DOWNLOAD JBL 1500.4 GTi Service Manual ↓ Size: 286.28 KB | Pages: 8 in PDF or view online for FREE

Thank you for purchasing one of the finest

automotive subwoofers that JBL has ever pro-
duced. As a GTi competition series owner, you are
a member of a select group of people who enjoy
the most advanced mobile audio products avail-
able. We call this group “Club GTi.” By sending in
the customer information card, that you will find
packaged with this product, we will automatically
register you as a Club GTi member which will give
you access to exclusive Club GTi apparel and
other special offers we will make available from
time to time. We suggest you fill the card out at
your earliest convenience.

Also, be sure to save your sales receipt in a

safe place. It will be necessary to use this docu-
ment as proof of purchase should you need war-
ranty service.

The GTi Subwoofer is built to meet the same

rigorous standards of construction and perfor-
mance that have long established JBL’s renowned
home and professional speaker systems. The GTi
Subwoofer achieves its exceptional performance
with leading edge technologies that push the per-
formance envelope in automotive sound.

The exceptional sound of the GTi Subwoofer

comes from an Aquaplas coated fiber cone for
extra stiffness and resonance elimination. The

cone motion is controlled by a synthetic rip-stop
suspension which resists tearing for safe, effortless
high-power use. The cone is driven by a massive
motor structure utilizing a 3" or 4" edge-wound
OFC copper wire, high temperature fiberglass,
voice coil. This motor structure includes JBL’s
patented Symmetrical Field Geometry (SFG) to
create a uniform magnetic flux field through the
coil and thus lower distortion. The frame and cone
are contained in a high strength die-cast aluminum
frame for maximum rigidity and higher magnetic
energy concentration on the coil.

What does all this technology add up to? A

driver with smooth frequency response, excellent
transient response, high output efficiency and high
power handling in a rugged package able to deal
with the rigors of high performance car audio. The
result is exceptionally powerful, dynamic and accu-
rate sound reproduction that cuts through the road
noise and provides a solid musical foundation.

Because this is a high performance product

which may be used in many different configura-
tions, we strongly recommend that you have the
subwoofer professionally installed to unleash its
full performance potential. However, if you feel
you have the skills necessary to build a subwoofer
box, read the following information on enclosures
before proceeding. 

The enclosure you build plays a profound role in

the performance of your subwoofer. The size and vent
tuning frequency, where applicable, determine the
low-frequency performance and output capability of
the subwoofer system. Your GTi subwoofer will pro-
vide excellent performance when used in Sealed,
Vented, Infinite Baffle, and Band-pass applications
that are properly designed. Each of these applications
have limits and advantages that should be considered.

GTi woofers are most commonly used either in

Vented enclosures or Infinite Baffle. Instructions which
follow will give you a simplified design method to build
a Vented enclosure for your subwoofer. Another
option is the Infinite Baffle (free air) installation which
doesn’t require complex design and construction, and
can sound very good when solidly installed. However,
because there is no enclosure to control cone motion,
the power handling and maximum output level of an
Infinite Baffle installation is reduced from that of a
Vented enclosure. Refer to the “Points on Power
Handling” section for more information.

For more complex designs, such as single and

dual-vented Band-pass types, we suggest you opti-
mize your design with JBL SpeakerShop, a speaker
system design software for a P.C. running Microsoft
Windows, available from your JBL dealer. If JBL
SpeakerShop or a similar program is not available,
the JBL Customer Service department will be happy
to assist you. 

3

Welcome to Club GTi . . . . . . . . . . . . . . . . . . . . . . . 3
Autosound at its Best . . . . . . . . . . . . . . . . . . . . . . . 3
Unleash Maximum Performance. . . . . . . . . . . . . . . 3
Enclosure Basics . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Vehicle Acoustics . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Enclosure Design. . . . . . . . . . . . . . . . . . . . . . . . . . 5
Enclosure Construction Tips . . . . . . . . . . . . . . . . . . 6
Points on Power Handling . . . . . . . . . . . . . . . . . . . 6
Speaker Connection and Combination . . . . . . . . . . 7

2

Staple or clip your original bill of sale here 

▼

GTi Subwoofer Owners Manual  7/17/98 9:51 AM  Page 2

Follow the simple steps below to design an

optimum ported box for your GTi driver. 

