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.