Enclosure Construction - JBL MkII automotive Owner's Manual

ENCLOSURE
CONSTRUCTION
Please observe the following suggestions
when building an enclosure for GTi
Series subwoofers.
1. Choose an enclosure design from the
Enclosure Design Sheet included with
your subwoofer.
2. Use at least 3/4" (19mm) MDF (medium
density fiberboard) or marine birch
plywood to build the enclosure.
Enclosures for 12" and larger sub-
woofers and smaller woofers driven
by high-power amplifiers should be
constructed using 1" (25mm) material.
3. Join pieces of wood with glue and
screws; do not use nails. Once the box
has been tested, seal all joints inside
the box with silicone caulk.
4
4. Fill the enclosure with damping
material (Dacron, fiberglass insulation
®
or long-fiber wool) according to the
design you have chosen from the
Enclosure Design Sheet. "0% fill"
indicates that no damping material
should be used; "50% fill" indicates
that all interior walls except the baffle
should be lined with 1"-thick damping
material, and "100% fill" indicates that
the box should be loosely stuffed with
damping material.
5. Use PVC or ABS plastic pipe for ports.
Keep in mind that the openings at
either end of the port must be at least
one port diameter away from any
obstruction.
6. Use the 10/24" machine screws and
T-nuts provided to mount the woofer
to the baffle. See Figure 1.
Figure 1. Mounting the GTi woofer in its
enclosure
10/24"
SCREWS
(PROVIDED)
T-NUTS
(PROVIDED)
DUAL COIL VERSUS
DIFFERENTIAL DRIVE ® *
Conventional dual-voice-coil woofers
use a pair of voice coils "interwound" on
the former and centered in the magnetic
gap. The two coils may be connected in
series or parallel in order to maximize an
amplifier's output power. Both coils drive
the cone forward and rearward. X
max
(one-way-linear) is determined by the
amount of voice coil exposed above and
below the top plate. A conventional
voice coil is made up of several layers of
windings, each transferring heat to the
winding next to it until the heat is finally
dissipated by the outside layer through
the top plate, magnet and backplate. This
arrangement is inefficient, and results in
low thermal power handling. See Figure 2.
* U.S. patent no. 5,748,760 and other patents pending
CONVENTIONAL
Figure 2. Conventional dual-voice-coil
DUAL VOICE COIL DESIGN
design
CONE
BOTH COILS
DRIVE CONE SPIDER
FORWARD
AND
OVERHANG DETERMINES
REARWARD
X
max
TOP PLATE
MAGNET
POLEPIECE / BACKPLATE
JBL GTi Series subwoofers employ
Differential Drive, a technology
developed by JBL Professional.
Differential Drive employs two voice
coils positioned at opposite ends of the
former, each suspended in a separate
magnetic gap. These two coils may be
connected in series or parallel, like a
conventional DVC woofer, to maximize
an amplifier's output power. Both coils
MUST be connected to the amplifier in
correct polarity! At low power, both
voice coils drive the woofer's cone, and
any motor nonlinearities are cancelled
by the out-of-phase coils and gaps. As
power input increases so that one coil
rides completely out of its gap, force is
still applied to the cone by the other coil.