1. Determine the maximum enclosure size 

available for your installation by measuring 
and multiplying the height x width x depth (in
inches) and then dividing by 1728. This will tell
you exactly how many cubic feet your enclosure
will be.

2. To determine the volume of an irregularly

shaped box, divide the box into sections, calcu-
late the volume of each section individually, then
add the separate volumes to determine the total
box volume. Here are some useful formulas to
help you calculate volumes:

USEFUL FORMULAS
1 cubic foot = 1728 cubic inches
1 cubic foot = 28.32 liters
1 liter = 0.353 cubic feet = 61 cubic 

inches = 1000cm

3

Radius = 1/2 diameter 
Box volume = height x width x depth 
Volume of triangular box section = 1/2 base x

height x length 

Volume of tubular duct = area of circle x length
pi (

π

) = 3.1416 

Area of circle = pi (

π

) x radius

2

Diameter of circle = 2 x area / pi (

π

) 

Area of square = height x width

3. Now that you have calculated the internal box

volume, you should subtract the DRIVER DIS-
PLACEMENT (listed on the specification sheet)
while also subtracting approximately 0.1 cubic
feet for the port tube (if one is being used). If
you’d like, you may calculate the exact dis-
placement of the port tube by utilizing the for-
mula listed above (Volume of tubular duct).  

4. Using the GTi Enclosure chart (shown on your

GTi driver specification sheet), choose a desired
box volume on the vertical chart axis, which fits
within the size limit you have established. Draw
a line across from this point until it intersects the
recommended tuning frequency. Draw a line
down to the horizontal axis and read the corre-
sponding tuning frequency.  We have drawn an
example on the chart.

5. Use the Port Nomograph on the Driver Specifica-

tion sheet to determine the port diameter and
length based on the enclosure volume and tuning
frequency determined in step 4. Follow these
steps to determine the port.

1. Draw a line from the chosen enclosure volume to

the desired tuning frequency.  Continue this line
until it intersects with the Lv/Sv construction line
(Lv = port length, Sv = port opening surface
area). This is called the construction point.

2. Draw a horizontal line starting from the con-

struction point straight through the chart. Note
that the Lv/Sv axis also appears at the right of
the chart to help keep your line parallel all the
way across the chart.

3. Select the curved line with the port diameter

you would like to use. Locate the point where
this curved line intersects with your horizontal
construction line and draw a vertical line from
this point down to the bottom of the chart. 

NOTE: Try to use a port tube no smaller than 1/3 the

diameter of your speaker. Using a smaller diameter

port can result in “whistling” or “breathing” noises.

4. The required port length is found at the point

where the vertical line you’ve drawn intersects
with the scale at the bottom of the chart.

NOTE: The recommended port tube dimensions

should be considered a starting point. Since it is

impossible to predict “in-car” response exactly, you

may find that a port length slightly longer or shorter

than that recommended may produce slightly better

results. Experiment! 

Lay out plans for your box using the information
determined in the above steps and the box con-
struction tips listed on page 6. 

5

The acoustics of your vehicle also influence

bass performance tremendously. Fortunately, the
small interior volume of most cars and trucks
boosts low bass output enough to allow for small-
er enclosure sizes while still achieving good bass
performance. As a general rule, below 50-80 Hz
(depending on interior size), bass response
increases at a rate which approaches 12dB per
octave as the frequency decreases. This adds a
significant bass reinforcement to the actual output
of the subwoofer. If this rise in bass is not
addressed in the design of your enclosure, bass
performance may be boosted to the point where it
is actually too strong for some tastes. For others,
there is never too much! The enclosure design
recommendations which appear on the specifica-

tion sheet for your particular model, were
chosen for optimum in-car performance.
You may find them to be different than the
recommendations from formulas used to
design enclosures for in-home or profes-
sional sound-reinforcement applications.
These “Transfer Function Compensated”
tuning recommendations result in small
enclosures with excellent in-car low fre-
quency performance.

4

This graph shows the result of tuning the enclosure to compensate for the effects 
of the vehicle transfer function. Note that the in-car response is smoother and the 
–3dB point moves down to approximately 22Hz.

This graph shows the effect of vehicle transfer function added to an enclosure 
using “standard” tuning. Note the peak at 50Hz, with a –3dB point of 
approximately 31Hz.

This is a comparison of in-car response curves with and without transfer function 
compensated tuning.

