JBL GT1200 - Manual (Page 4)

(i.e., entire box is stuffed with
loosely-packed polyester fiberfill).
4. Use PVC or ABS 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 obstructions, including
filling material inside or outside
the box. Rectangular vents can
be used as long as the cross-
section area matches the
recommended port-area values
in the enclosed data sheet.
2. Seal all joints with glue and
screws; do not use nails. We
recommend “deck” or “zip” screws
since they have coarse threads
for better grip and don’t require
pre-drilling holes. Once the box
has been tested, seal all interior
joints with silicone caulk.
3. Depending on the application,
fill the enclosure according to the
design you have chosen from the
enclosed data sheet in one of three
ways: zero-percent fill (i.e., no
fill), or 50-percent fill (i.e., 1"-
thick polyfill sheets on all inside
walls except where subwoofer is
mounted), or 100-percent fill
Sealed enclosures exert the most
control over the motion of a sub-
woofer because the air acts like
a spring against the motion of
the woofer cone. Larger boxes
allow for more excursion (cone
movement), thus more low-
frequency output for the amount
of power used. When placed in a
sealed box larger than the com-
pliance (Vas) of the subwoofer, it
will perform as if it were in an
infinite baffle installation.
Vented and bandpass enclosures
have the lowest amount of excur-
sion for the amount of sound out-
put. This is a result of port tuning
reinforcing the sound output.
Vented boxes will not provide
adequate woofer control when dri-
ven below the port tuning range,
so proper design is important. A
fourth-order bandpass box will
have the lowest overall cone
excursion at the expense of limited
bandwidth.
In addition, there are a number of
points you’ll want to keep in mind
as you construct an enclosure:
1. Use 3/4" (19mm) MDF (medium-
density fiberboard) or marine
birch plywood to build an enclosure.
Enclosures for 12" and larger
subwoofers, or small subwoofers
driven by high-power amplifiers,
should be constructed using 1"
(25mm) material.
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GTSubwoofers Owner's Manual 7/14/98 2:19 PM Page 4