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12.21 What is the best material to make speaker boxes out of? Why?




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This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.

12.21 What is the best material to make speaker boxes out of? Why?

An ideal speaker cabinet material would be very stiff, so that
it would not tend to move with variations in box air pressure.
It would also be very well damped, so that if it ever does
deflect from air pressure, it will come back to the original
position without resonating. It would also have a very high
resonant frequency (supersonic), so that low frequency box air
pressure would not cause it to resonate. An attractive material
is preferred, and additional credit is given for a material
which is easy to cut, glue, and finish. A great material would
be cheap, too. Finally, it would be nice if the material were
light, because we all have to move our speakers sometimes,
and it's hard to appreciate good speakers with a sore back.

With all of those attributes, it would seem that no
material is perfect. However, there are many materials that
have enough of the above good attributes to make excellent
speaker cabinets. Yet each has advantages and disadvantages.

In the list of good speaker box materials below, letters are
used to indicate which attributes the material possesses.

S = Stiff
D = Damped
H = High Resonance
A = Attractive
M = Machinable
C = Cheap
L = Light

MEDIUM DENSITY FIBERBOARD (MDF): SDMC This is the most practical
material for quality speakers. It is harder to find than plywood,
but most lumber yards can special order it. It cuts very nicely
and has a smooth surface. It takes veneer very well. However,
bring a helper when you pick the stuff up. One sheet is very
heavy. MDF is harder on tools than common wood, but easier than
particle board. This is the material that many great speaker
makers use. US $45 for a 4'x8'x1" sheet. Density: 50 lbs/cu ft.

POLYCARBONATE (LEXAN): DM A clear or solid-color polycarbonate
box can look strikingly good. However, this is not a cheap
material. To locate it, look in the classified directory under
PLASTICS. US $400 for a 4'x8'x0.5" sheet. Density: 75 lbs/cu ft.
Acrylic (Plexiglass) is cheaper than Polycarbonate, but weaker
and poorer damped (not recommended).

CORIAN (tm), FOUNTAINHEAD (tm), AVONITE (tm), SURELL (tm),
GIBRALTAR (tm): SDA Regardless of the brand, these synthetic
countertop materials come in a wide array of colors and look
beautiful. They are hard to buy, and different to work with.
They take special glue to bond and require wet sanding with
very fine paper to finish. You can tap it, but it's too brittle
for wood screws. Helicoil inserts are very effective. Yet an
experienced builder can complete a cabinet in under an hour,
from raw material to final finish. Corian is acrylic mixed with
powdered aluminum trihydrate clay filler. Avonite, Gibraltar,
and Surell are polyester resin mixed with filler. One user
commented that Corian is easier to use and is easier to make
invisible seams than the other synthetics. It has been said
that Corian is actually easier to use than wood, but that
depends on your equipment and experience level. Estimated cost
for Corian is US $20 per 1'x1'x0.5". Density: 100 lbs/cu ft.
Available from:

Art Specialties
74 North Aurora St
Lancaster, NY 14086
800-724-4008

Ask for their free information pack on working with Corian.
Note: These product names are registered trade marks and apply
to specific materials from specific manufacturers.

MARBLE: SDHA One challenge with marble speaker enclosures is
cutting holes for the drivers. A carbide bit on a router will
work, but it will dull quickly. Marble is also difficult to glue,
so bracing is difficult. But it sure is pretty when you're done!
US $25 to $45 per 1'x1'x1.25". Density: 160 lbs/cu ft.

PLYWOOD SHEETS SPACED AND FILLED WITH SAND OR LEAD SHOT: SDAMC
If you have time on your hands and want a great impractical box,
try this. Make a simple box out of common plywood. Then glue
cleats on the outside of the box to space the outside plywood
from the common plywood. Glue hardwood-veneered plywood to the
cleats and pour sand or lead shot into the spaces between the
cleats. It won't be light, but with the filler, it will be
extremely well damped. In addition, if you use strong cleats
and glue well, the box will be extremely stiff. One person used
different size Sonotubes as an alternative to plywood, and
filled the space between them with sand. Be sure to sterilize
the sand in your oven before putting it in the box.

ALUMINUM SHEETS SPACED AND FILLED WITH ALUMINUM HONEYCOMB
(Aerolam): SDHL Airplanes use this material for flooring. Next
time a plane crashes in your neighborhood, see if you can get
the wreckage for your next speaker project. You can't get a
better, light-weight material. Celestion has exploited this for
some great products. If you're really ambitious, you can make
your own sandwich out of high-quality plywood faces and a thick
honeycomb core. You will probably need an epoxy to glue the
honeycomb to the plywood. A home-brew sandwich is easier to cut
and glue than Aerolam.

FORMED CONCRETE: SDHC There are tricks to working concrete, such
as to cast braces, rebar, and steel-wire right into the mix.
Also, some concrete is better damped than other. Remember to oil
your concrete forms so that they can be removed. Most concrete
speakers use an MDF front panel, but you can pour one if you use
cardboard tubes or plywood rings to mold the concrete into the
shape of a speaker cutout. Alternately, you can make a common
veneered plywood speaker box and cast concrete inside it for
stiffening.

Any box can be improved by making the walls thicker, by bracing
the walls, and by stiffening the walls. The stiffness of a
material goes up as the cube of the thickness, so a slightly
thicker material is much stiffer. A thicker panel will also have
a higher resonant frequency because the stiffness goes up faster
than the mass.

Consider lining the inside of your speaker with ceramic tile,
attached with thinset mortar. You can get tile remnants cheaply.
They are easy to apply and can be added as an afterthought to
an imperfect box. However, be sure to attach all braces before
tiling, because it is hard to attach anything to tile.

Also consider bracing any weak parts of the box. For example,
all joints will benefit from a wooden cleat. The back of the
box will benefit from stiffeners where the speaker terminals
are attached. Most importantly, brace the front panel, or
make it out of a double thickness of material.

 

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previous page: 12.20 What is a Sealed, Ported, Bass Reflex, Acoustic Suspension, Bandpass, and Coupled Cavity Speaker? Which is better?
  
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next page: 12.22 What size fuse or circuit breaker should I put in my speaker to protect it from damage?