P1242 - JBL Car Audio User Guide / Operation Manual. Page 2

Thank you for purchasing a new
JBL GTp subwoofer. Subwoofer
installation requires woodworking
skills and some experience 
disassembling and reassembling
automotive interiors. If you lack 
the tools or necessary skills, have
your subwoofer installed by an
authorized JBL dealer. 

Warning: Playing loud music in 
an automobile can permanently
damage your hearing as well as
hinder your ability to hear traffic.
We recommend listening at low 
levels while driving. JBL accepts no
liability for hearing loss, bodily injury
or property damage resulting from
use or misuse of this product.

GTp subwoofers are optimized 
to perform best in small, sealed,
vented and prefabricated bandpass
enclosures. While infinite-baffle
mounting of GTp subs is possible,
power handling will be greatly 
compromised because there’s no
enclosed volume of air to prevent
the speaker’s cone from moving
past its limit. For this reason, we 
do not recommend infinite-baffle
mounting for GTp subwoofers.

You should choose the enclosure
you will use based on the type of
music you listen to, how much
amplifier power you will use for the
subwoofer and how much space
inside the vehicle you can devote 
to a subwoofer enclosure.

Because a sealed enclosure 
provides the most control over the
woofer’s movement, a woofer
mounted in a sealed enclosure will
handle more power than a woofer
mounted in another enclosure type.
Sealed enclosures provide more
accurate sonic reproduction than
other enclosure types, so they are
well suited to all types of music.
Sealed-enclosure construction 
is straightforward and there are

many prefabricated sealed 
enclosures available. An optimum
sealed enclosure is always smaller
than other types of enclosures 
optimized for a particular speaker,
so they require the smallest amount
of space inside the vehicle.

Vented enclosures provide better 
efficiency in the 40Hz – 50Hz range
but this efficiency comes at the
expense of sound in the lowest
octave (below 40Hz) and at the
expense of some control and power
handling. If you are using a small
amplifier, a vented box will provide
more bass output from less power.
Vented enclosures are also well
suited to a variety of music types.
Because vented enclosures require
the volume of the enclosure and
the size of the port to have a 
specific relationship with the 
characteristics of the woofer, the 
enclosure must be built exactly to
the specifications provided. While
there are some prefabricated 
vented boxes available, matching a
prefabricated box to a particular
woofer is difficult. If you wish to use
a vented enclosure, we strongly
recommend having your authorized
JBL dealer build it or verify that

your design is correct if you wish 
to build it yourself. An optimum
vented enclosure is always larger
than the optimum sealed box for
the same woofer and will require
more space inside the vehicle. 

Bandpass enclosures often 
provide the most output available
from any amplifier and subwoofer
combination at the expense of
sonic accuracy. If sheer SPL
(sound-pressure level) is what you
desire most, choose a bandpass
enclosure. Bandpass-enclosure
design is very tricky and the aid 
of a computer and enclosure
design-software is necessary. If 
you are an experienced installer 
or have some woodworking 
experience, you may wish to build
the enclosure described in the
enclosure design sheet included
with this woofer. Fortunately, there
are many prefabricated bandpass
boxes available and they are all
optimized to extract the most 
output possible from any woofer.
Bandpass enclosures can be quite
large and may require a lot of space
inside your vehicle.

02

Choosing an Enclosure 

JBL Pro Power GTp Series