GTi Subwoofer Owners Manual  7/17/98 9:51 AM  Page 4

Follow the simple steps below to design an

optimum ported box for your GTi driver. 

1. Determine the maximum enclosure size 

available for your installation by measuring 
and multiplying the height x width x depth (in
inches) and then dividing by 1728. This will tell
you exactly how many cubic feet your enclosure
will be.

2. To determine the volume of an irregularly

shaped box, divide the box into sections, calcu-
late the volume of each section individually, then
add the separate volumes to determine the total
box volume. Here are some useful formulas to
help you calculate volumes:

USEFUL FORMULAS
1 cubic foot = 1728 cubic inches
1 cubic foot = 28.32 liters
1 liter = 0.353 cubic feet = 61 cubic 

inches = 1000cm

3

Radius = 1/2 diameter 
Box volume = height x width x depth 
Volume of triangular box section = 1/2 base x

height x length 

Volume of tubular duct = area of circle x length
pi (

π

) = 3.1416 

Area of circle = pi (

π

) x radius

2

Diameter of circle = 2 x area / pi (

π

) 

Area of square = height x width

3. Now that you have calculated the internal box

volume, you should subtract the DRIVER DIS-
PLACEMENT (listed on the specification sheet)
while also subtracting approximately 0.1 cubic
feet for the port tube (if one is being used). If
you’d like, you may calculate the exact dis-
placement of the port tube by utilizing the for-
mula listed above (Volume of tubular duct).  

4. Using the GTi Enclosure chart (shown on your

GTi driver specification sheet), choose a desired
box volume on the vertical chart axis, which fits
within the size limit you have established. Draw
a line across from this point until it intersects the
recommended tuning frequency. Draw a line
down to the horizontal axis and read the corre-
sponding tuning frequency.  We have drawn an
example on the chart.

5. Use the Port Nomograph on the Driver Specifica-

tion sheet to determine the port diameter and
length based on the enclosure volume and tuning
frequency determined in step 4. Follow these
steps to determine the port.

1. Draw a line from the chosen enclosure volume to

the desired tuning frequency.  Continue this line
until it intersects with the Lv/Sv construction line
(Lv = port length, Sv = port opening surface
area). This is called the construction point.

2. Draw a horizontal line starting from the con-

struction point straight through the chart. Note
that the Lv/Sv axis also appears at the right of
the chart to help keep your line parallel all the
way across the chart.

3. Select the curved line with the port diameter

you would like to use. Locate the point where
this curved line intersects with your horizontal
construction line and draw a vertical line from
this point down to the bottom of the chart. 

NOTE: Try to use a port tube no smaller than 1/3 the

diameter of your speaker. Using a smaller diameter

port can result in “whistling” or “breathing” noises.

4. The required port length is found at the point

where the vertical line you’ve drawn intersects
with the scale at the bottom of the chart.

NOTE: The recommended port tube dimensions

should be considered a starting point. Since it is

impossible to predict “in-car” response exactly, you

may find that a port length slightly longer or shorter

than that recommended may produce slightly better

results. Experiment! 

Lay out plans for your box using the information
determined in the above steps and the box con-
struction tips listed on page 6. 

5

The acoustics of your vehicle also influence

bass performance tremendously. Fortunately, the
small interior volume of most cars and trucks
boosts low bass output enough to allow for small-
er enclosure sizes while still achieving good bass
performance. As a general rule, below 50-80 Hz
(depending on interior size), bass response
increases at a rate which approaches 12dB per
octave as the frequency decreases. This adds a
significant bass reinforcement to the actual output
of the subwoofer. If this rise in bass is not
addressed in the design of your enclosure, bass
performance may be boosted to the point where it
is actually too strong for some tastes. For others,
there is never too much! The enclosure design
recommendations which appear on the specifica-

tion sheet for your particular model, were
chosen for optimum in-car performance.
You may find them to be different than the
recommendations from formulas used to
design enclosures for in-home or profes-
sional sound-reinforcement applications.
These “Transfer Function Compensated”
tuning recommendations result in small
enclosures with excellent in-car low fre-
quency performance.

4

This graph shows the result of tuning the enclosure to compensate for the effects 
of the vehicle transfer function. Note that the in-car response is smoother and the 
–3dB point moves down to approximately 22Hz.

This graph shows the effect of vehicle transfer function added to an enclosure 
using “standard” tuning. Note the peak at 50Hz, with a –3dB point of 
approximately 31Hz.

