04/03/2000 Design News

Sound system

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," Rusche says.

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 says.

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."