Polk Audio dX8 - Manual (Page 3)

4 Premium Performance Loudspeakers
dX Series
Premium Performance Loudspeakers 5
dX Series
Who Is Going To Build Your Subwoofer Enclosure?
Since the subwoofer enclosure is so critical to getting the best performance from your
dX subs, you should ask yourself the following questions:
1. Do I really enjoy working with my hands?
2. Do I have good woodworking and mechanical skills?
3. Do I have, or have access to, woodworking and electrical tools?
4. Do I have a solid understanding about musical reproduction in an automobile?
3 If you answered NO to any of the above questions, we recommend you choose
one of these two possible alternatives: First, there are pre-built subwoofer enclosures
on the market from manufacturers like Q-Logic and R/T, or enclosure kits from
BassLine and others. The second alternative is to have your authorized Polk Audio
dealer design and build a woofer box for you.
3 If you answered YES to all of the above, let’s review some basics before you begin.
Building Your Own Enclosure
The dX subwoofers have been optimized to give you flat bass response in small sealed
enclosures. Some listeners may want response other than “flat,” or may want to trade-off
depth of response for greater efficiency by using a vented enclosure. There is no way we
could provide all the information for all of the options here. This manual will give you
enough information to build a great sounding, small sealed enclosure. If you are interested
in a greater choice of enclosure designs, your Polk dX Series dealer has extensive experience
designing woofer boxes and will be more than happy to work with you. If you prefer, you
can call our Customer Service Department from 9AM to 6PM, Monday through Friday,
Eastern Time at (800)377-7655.
Acoustic Suspension Enclosures
As the dX subwoofers are optimized for acoustic suspension
enclosures, we suggest you use this type of design. The
acoustic suspension cabinet is a sealed airtight box, and
is the easiest box to build. It also is a very predictable
enclosure with easily calculated parameters, and it has a
smooth natural sound. Properly built acoustic suspension
cabinets have a flat frequency response that begins rolling
off at 12 dB per octave at the frequencies below its cabinet
resonance. This works very well inside a car because of a
natural phenomenon called “room gain” or “transfer function”
that gives you a 12 dB per octave increase in bass frequencies. You can
roughly calculate at what frequency this gain begins by using the equation F= 565 / L.
F is the frequency at which bass gain begins, and L is the longest dimension of your
“room.” If, for example, you measured the longest dimension of your car as 5.65 ft., the
room gain begins at 565 / 5.65 or 100 Hz. If your goal was perfectly flat frequency response
you would design your cabinet for this particular car to have a resonance frequency of 100
Hz. Since most people want more bass than a flat frequency response yields, tuning the
cabinet at a lower frequency, say 50 Hz, would give you a gain of 12 dB per octave between
100 and 50 Hz and flat response from 50 Hz down. The larger the cabinet, the lower the
resonant frequency, and the lower the efficiency. Two identical systems will sound very
different in a Honda vs. a Cadillac. The bigger the car the lower the frequency at which
room gain begins.
Tools You Will Need To Build Your Enclosure
If you have decided to build your own enclosure for your Polk dX subwoofers,
here is a list of the tools you should have available to you.
❏ Calculator
❏ Assorted Drill bits
❏ Screwdriver bit for drill or manual screwdriver
❏ Circular saw
❏ Tape measure
❏ Drill - electric or cordless
❏ Jigsaw