sealed
enclosure. Here a woofer is mounted in a completely
sealed box that isolates the front and rear waves of
Advantages of the sealed design include ease of
assembly, more lenient box size requirements,
excellent transient characteristics, high power
handling, and a typically smaller enclosure.
Disadvantages are less efficiency (requires more
amplifier power) than vented enclosures and less low
bass as compared to a properly tuned vented
enclosures above the port tuning frequency.
Vented enclosures are similar to a sealed
enclosure in design except that they add a “port”
that is a certain length and a certain area around.
This port, usually a length of PVC pipe, is tuned to
a certain frequency by variation in the two
parameters of area and length. By adding this port,
the rear wave of the cone motion is used to
reinforce the front wave. When done properly the
subwoofer system becomes more efficient than a
comparable sealed enclosure above the port's tuning
frequency. However, below the port tuning frequency
the woofer will become unstable and "unload" causing
the woofer to oscillate violently, leading to
premature woofer failure when driven with excessive
Advantages of this design are increased
efficiency and a lower frequency extension above the
port tuning frequency.
Disadvantages are a more complex design, a larger
enclosure as compared to a sealed design, lower
power handling, and slightly less transient
(4th order) Enclosure
The most complex design of the common enclosures
is a band pass system. The purpose of the band pass
system is increased efficiency over a small range of
frequencies. This design also helps filter out
unwanted midbass and upper frequencies. There are
variations on the band pass design known as orders.
The simplest design is the single reflex (4th order)
in which a woofer in a sealed enclosure fires into a
Dual Reflex Bandpass
(6th order) Enclosure
Series Tuned Dual Reflex
There are also dual reflex designs (6th order) in
which both enclosures are vented and series tuned
designs in which the woofer and first vent fires
into the second vented enclosure.
Advantages of this design are a high efficiency
over a small range of frequencies and some filtering
Disadvantages are decreased frequency response,
less accurate reproduction, extreme complexity in
design and assembly, larger enclosure size, greater
woofer unloading and decreased power handling.
Another enclosure design that you may hear of is
isobaric. Isobaric is not actually an enclosure type
but a woofer loading method. Isobaric loading can be
used in conjunction with any of the enclosure types,
except perhaps infinite baffle. Isobaric loading
consists of two woofers coupled together by a short
enclosure which is only long enough to prevent the
two woofers from striking one another. The woofers
may be mounted face to face, back to back or front
to back. It is important however to have the woofers
firing in phase with one another. In order to
accomplish this with the woofers firing front to
front or back to back one of the woofers speaker
leads must be reversed with respect to the other
Isobaric loading is usually used when space is at
a minimum or a maximum number of woofers wish to be
used in a certain volume of space. By isobarically
loading the woofers, a box of only half the size of
a one woofer box is needed. For example, if a 12"
woofer requires a two cubic foot box, the same 12"
woofer isobarically loaded only requires a one cubic
foot box. The cost of this design, other than
doubling the woofer cost, is a reduction in the
subwoofer system efficiency by three decibels (if
both woofers are given the same power as a single
woofer), which is equivalent to halving the
amplifier power. New, small box subwoofer designs
have all but negated the need for isobaric loading
in car audio and it has fallen out of popularity.
Advantages of this design are increased linearity
in the speaker movement, lower space requirements,
and increased power handling.
Disadvantages are increased cost, increased
design and assembly complexity, and decreased
You should also check out Advanced
Enclosure Design and Fabrication. It
covers designing enclosures using free computer
software, maximizing the output of a subwoofer
system, building the box like a pro and testing the
output using inexpensive equipment.