textfiles/music/nclosure

     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.