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Sub-30Hz Behavior
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Sealed box designs and single-reflex
bandpasses are much better at controlling excursion at extremely low-frequencies
(below 30Hz.) For this reason, they can usually handle more power in these
frequency ranges than ported designs and dual-reflex bandpass designs which
makes them less prone to low-frequency induced speaker damage. At frequencies
below the tuning frequency of the port, a woofer in a ported box (or a
dual-reflex bandpass) starts to de-couple. This means that the controlling
function of the enclosure begins to disappear. The collapse is gradual
rather than immediate, but at some point below the tuning of the port,
the speaker behaves as if it were operating without an enclosure and suffers
from potentially damaging over-excursion. (This is why it is a good practice
to use a sub-sonic filter when running a ported enclosure or a dual-reflex
bandpass. Some high-quality electronic crossovers like the AudioControl
4XS incorporate a programmable subsonic filter circuit.)
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Related to the loss of enclosure
damping, ported and dual-reflex bandpass designs also exhibit higher distortion
levels at very low frequencies than sealed or single-reflex bandpass designs.
The importance of this is questionable, however, since little program material
extends to below 30Hz.
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Sealed enclosures and single-reflex
bandpass designs have a rather shallow low-frequency roll-off rate of around
12dB/octave, whereas ported enclosures and dual-reflex bandpasses typically
exhibit 18- 24dB/octave roll-off. For this reason, sealed enclosures and
single-reflex bandpass boxes can have much higher -3dB points (the frequency
at which the output dips 3dB below the reference efficiency of the speaker)
than ported designs while still producing very good ultra-low frequency
output.
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30-80Hz Behavior
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This is the frequency range that
is most important in that it encompasses the vast majority of low-frequency
information present in music. Serious audiophiles assign much more importance
to good performance in this range than in the extreme low-frequency range.
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At moderate power levels all of
these enclosure types exhibit pretty decent manners. The ported box and
the bandpass designs produce less distortion than the sealed box, but the
difference is marginal.
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At higher power levels things change
considerably. The dual-reflex bandpass, due to the fact that its ports
control cone motion over a wider range of frequencies, produces the least
distortion and exhibits the best power-handling characteristics. The ported
enclosure and the single-reflex bandpass also do a very good job producing
high-levels of undistorted bass output, again due to reduced cone motion
in this frequency range. Bringing up the rear in this category is the sealed
enclosure, which produces higher levels of distortion at high power levels.
There is a common misconception that ported designs produce more distortion
than sealed boxes. As you can see this is not entirely accurate; it depends
on the frequency and the power level.
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Transient Response
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Transient response refers to the
ability of the subwoofer system to reproduce quick changes (transients)
in the program material accurately. This is often interpreted as "tightness"
or "looseness" which is maybe a dangerous terminology since many people
are more influenced by tonal characteristics when asked to qualify the
"tightness" of the bass. Transient response is actually a function of accuracy
in relation to time rather than frequency. In music, sounds like drum strikes
and quick bass guitar pulses are good tests of a subwoofer system's transient
performance. A system with good transient response will reproduce these
sounds with clear, "tight" definition. A system with poor transient response
tends to blur these sounds over time, due to the speaker's inability to
stop and start quickly enough to react to the signal accurately.
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It is generally accepted that an
optimized sealed enclosure exhibits the best transient response characteristics.
The control provided by the air-spring in a good sealed system contribute
to generally outstanding transient behavior (at very high power levels,
the increased distortion can overshadow this advantage, however.)
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A ported enclosure can also achieve
good transient behavior but never as good as an optimized sealed enclosure.
It is possible, however, for a well-designed ported enclosure to have better
transient response characteristics than sealed enclosures with higher Qtc's
(above 1.0.) The specific alignment of the sealed and ported enclosures
plays a huge role in determining the transient characteristics of each
individual subwoofer system.
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Single-Reflex bandpass designs can
also have good transient characteristics if their bandwidth is fairly narrow,
but again, not as good as an optimized sealed enclosure. As the bandwidth
becomes wider, their transient response can degrade considerably.
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Dual-Reflex designs generally exhibit
inferior transient response characteristics when compared to the other
designs. As with single-reflex designs, narrower bandwidths produce better
transient performance than wider ones.
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Efficiency
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The term "efficiency" refers to
the ability of a speaker system to convert electrical energy (power from
the amplifier) into acoustic output. Consequently, it also serves as an
indication of which system will produce the loudest possible output given
the same size amplifier (assuming they can all handle the power.) For the
purposes of this comparison, we are looking at efficiency in the 40-80Hz
octave.
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Generally speaking, the most efficient
enclosures are the two narrow-bandwidth bandpass designs with the dual-reflex
version having a slight edge. Next in line, the wide-bandwidth dual-reflex
and the ported enclosure exhibit very good efficiency as well. The sealed
enclosure and the wide-bandwidth single-reflex bandpass are the least efficient
designs.
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Midbass transition
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For sub-bass to sound natural, the
system must have good midbass capability as well. These two are interrelated
because harmonic components of the sounds produced by instruments that
play in the sub-bass range must be accurately reproduced in the mid-bass
range for a system to sound accurate.
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In car audio, we normally don't
have the luxury of using very large drivers to reproduce midbass. For this
reason, the ability of a subwoofer system to smoothly transition to the
mid-bass region becomes very important to achieving top-notch fidelity.
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The sealed and ported enclosures,
because the speakers play directly into the listening environment usually
produce the smoothest midbass transition. Wide bandwidth bandpass designs
are a little more ragged, but still deliver good midbass reinforcement.
The narrow bandwidth bandpass designs can create serious problems because
their high-frequency roll-off can begin as low as 75-80 Hz and the amplitude
of their response peaks is very high, which necessitates the use of larger,
very capable midbass speakers in order to blend smoothly with the sub-bass.
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There Is No Free Lunch
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As you can see by the comparison,
no enclosure design is superior in all respects. They all have advantages
and disadvantages. Analyzing the characteristics of each enclosure type
will help you decide which enclosure type is right for your application.
An informed decision involves an analysis of the following factors: the
space that you want to make available in your car for the enclosure, your
performance expectations (loudness, tonal qualities, etc,) the amount of
amplifier power you will be using, and of course, your budget. Top-notch
car audio specialists will weigh all the factors and consider all enclosure
types before recommending a subwoofer system. Many will even show you specific
data to support their suggestions.
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Remember that the information presented
here assumes that each enclosure type has been properly designed and executed.
This means that the speaker and the enclosure are carefully matched as
a system. The skills of a competent designer, installer and cabinet builder
are every bit as important to the end result as the design of the box or
the type of woofers that you buy.
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Despite the very general scope of
this piece, we hope it leaves you with a better understanding of subwoofer
systems. At the very least, we hope that the next time you hear someone
say "whatever you do, don't port the box" or "sealed boxes don't sound
good," you will ask them to thoroughly explain their position. It could
be amusing.
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