This is a comparison of in-car response curves with and without transfer function 
compensated tuning.

GTi Subwoofer Owners Manual  7/17/98 9:51 AM  Page 4

• At the same power levels Infinite Baffle designs
(no enclosure, sometimes called “Free-Air”) allow
greater cone motion and the power rating of the
subwoofer must be reduced to reflect this (see the
specifications). At the same power levels Infinite
Baffle subwoofer installations will have much longer
cone excursion than subwoofers mounted in an
enclosure. 

• Sealed enclosures exert some contro

l 

over excur-

sion as a result of the air inside the enclosure acting
as a spring against the woofer cone motion. Large
enclosures typically allow higher excursion than
smaller boxes. 

• Vented and Band-pass enclosures have the lowest
excursion as a result of the port tuning effects.
However, vented enclosures will allow for high
excursion outside the port tuning range. Single-
tuned Band-pass enclosures provide the lowest
overall cone excursion.

See the specifications sheet for details on the GTi
subwoofer power handling capabilities in different
enclosure configurations. 

GTi subwoofers are 4 ohms nominal imped-

ance. If you are connecting two speakers in
parallel to an amplifier channel, make sure your
amplifier will drive a two ohm load. 

If your amplifier
will not drive a
two ohm load, we
recommend that
you connect the
two speakers in
series to create
an 8 ohm load as
shown here. 

Four woofers can also be wired (as shown below)
in parallel/series or series/parallel so that the
amplifier will see four ohms nominal impedance.

Please consult your JBL dealer for information on
more complex speaker combinations.

7

The GTi Subwoofer will only sound as good

as the enclosure it is mounted in. Build an enclo-
sure that meets the guidelines listed below and
you will get the high performance bass you sought
when you stepped up to GTi.

1. Box Dimensions - Try to choose subwoofer enclo-
sure dimensions (height, width and depth) which are
not equal or multiples of each other. This will mini-
mize standing waves and resonance in the cabinet.
Example: Ideal - 26"h x 17"w x 10"d; Not ideal: 30"h
x 15"w x 9"d (30 is a multiple of 15).

2. Enclosure Material and Thickness - Use MDF
or particleboard at least 3/4" (19mm) thick. With
high powered amplifiers or subwoofers of 12"
diameter or larger, use 1" (25mm) thick MDF or
particleboard.

3. Joint Construction - All joints should
be glued and screwed (no nails). The
cabinet should be airtight and must
include adequate bracing to minimize
resonance.

4. Fiberglass Application - Apply a 1"
blanket of fiberglass or polyester to
interior walls (except the baffle board)
of all vented enclosures. For sealed
enclosures (no ports) fill the entire volume with
fiberglass or polyester (12-16 oz. per cubic foot). 
If you do not wish to use damping material then
make the box size 10% larger than recommended
by the graphs.

5. Ports - Construct port tubes from PVC pipe or
cardboard tubing with a wall thickness of at least
1/16". The end of the port tube must be kept one
port diameter away from either the inside of the 
box or any surfaces in the car.

If a port tube is not available, you may also 
construct a square/rectangular vent. Changing 
the shape of the vent will not effect the length as
long as the surface area of the opening stays the
same. We’ve supplied a chart that lists some 
standard dimensions for square/rectangular vents.

The power handling of any subwoofer is

related both to its ability to dissipate heat and to
the maximum cone excursion limits.

• Too much electrical power can cause the wire in
the voice coil to overheat and burn out.

• Too much cone motion can cause the cone to
mechanically limit and damage the subwoofer.

• Depending on the subwoofer model, the GTi motor
structure can easily handle an amplifier rated to
deliver from 300 – 600 watts of continuous power.
This is an electrical rating dependent upon an enclo-
sure design which keeps the woofer cone from
reaching its mechanical limits when the amplifier
delivers high power. Properly designed Vented and
Band-pass enclosures are capable of this control.

6

3.14 sq. in.

= 

2"

= 

1.75" x 1.75"

7.07 sq. in.

=

3" 

=

2" x 3.50"

12.57 sq. in.

= 

4" 

=

2" x 6.25"

19.63 sq. in.

= 

5"

=

3" x 6.50"

28.27 sq. in.

=

6" 

=

3" x 9.50"

GTi Subwoofer Owners Manual  7/17/98 9:51 AM  Page 6