CABINET SHAPES
Modern analysis seems to suggest that special
cabinet dimension ratios do not have as much of
an effect on performance as what was once
assumed. Standing waves are caused by the
reflection of frequency wavelengths that happen
to be the same size as the distance between
parallel surfaces, or surfaces that are at right
angles to the first surface and of the same
size. This effect can be minimized by using
enclosure designs with non-parallel panels, or
using proportions that can reduce the
opportunities for the offending frequencies to
reinforce themselves. The generally accepted
"golden ratio" for these proportions is said to
be 2.6/1.6/1, but the use of this ratio is not
critically important when designing and building
subwoofers. An airtight box, well braced, and
adequately stuffed with damping material, will
be as important as the shape of the enclosure. A
free standing subwoofer enclosure must by
necessity complement the environment in which it
is used, so end table or coffee table shapes are
common. Even built in subs face constraints
imposed on them by the structure into which they
are built.
CABINET
MATERIALS AND CONSTRUCTION TECHNIQUES
The material of choice for subwoofer enclosures
is MDF or medium density fiberboard. It has
greater mass and density than particle board,
which comes in second due to the use of larger
particles or flakes of wood in its composition
and lower glue content. Void free birch plywood
is more appropriate for portable speakers
because it is stronger and lighter than
composition products, but unfortunately is not
as dense. An airtight enclosure with well braced
and damped panels will ensure that more sound
energy reaches the listener instead of simply
vibrating the cabinet walls or pushing the air
out of small gaps. The use of a silicone or
latex caulk should be used for all cabinet
joints and even input terminals, no matter how
solid things seem to be. Speaker sealing caulk
or foam tape are convenient products that can
ensure that the driver's mounting will not
contribute to leaks. Special care should also be
given to the grill cloth or screen that many
users install to prevent the exposure of the
driver to dust or foreign objects (or the
probing fingers of inquisitive small children).
The excursion limits of the subwoofer can be
quite considerable and could exceed the
clearances provided by some methods of
protection, resulting in buzzes, rattles, or
even damage to the driver.
The finishing of the enclosure is as
important to some users as the actual acoustical
performance of the subwoofer system. Popular
finishes include genuine wood veneer, simulated
wood laminates, paint, carpet, even spray on
truck bed liner. Each has its own advantages and
disadvantages, but it is probably one of the
most subjective decisions that will be made
during the whole subwoofer project. The one that
you like the most or feel most comfortable
applying is the best one for your project, as
exterior finish will have little noticeable
effect on subwoofer system performance.
What will have a serious effect on performance
is stuffing. Stuffing, or filling, is used
primarily to absorb or minimize standing wave
reflections within the box. The proper use of
stuffing can also affect the virtual box size
that the subwoofer "sees" and can provide an
increase in effective volume of 25% or more.
This can be useful for applications where the
enclosure size is marginally smaller than
optimum, or to make up for internal volume
losses from driver or brace displacements.
Fiberglass, Dacron, Acousta-Stuf, acoustic foam,
and long fiber wool are all popular and readily
available acoustic damping materials. Be sure to
identify the basic stuffing requirements or
recommendations for your chosen project, but
consider those to be a suggested starting point.
Experimentation can lead to an amount of filling
somewhat greater or smaller than originally
specified, but can result in performance that
satisfies the user's personal requirements. |
