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SEALED BOX
Sealed box subwoofer enclosures have
advantages that appeal to beginners because of
the simplicity of box design and construction,
and to experts because of the compact enclosure
dimensions, very good power handling, and
outstanding transient response. They are noted
for their tight "non-boomy" sound, and
relatively small size compared to other designs.
The driver will tend to have a fairly low free
air resonance (Fs), long excursion capability (xmax),
and loose suspension (the air loading of the
small box provides restoring force). A bigger
version of the contemporary compact sealed box
is the classic "infinite baffle" which, because
of the increased compliance of the air in the
larger enclosure, requires a stiffer mechanical
suspension to provide loading for the woofer
cone. The larger cabinet volumes for infinite
baffles are not always practical for all
applications. All sealed box designs, large or
small, tend to have lower sensitivity and higher
bass roll off characteristics than other
designs.
VENTED BOX
Vented, ported, ducted, bass reflex; these terms
all describe the same type of enclosure. Vented
designs have been around for quite a while, but
really came on with a bang after acoustic
researchers devised a way to effectively
identify speaker parameters (the mechanical and
acoustical characteristics of the driver in
question) and use those parameters as elements
in mathematical formulas that consistently model
or predict the response of a given driver in a
given enclosure. Simply put, the driver is
matched to what is essentially a tuned resonant
air chamber. As the driver goes lower in
frequency, the driver excursion is reduced and
the air in the vent proportionately increases
its pressure on the air outside of the
enclosure. This "high pressure air" in the port
is just as effective a diaphragm at those
frequencies as an actual hard diaphragm made of
solid material. Talk about getting something for
nothing! The vented box has a lot to offer, with
advantages like flatter response down to the
cutoff frequency or lower limit (although this
is generally higher in frequency than a sealed
box), reduced cone excursion near the box
resonance frequency, and overall higher
efficiency than most sealed box designs. There
are also some significant limitations to this
type of design. The transient response of the
subwoofer is generally not as good as a sealed
box design, but this can be minimized by careful
driver selection and critical box tuning.
Another problem is that there is no acoustic
loading on the driver below the specific tuning
frequency of the enclosure, meaning there is no
control over cone motion beyond the subwoofer's
own mechanical suspension at those frequencies
below the box resonance. This results in
"bottoming out" and the ultimate destruction of
the driver. A simple solution is to use a low
cut or rumble filter before the amplifier to
prevent the subsonic frequencies from affecting
the speaker below cutoff. The overall design of
a vented box can be a bit trickier for the
casual speaker builder, but modern software
programs like BassBox Pro or the technical
support staff at Parts Express can help you to
develop a high performance project.
PASSIVE
RADIATORS
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Passive radiators are an alternative to vented
designs and are in fact part of the same family
of enclosure design. It is the simple
substitution of an actual diaphragm (usually
similar to a speaker without a magnet and voice
coil) for the virtual diaphragm of air in the
vent that we just described in the last section.
Advantages would include the absence of wind
noises from a vent, and the ability to block
higher frequency sound reflected from the back
of the driver and out through the vent. They are
also useful when a particular tuning requires a
vent length longer than what the enclosure can
accommodate. Some disadvantages are inferior
transient response due to the actual mass of the
passive radiator diaphragm, and greater
difficulties in tuning compared to vented
designs.
BANDPASS BOXES
Bandpass boxes are designs that use multiple
chambers to create an acoustical filter that
will focus and increase sound output within a
specific frequency range. The most common of the
many variations is the 4th order bandpass, which
consists of a woofer loaded in a conventional
sealed box enclosure, but the output of the
woofer is directed through a second vented
chamber in front of the driver. The port
increases gain in a fairly narrow range, but
also acts as a low pass filter to help reduce
high frequency output above the bass
frequencies. The actual vent diameter should be
as large as possible or practical, as these
designs are particularly susceptible to vent
noise- the whole output of the woofer is passing
through that opening! Bandpass designs offer
transient response approaching that of a
conventional sealed box, combined with output
that can actually be greater than a vented box.
The downside of this family of boxes is a
tendency toward an unnatural or "one note"
sound, due to the peaky nature of their
operating range.
TRANSMISSION
LINES
The transmission line, or TL, is also known as
an acoustic labyrinth because of the long and
somewhat complicated path that the rear
radiation of the speaker must pass through
before exiting the enclosure. The idea is to use
the long "tunnel" to selectively absorb higher
bass frequencies through the use of damping
material, while tuning the labyrinth length to
equal a quarter wavelength of the resonant
frequency of the driver. The rear pathway
creates a 90 degree shift in the speaker's rear
radiation to constructively combine with the
front radiation, while damping and controlling
cone motion at the driver's resonant peak. They
are many variations on the basic TL concept, but
not as much data to mathematically model or
predict the performance of a particular design
compared to closed box or vented systems. The
concept demands especially careful analysis of
loudspeaker parameters, a fair bit of testing
and tuning, and the space to accommodate a
design that may turn out to be somewhat large
and heavy. As a result, the transmission line
enjoys a bit of a cult following, but when
properly executed the results can be very
impressive.
HORN LOADED
If we looked back through the mists of time, we
would see small power amplifiers and low wattage
speakers requiring a little bit of a lift from
the Laws of Physics to get the kind of sound
pressure level demanded by many playback
environments. The use of horns to boost audio
output certainly became very popular during that
era, and horns still have a place in some
specific modern audio applications. If you have
ever cupped your hands in front of your mouth to
amplify your own voice ("Hey, you kids, cut that
out!") you will have employed basic horn loading
techniques. Horns increase the gain or output of
a source, and also improve its directivity or
directionality. The horn serves as a transformer
that matches the high acoustical impedance or
pressure presented by the driver at the small
throat of the horn to the lower acoustical
impedance seen by the air at the larger mouth of
the horn. Linearity is maintained by a carefully
calculated "flare rate" for the horn path that
ensures that the sound will travel with
consistent expansion. The overall directivity
and low frequency response of the horn are both
determined by the length and mouth area of the
horn. A large low frequency horn can be made
smaller by "folding" it, which can get some of
the larger designs down to a more manageable
shape and size. A particular "K" brand has
certainly become identified with the successful
employment of horn loaded designs in high
quality home stereo systems. Although horns have
many attributes including high efficiency,
excellent transient response, and great
dispersion control, their size and complexity
tends to make them practical for only a very
small minority of home stereo and home theatre
users.
BASS ACTUATORS
The small but mighty bass actuator does not
require an enclosure of any kind, but still has
a place in our general discussion concerning sub
bass performance. A bass actuator is a small
servo device or motor that converts an audio
input signal into a mechanical force that is
placed in direct contact with a solid surface.
Conventional subwoofers transmit information
through air, while a bass actuator delivers low
frequency program directly through solids. The
sensation is very much the same as it would be
if the sub bass had come from a speaker first
and then vibrated the solid surface, so
actuators can be useful in situations where
space is too limited for large speaker
enclosures. The actuators connect to an amp just
like a speaker, but you should be prepared to
use them in multiples because the coverage of
the effect is limited by the nature of the solid
object to which the bass actuator is attached. |
