JBL GTiAutomotive - Manual (Page 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.
• 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.
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.
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
Enclosure Construction Tips
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.
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.
• 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
AREA CIRCLE DIAMETER VENT (h x w)
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"
D
H
W
–
+
–
+
(+)
2 ohms
Nominal
Impedance
(–)
–
+
–
+
(+)
(–)
8 ohms
Nominal
Impedance
Parallel Wiring
Series Wiring
Parallel/Series
Wiring
Series/Parallel
Wiring
4 ohms
Nominal
Impedance
–
+
–
+
(+)
–
+
–
+
(–)
4 ohms
Nominal
Impedance
–
+
–
+
(+)
–
+
–
+
(–)
GTi Subwoofer Owners Manual 7/17/98 9:51 AM Page 6