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These enclosures seem to be the
latest rage in the car audio world. It would probably surprise many people
to know that these designs have been around for many years. The first patent
for a bandpass enclosure was filed in 1934 by Andre d'Alton. In the last
ten years, interest has been renewed in these enclosure designs and substantial
strides have been made in defining their behavior. Many home sub/satellite
speaker systems currently use bandpass designs for low-frequency reproduction.
Designs from Bose, KEF, AR, and many others have become very popular in
home audio circles.
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In a bandpass box design, the woofer
no longer plays directly into the listening area. Instead, the entire output
of the subwoofer system is produced through the port or ports. In a conventional
sealed or ported subwoofer system the low-frequency extension is controlled
by the interaction of the speaker and the enclosure design, but the high
frequency response is a result of the speaker's natural frequency response
capability (unless limited by a crossover.) In a bandpass enclosure, the
front of the speaker fires into a chamber which is tuned by a port. This
ported front chamber acts as a low-pass filter which acoustically limits
the high- frequency response of the subwoofer system. The name "bandpass"
is really pretty descriptive in that it refers to the fact that the enclosure
will only allow a certain frequency "band" (range) to "pass" into the listening
environment.
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So what? Couldn't the same thing
be accomplished by placing a low pass crossover on the subwoofer system?
Yes, it could, but a bandpass enclosure can produce significant performance
benefits in terms of efficiency and/or deep bass extension that would not
be possible in conventional designs of equal size.
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By adjusting the volumes of the
front and rear chambers and the tuning of the port or ports, significant
performance trade-offs can be created. When box parameters are adjusted
for a narrower bandwidth, the efficiency of the subwoofer system within
that bandwidth increases and can reach gains of up to 8dB (sometimes even
higher.) As box parameters are adjusted for wider bandwidths, very impressive
low-frequency extension can be produced from extremely compact enclosures
at the expense of efficiency and good transient response. Intermediate
bandwidths can also be designed which create a compromise between all these
characteristics. As if that is not confusing enough, within each bandwidth
range, the designer can also manipulate box parameters to shift the range
of operation up or down the sub-bass range which also has an effect on
efficiency.
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As you can see, bandpass enclosures
can have very different sound characteristics based on the designer's choice
of box parameters. As such, it is not always possible to make blanket statements
as to the performance benefits and drawbacks of bandpass enclosures in
general.
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One characteristic of bandpass enclosures
which is universal is that they exert greater control over cone motion
over a wider frequency band than conventional designs. Due to controlled,
rapidly changing air pressure on either side of the woofer, the woofer
is capable of producing high levels of acoustic output without physically
moving very much. This means that the woofer is less likely to encounter
excursion limits in the main part of the sub-bass range. However, just
because the cone isn't moving as much doesn't mean that the speaker's motor
assembly isn't still trying to drive the cone hard; it just means that
the speaker cone is encountering resistance to motion. This resistance
can be very hard on speakers, especially when crazy car audiophiles are
at the controls. The conflict between the force generated by the motor
assembly and the air pressure in the enclosure can impose extreme stress
on the glue joints and suspensions of the woofers. You can literally tear
a speaker apart in a bandpass enclosure if you apply too much power. Because
the speaker is not moving as much and because noises are masked by the
front chamber, it is also very difficult to hear when a woofer is in serious
trouble. Many people have been known to crank bandpass enclosures up and
blow the speaker to bits within a few minutes because they did not realize
that the speaker was having a heart attack. Choosing the right amount of
power and carefully setting amplifier gains is very important in order
to ensure long- term reliability.
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Bandpass enclosures can be divided
into two basic types: single- reflex and dual-reflex. In a single-reflex
design, the rear chamber is sealed and the front chamber is ported. In
a dual-reflex design, both front and rear chambers are ported into the
listening area. A variation of the dual-reflex and single-reflex, known
as "series-tuned," has a port which connects the rear and front chambers.
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The differences between single-reflex
and dual-reflex bandpasses are similar to the differences between sealed
and ported enclosures. A single-reflex typically exhibits a shallower low-frequency
roll- off rate (approximately12dB/octave) and better transient response.
A dual-reflex is more efficient and controls cone-motion over a wider range
but typically has a sharper (18-24dB/octave) low- frequency roll-off. Because
of the difference in low-frequency roll- off rates, a dual-reflex usually
has to be larger in size to produce the same low-frequency extension as
a single-reflex design.
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As compared to more conventional
enclosure designs, bandpass enclosures are very complex to design and build.
The rules governing the performance of bandpass enclosures leave no room
for error. Slight volume miscalculations or sloppy construction can turn
a good design into a poor-performing box. Integrating the proper size port
or ports can be extremely challenging and often renders designs that looked
great on paper completely impractical. The design of these boxes should
definitely be left to people with extensive enclosure- building experience.
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BandPass
Enclosures
