JBL Speaker GTiAutomotive User Manual

GTi Automotive  
Intercooled Subwoofer  
Application and Installation Guide  
 
Staple or clip your original bill of sale here  
Table of Contents  
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
 
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.  
Welcome to Club GTi  
Enclosure Basics  
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-  
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.  
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.  
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.  
Autosound at its Best  
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.  
Unleash Maximum Performance  
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.  
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.  
The exceptional sound of the GTi Subwoofer  
comes from an Aquaplas coated fiber cone for  
extra stiffness and resonance elimination. The  
3
 
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.  
Normalized Amplitude Response (dB/Hz)  
Vehicle Transfer Function  
JBL SpeakerShop 1.0  
Vehicle Acoustics  
dB  
6
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  
In Car Response Curve  
0
–6  
–12  
–18  
–24  
–30  
–36  
Enclosure-only response  
Transfer Function  
Compensated Tuning (33Hz)  
5 Hz  
10  
50  
100  
500  
1000  
2000  
Frequency  
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.  
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-  
Normalized Amplitude Response (dB/Hz)  
JBL SpeakerShop 1.0  
Normalized Amplitude Response (dB/Hz)  
JBL SpeakerShop 1.0  
dB  
6
In Car Response with Transfer  
Function Compensated Tuning  
dB  
Vehicle Transfer Function  
6
0
0
–6  
In Car Response Curve  
–6  
–12  
–18  
–24  
–30  
–36  
–12  
–18  
–24  
–30  
–36  
In Car Response with  
Standard Tuning  
Enclosure-only response with  
Standard “Optimum” Tuning (42Hz)  
5 Hz  
10  
50  
100  
Frequency  
500  
1000  
2000  
5 Hz  
10  
50  
100  
500  
1000  
2000  
Frequency  
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.  
4
 
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).  
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.  
Enclosure Design  
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.  
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.  
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.  
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:  
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.  
USEFUL FORMULAS  
1 cubic foot = 1728 cubic inches  
1 cubic foot = 28.32 liters  
1 liter = 0.353 cubic feet = 61 cubic  
inches = 1000cm3  
Radius = 1/2 diameter  
Box volume = height x width x depth  
Volume of triangular box section = 1/2 base x  
height x length  
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.  
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!  
Port Dimension Calculation  
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.  
Volume of tubular duct = area of circle x length  
pi (π) = 3.1416  
Lay out plans for your box using the information  
determined in the above steps and the box con-  
struction tips listed on page 6.  
Area of circle = pi (π) x radius2  
Diameter of circle = 2 x area / pi (π)  
Area of square = height x width  
5
 
AREA  
CIRCLE DIAMETER  
VENT (h x w)  
Enclosure Construction Tips  
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.  
3.14 sq. in.  
7.07 sq. in.  
12.57 sq. in.  
19.63 sq. in.  
28.27 sq. in.  
=
=
=
=
=
2"  
3"  
4"  
5"  
6"  
=
=
=
=
=
1.75" x 1.75"  
2" x 3.50"  
2" x 6.25"  
3" x 6.50"  
3" x 9.50"  
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.  
4. Fiberglass Application - Apply a 1"  
blanket of fiberglass or polyester to  
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).  
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.  
Points on Power Handling  
The power handling of any subwoofer is  
related both to its ability to dissipate heat and to  
the maximum cone excursion limits.  
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.  
Too much electrical power can cause the wire in  
the voice coil to overheat and burn out.  
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.  
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.  
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.  
H
W
D
6
 
• 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.  
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.  
Speaker Connection  
and Combination  
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.  
Parallel/Series  
Wiring  
Series/Parallel  
Wiring  
(+)  
(+)  
Parallel Wiring  
(+)  
• Sealed enclosures exert some contro over excur-  
l
+
+
+
+
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.  
2 ohms  
Nominal  
Impedance  
+
+
4 ohms  
Nominal  
4 ohms  
Nominal  
Impedance  
Impedance  
(–)  
• 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.  
Series Wiring  
+
+
+
+
(+)  
If your amplifier  
will not drive a  
two ohm load, we  
recommend that  
you connect the  
two speakers in Impedance  
series to create  
an 8 ohm load as  
shown here.  
+
(–)  
(–)  
See the specifications sheet for details on the GTi  
subwoofer power handling capabilities in different  
enclosure configurations.  
8 ohms  
Nominal  
Please consult your JBL dealer for information on  
more complex speaker combinations.  
+
(–)  
7
 
A Harman International Company  
NMGP/N:323967-001  
9/95  
 

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