A molded polyurethane composite helps
one company make automotive audio enclosures that meet
the acoustic and physical challenges of the road
By Joseph Ogando, Materials Editor
Hopkinsville, OH-Next time a passing
car with a ground-shaking sound system literally rocks
your world, Tim Rusche might be to blame. His company,
Power Composites Inc., designs and manufactures speaker
enclosures for powerful aftermarket audio systems. And
one thing comes through loud and clear in Rusche's approach
to enclosure design: high-end car speakers demand more
than a simple wooden box.
For one thing, automotive enclosures encounter tight
packaging constraints made worse by the fact that the
loudest aftermarket sound systems have long since outgrown
the physical dimensions of speaker openings built into
most cars. "The space available for installing audio
components is really limited and changes from vehicle
to vehicle," Rusche says. "So I have to make all sorts
of odd enclosure shapes." Some of the speaker housings,
for example, must fit within the center console between
the front seats. Other models, including an unusual
horn-like enclosure for bass speakers, install in the
relative spaciousness of the trunk but still have to
match up with six-by-nine-inch speaker openings that
connect the trunk and the passenger compartment. In
all, Power Composites produces dozens of models to keep
up with all the different types of speakers and vehicles.
Regardless of their size or location in the car, all
of the enclosures have to balance the packaging requirements
with acoustical properties that will foster a pleasant
sound as well as optimize the speaker output for a car
battery's 12 volts of power. Rusche's efforts at fine-tuning
the acoustics take two directions. For one, he has tried
out different enclosure geometries-which lead him to
the bass-speaker "horn." And he has searched for new
enclosure materials that will enhance the acoustics.
"Some materials sound better than others," he says,
noting that wood has a pleasing resonant frequency,
while many plastics efficiently transmit high-frequency
sounds for an "unpleasing" acoustic character.
In mobile audio, sounding good is only half the battle.
The enclosures also have to be strong enough to resist
the often bruising treatment inside the car-especially
in the trunk, where temperature extremes and heavy objects
can batter bass speakers. And finally, these aftermarket
enclosures have to appeal to the consumer, so they must
have the right surface finish and heft.
To balance all these requirements, Rusche has developed
a whole new breed of speaker enclosures. Steering clear
of "wooden-box" housings, he instead rotationally molds
one-piece, seamless speaker enclosures from ceramic-filled
polyurethanes. According to Rusche, the polyurethane
composite imparts the right mix of acoustical properties,
strength, and aesthetics, while rotomolding lends itself
to economically making dozens of complex enclosure shapes
at low production volumes.
The right material. In picking a material for
the speaker housings, Rusche first had to think about
the acoustical properties. Guiding him was the belief
that the denser and stiffer the enclosure material,
the better it would transmit sound. At the same time,
the material couldn't be so heavy that it would scare
off consumers used to wood enclosures. "Concrete would
be a good choice, but nobody would buy it," he quips.
The material also had to be tough enough to survive
the trunk treatment.
At first, Rusche considered "cultured marble," which,
despite its name, is a polyester filled with calcium
carbonate. "It's a nice dense material," he says. But
Rusche ultimately rejected it as "too brittle" and "too
heavy" for this automotive application. Instead, he
found that polyurethane-Baytec 140 P from Bayer Corp.
(Pittsburgh)-fit the bill when filled with ceramic microspheres
derived from power plant "fly ash." The resulting polyurethane
composite, "has more impact resistance and weighs less
than cultured marble, though you trade off some rigidity,"
As an entrepreneurial effort, Power Composites lacks
the equipment to measure the acoustic performance of
the speakers or the impact strength of each and every
enclosure design. Instead, Rusche dials in the desired
end-use properties by experimenting with different loadings
of the micro- spheres. "Varying the amount of filler
changes both the acoustics and the strength of the enclosures,"
he explains. Using his ears and those of some musician
friends as a guide, Rusche found that 33% filler by
weight produced the best-sounding enclosures, and observations
of the finished product on the road show that this loading
results in a rugged enclosure, Rusche reports.
As an indication of just how much of the composite
the filler occupies, Rusche notes that a 33% loading
translates to three parts filler to one part urethane.
Such a high filler-to-plastic ratio helps the enclosures
resist temperature extremes. "The filler is not going
to shrink or expand as much as the plastic," Rusche
Turning out the parts. Of all the possible ways
to make plastic speaker enclosures, Rusche chose rotomolding
because it offered him the flexibility to produce dozens
of enclosure geometries quickly and inexpensively even
at low volumes.
Part of this capability springs from the company's
distinctive tooling method. Instead of aluminum tools,
Rusche employs glass-reinforced epoxy tooling, which
not only costs less than metal but can be created from
a wood model in as little as 15 hours-over the course
of 3 days to allow for cure time. With such a brief
tooling lead-time, Rusche can bring out new designs
in a week or less. Also, the fact that the fiberglass
molds run at room temperature and pressure contributes
to a speedy production period by keeping the cycle time
to about 15 minutes.
Rusche, whose audiophile tendencies led him into the
enclosure business in the first place, doesn't disparage
wood as an enclosure material. "We spent a lot of time
trying to match the pleasing acoustic qualities of wood,"
he admits. But, "molding speaker enclosures from a polyurethane
composite gives you the ability to do things differently,"
Rusche sums up. "You're no longer restricted by the
flatness of wood."