Polk Audio EX Series II Subwoofer EX652a - Manual (Page 4)

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together and cancel themselves out, making no bass at all. If you plan on building an
infinite baffle system you must plan on sealing all of the air passages between the trunk and
the passenger compartment (not an easy job). Also the metal used on the rear decks of cars
does not make for a very good baffle board; the metal will tend to resonate and again cancel
out the bass frequencies. You should plan on mounting your woofers on a non resonant
wooden baffle. A properly designed infinite baffle enclosure will roll off at 6 dB per octave
below cabinet (speaker) resonance, and will have a very smooth frequency response. The
infinite baffle is the least efficient box design.
TUNED PORT ENCLOSURES
The tuned port enclosure is similar to the acoustic suspension box
except that it uses a port to take the back wave from the
speaker and “tune” it to reinforce the front wave. A
system becomes more efficient when the
frequencies of the port and the front wave are in
phase. It is absolutely critical that the dimensions
of the box and the port are perfect, or the back and
front waves will cancel each other out, reducing
your bass output. Tuned port enclosures tend to be
peaky in the bass, and many people (particularly
Bassheads) like the punchiness of the sound. Tuned port
enclosures roll off at a very steep 18 dB per octave below cabinet
resonance, which means that the room gain of the car will not help to extend the lowest of
frequencies. In addition, the woofer becomes “unloaded” below the tuning frequency of the
port and very loud musical information below these frequencies can damage a woofer.
BAND-PASS ENCLOSURES
Band-Pass boxes can be built in a near infinite amount of
configurations. They incorporate at least two
individual chambers and are by far the most
complicated box to model and build. The theory
behind band-pass boxes is to limit the frequencies
produced by the subwoofer, thereby making it more
efficient in the frequencies that it produces.
A by-product of this efficiency is that it is often
possible to build a band-pass box much smaller
than its equivalent acoustic suspension box and get
the same quantity of acoustic output. If you have a very
small car with limited room for a subwoofer box, a band-pass box
may be a good choice. The downsides of the band pass box are numerous. First, the
construction must be flawless, any airleaks or rattles will seriously degrade performance.
FIGURE 4.
FIGURE 3.
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ACOUSTIC SUSPENSION ENCLOSURES
The acoustic suspension cabinet is a sealed airtight box,
and is the simplest, easiest box to build. It is a very
predictable enclosure with easily calculated
parameters and a smooth natural sound. Properly
built acoustic suspension cabinets have a
reasonably 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 in the
interior 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.
INFINITE BAFFLE
Also known as free air enclosures, this essentially means mounting
your woofers in the rear deck of your car, and not building a box
around the speakers. At first glance, this would seem to be a very easy
way to install your new speakers, but many problems can occur if this
is not done exactly right. In the acoustic suspension design described
above, the box is providing two major functions: first, the air that is
sealed within the box acts as a spring to help support the suspension of
the speaker. As the woofer cone moves in towards the box, the air in the
box pressurizes; and as the cone moves out a vacuum forms. This helps to
keep the cone from traveling too far and destroying itself. An Infinite Baffle design
has no box; therefore, it has no trapped air behind the speaker to act as a spring. If your
speakers are not specifically designed for infinite baffle use they can be damaged. Your new
EX Series II woofers are designed for use in enclosures and are not recommended for
infinite baffle use. The second thing that a sealed box provides is sound wave isolation
created from the front of the speaker cone due to the waves that are created from the back of
the speaker cone. If the two wave fronts are not isolated from each other they will mix
FIGURE 1.
FIGURE 2.