104 lines
5.6 KiB
Plaintext
104 lines
5.6 KiB
Plaintext
|
||
|
||
|
||
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.
|
||
|
||
|
||
|