GT4-12 - JBL Car Audio User Guide / Operation Manual. Page 2

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Thank you for purchasing a 
new JBL GT4 Series 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 hin-
der 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.

GT4 Series subwoofers are opti-
mized to perform best in small,
sealed, vented and prefabricated
bandpass enclosures. While infinite-
baffle mounting of GT4 Series 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 GT4 Series sub-
woofers.

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.

Choosing an Enclosure 

JBL GT4 Series