•
VENTED BOX:
Also known as a ported or bass reflex enclosure, this type of box is almost as easy
to construct as a sealed enclosure. This design however is more efficient and results in an increase
of bass output at certain frequencies. Although vented enclosure has an output advantage at some
frequencies, it also has a steeper roll off, which will only affect bass frequencies below port tun-
ing. Air inside the port will move in phase with the subwoofer reinforcing frequencies above the F3
(-3dB point). The SPL advantage of the vented enclosure comes from the fact that the effect of the
vent tuning reduces the excursion of the subwoofer near the tuning frequency allowing more SPL
to be produced before the subwoofer bottoms. Damping material should also be used to line the
internal walls to reduce any sonic reflection. Because of the characteristics of a vented enclosure,
subwoofers typically will become unloaded (act as if it is not in a box) at frequencies of ½ octave
below the enclosure tuning frequency. Using a high pass filter / subsonic filter will help prevent the
subwoofer from unloading or exceeding its excursion capabilities. It is important to stay within the
recommended enclosure designs.
(PowerBass recommends port placement to coincide on
the same panel or baffle that the subwoofer resides on.)
•
BANDPASS ENCLOSURE:
The most common band-pass enclosure is a combination of a sealed and
a vented enclosure working together (4th order or single reflex band-pass system). The rear of the
speaker goes into the sealed chamber while the front of the speaker goes into the vented chamber.
Because the vent produces all the sound generated from this type of enclosure, sound quality may
not be as good as a well-designed ported or sealed design. Often the sacrifice for sound quality and
subwoofer cone control is greater SPL output. However unlike a sealed or vented enclosure, you can
effectively increase the enclosure sensitivity with the appropriate tuning choices. You can choose
to design a narrow frequency band with very high sensitivity or choose a wide frequency band with
the sacrifice of lower sensitivity. The damping effect of the vented chamber reduces the motion of
the cone so that potentially higher sound output is achieved without the subwoofer reaching its
mechanical limits. Minor deviations from design recommendations however may result in frequency
response and distortion problems.
(If you are intending to use multiple subwoofers in your system, PowerBass recommends hav-
ing separate chambers for each subwoofer if the subwoofers will be operated by two different
amplified signals. This will prevent any possible cancellation.)
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