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
resonance 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.
