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VENTED LOUDSPEAKER ENCLOSURE CONSTRUCTION AND OPERATION
Vented or "bass-reflex" enclosures require special
construction due to the large forces that can be developed by the
drivers installed inside that act on them. This is particularly
true of large subwoofer enclosures. It is important for cabinet
builders to be aware of construction techniques that are peculiar
to loudspeaker enclosures in order to build an extremely rigid
and secure enclosure that will not detract from the potential of
the drivers installed in it. Some background on how vented
speaker enclosures work will help you understand what
construction requirements are unique to this type of cabinet.
Vented loudspeaker enclosures have two primary functions:
the separation of vibrations from the front and rear of the
loudspeakers, and the containment of air so that the air can act
as a resonating elastic medium inside the enclosure. Vented
enclosure operation is analogous to the way a bottle will behave
as a whistle. You will note when blowing air across the top of a
bottleneck that a certain pitch is generated in the air
resonating inside the bottle. This effect was among the subjects
of a scholarly scientific paper published by German scientist
Hermann Helmholtz in 1859, and has long since come to be known as
the "helmholtz frequency" or the "helmholtz resonator." If you
add water inside the bottle displacing air, (make the inside
volume smaller) the pitch goes up. If you cut off part of the
bottleneck (the duct) the pitch goes up. If you increase the
diameter of the bottleneck the pitch goes up. If you pour out
water or make the neck longer or decrease the neck's diameter,
the pitch goes down. You can thus tune the bottle (enclosure)
higher or lower by adjusting the ratio of vent volume and
enclosure interior volume. The particular pitch obtained depends
on the ratio of the the mass of the air in the enclosure and the
mass of the air in the much smaller vent.
In a tuned system it's important to avoid air leaks, since
the vent produces most of the sound at the frequency of resonance
(helmholtz frequency) and the pressure inside the enclosure can
be substantial. Air leaks in the enclosure's seams or walls can
cause the tuning of the system to shift in frequency, producing
other undesirable effects as well.
In a very large bottle--for example, several cubic feet--
there is space on the wall or on the end of the bottle to install
a loudspeaker. Instead of having to blow air across the duct to
produce resonance, the resonance can be stimulated by excitation
from the loudspeaker within. The duct can also be turned around
and pointed inside the bottle and the bottle's outside surfaces
can be flattened to form a conventional box-shaped loudspeaker
enclosure. This, then, is the typical nature of a vented
loudspeaker enclosure.
The material used for enclosure walls should be solid and
dense and should be free of voids or warps. The ideal speaker
enclosure would have no wall resonances at frequencies that fall
within the frequency range of loudspeakers mounted in it. 25 mm
(1") solid lead plate would make an excellent loudspeaker
enclosure.
19 mm (3/4") Finland or Baltic birch type plywood is
recommended where enclosures will be transported frequently,
while high-density particle board (not chip board) can be used
for permanently installed use. Corners must be strong and air
tight and should not have any air leaks or openings. Glued
joints should be properly filled with glue that will not crack
under high stress or impact. If the integrity of the glue seal
can't be determined, hot glue or RTV caulking should be used to
seal all seams. Bracing made of 2x4's or 75 mm (3") pieces of
the birch ply should be liberally applied either inside or
outside the cabinet, depending on whether the cabinet is to be
permanently installed or portable. The braces should be
liberally glued and screwed down on edge. Edge-wise drilled and
countersunk holes through the braces can be used for #10-2
flathead wood screws to avoid the use of more expensive lag
bolts. The glue on the braces accomplishes all the stiffening
needed so screws may be removed once the glue is dry if there is
any doubt about them coming loose from vibration. If butt-joint
cabinet edges are used, care should be taken to apply cleats
inside the corner edges to pull the edges tight with wood screws,
assuring air-tight corners and edge joints.
Although the sound waves in the subwoofer's frequency range
are very long, typically longer than 4.3 m (14') 1/4-wavelength
increments in interior cabinet dimensions should be the size
limit; in other words, if you will be using an 80 Hz crossover
frequency, let 1.07 m (42") or about a 1/4-wavelength of the 80
Hz sound wave, be the maximum dimension of any single loudspeaker
compartment within your enclosure. If enclosure volumes require
larger sizes, then use an interior dividing wall to separate the
volume into equal smaller compartments. Chances are if your
enclosure is that large, you need the extra enclosure stiffening
this will provide. Once the enclosure has been divided, each
compartment should be treated as an individual enclosure in both
bracing and porting. For example, a 1133 l (40 ft^3) enclosure
designed to house four 2245H subwoofer drivers should be divided
so that two compartments each contain two drivers. Each
compartment is then braced and vented as if it were a separate
566 l (20 ft^3) enclosure.