Loudspeaker loses weight due to polystyrene

Molded-in capability of material enhances acoustics and improves design.

Plastic speakers? Precisely.

JBL, Inc., Northridge, Calif., has developed two professional-quality loudspeakers that offer the acoustic quality and ruggedness of high-density particleboard.

The difference is the enclosure is made of a weather-resistant structural foam polystyrene resin from Mobil Chemical Co., Edison, N.J.

Molded by Xytec, Inc., Tacoma, Wash., the new cabinets are found on JBL’s Control 10 stereo loudspeaker and Control 12 professional stage monitor/loudspeaker. Each weighs 17 lbs.–40 percent less than a conventional particleboard-based cabinet.

The design flexibility of plastic also allowed designers to mold-in specific acoustic features to enhance sound and improve design.

“Structural foam polystyrene reduces cabinet weight significantly while retaining the necessary wall stiffness and load-bearing strength,” says Dan Siefert, senior systems engineer. “We also gain by using a waterproof material that enables the cabinet to withstand the temperature and humidity extremes encountered in indoor and outdoor concerts.”

Another plus is cabinet service life, especially when speakers go “on the road.”

“Particleboard cabinets used in professional concert loudspeakers look old very fast after they go on tour,” says Chuck Willard, director, product planning. “Precolored plastic, coated with a durable, black polyurethane finish, helps hide the dents and scratches without revealing the underlying material. And the high melt viscosity of the resin results in a very smooth, high-quality surface.”

Polystyrene vs. engineering materials

After the initial decision for plastic construction was approved, JBL audio engineers conducted an extensive cost/benefit analysis on a number of materials.

“While ABS and polycarbonate have higher strength ratings, we determined they would have cost 30 percent more without offering any needed properties,” says Willard.

“It wasn’t a contest to build the strongest possible cabinet. For the cabinet to work economically, as well as acoustically, we wanted it to have the best cost/performance profile. We soon realized polystyrene would meet our needs without the expense of engineering materials.”

Wall thicknesses in the trapezoid-shaped cabinet vary from 0.125 in. to 0.025 in. Cabinet dimensions are 24 in. x 17 in. x 12 in.

About 20 ribs, ranging in width from 0.75 in. to 2 in., stiffen the cabinet to limit resonances from sustained low-frequency sounds. Further stiffening is accomplished with a hollow PVC tube as a transverse brace and a lateral brace of hard wood.

A proprietary sodium borosilicate-based coating, Aquaplas [TM], is also applied after molding to dampen high-frequency resonance.

The front panel and enclosure for the L-Pad volume controls are the only parts of the cabinet that require added heat resistance to withstand temperatures of 275 [degrees] F or more. These components are molded from polycarbonate.

Supports molded in the rear of the cabinet accommodate hardware for wall-mounting, and are conservatively rated to handle the 44-lb. weight of the system with a 4:1 safety factor.

Future system enhancements include plans for a rear-mounted integrated amplifier to make the speakers self-powered for simplified use during concerts.

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