Sound Solution

Sound Solution This article appeared in Isopolyester News, June 1998

Sound Solution Found in Isopolyester Composite

The fiber-reinforced polymer composites industry attracts highly creative people who dramatize the versatility of composites. William Kreysler is such an person. The acoustic panels he creates for high-profile symphonic locations exemplify his inventive genius with isopolyester composites and allows music lovers to discern a score's subtle tonalities that great composers infuse into their scores.

His company, William Kreysler & Associates, Inc., Penngrove, California, combined isopolyester composite with a professionally planned design to enhance the acoustics at the Ravinia Festival Pavilion, Highland Park, Illinois. The Kreysler-made panels from the shell of the Pavilion, an outdoor stage for the performing arts.

Ravinia Festival Pavilion The Ravinia Festival engages the Chicago Symphony to perform at the site during the summer. The season also includes pop concerts and dance productions. With one side of the four-sided stage open, the acoustic panels must maintain their dimensional stability while being exposed to UV light, humidity and passing thunderstorms.

"Bill Kreysler is as clever as they come," says Pat Sanders, Director of the Ravinia Festival. "He devised a way to deliver the acoustic performance we needed at a lower cost. The panels were made at one-third to one-fourth the cost that others quoted."

The Ravinia panels are designed by world-renown acoustician Daniel Commins of Commins Acoustics Workshop of Paris, France, and New York City, USA. For European opera house acoustic systems, Commins first designed a composite panel of fiberglass/epoxy over a PVC foam core.

Isopolyester Helps Lower Cost
For the Ravinia project, Kreysler found a PVC foam core of equal performance but at a lower price. In another change, Kreysler replaced epoxy with isopolyester from both Ashland Chemical Company's Composites Polymer Division, Dublin, Ohio, and McWhorter Technologies, Carpentersville, Illinois. To achieve desired physical and mechanical properties, resin manufacturers formulate isopolyesters with purified isophthalic acid.

"Isopolyesters are much easier to process and handle than epoxies," says Kreysler. "Isopolyesters also have better UV resistance and more cost-effectively meet requirements for moisture resistance. Tests by Riverbank Laboratories of the Illinois Institute of Technology show the replacement materials produce a panel with better qualities than a panel made to the original specification. As a result, the isopolyester version of the panels is being used for new European projects."

Compared to epoxies, isopolyesters generally produce more favorable fire and smoke ratings. Individually, the resin, glass, and foam met the standards of the local fire code. "However," Sanders points out, "when the materials were combined, the finished panel failed to meet the code. We started to wonder if we were going to have to install a sprinkler system at an outdoor facility."

The problem was solved by coating the panels with a flame-resistant intumescent paint. It marked the first commercial use of the paint originally developed by the U.S. National Aeronautics and Space Administration to protect structures exposed to high heat. With a consistency of latex house paint, the material was easily pigmented. Scenic artists used the different colors to produce a mottled marble effect on the isopolyester composite surface.

Besides using less costly materials, the project saves money with an ingenious tooling idea. "The exact shapes of the panels are computer-driven to get the optimum acoustics," says Sanders. "Others told us it would take a consortium of composite manufacturers and several molds to get what we needed. Those companies weren't trying to gouge us. They just didn't think like Bill Kreysler."

Kreysler's thinking led to a single mold that produced identical eight-by-eight-foot (240 by 240 centimeter) square panels. To create three dimensional shapes, the mold was hinged on a diagonal. Each panel could be molded as if it came from a different mold, while in reality only one mold was needed.

The panels were produced by open molding laminates of resin-impregnated fiberglass fabric on each side of the foam core. Vacuum bags were used to ensure void-free laminates and to achieve proper adhesion between the laminate and core. When the resin cured, the finished panel had the high rigidity that is essential to effective acoustics.

"The performance is what we hoped for," says Sanders. "The composite panels are performing like wood without the problems. We even considered putting a wood veneer on the fiberglass panels. But when exposed to the rain and humidity of Chicago, wood warps enough to knock your socks off. The composite panels don't warp from moisture or buckle with changes in temperature."

Performance Verified
During the first Chicago Symphony rehearsal after the panels were installed, music professionals commented that the acoustics were better than they had experienced at many indoor halls. The performance of the new acoustics system was also verified by Commins, the system designer.

He measured the acoustics of the Pavilion before and after the new design with the composite panels. The old acoustic shell system used a combination of steel and transparent plastic sheet. In a report on the new panels and design, Commins writes: "Early decay time, or the early reverberation time is shorter, in particular at low frequencies. Its values are closer to the reverberation time values, an important asset."

Commins says the clarity of the music is far better with the new design and composite panels. Clarity is rated good or very good at almost all locations of the new pavilion. Before the new panels were installed, this trait was rated "consistently poor."

Energy-Time-Frequency curves Energy-Time-Frequency curves are used to "visually" measure the improvement with the isopolyester composite panels. The curves indicate the "richness" of sound from a specific musical source position to a specific seat position. After analyzing results, Commins states, "It is quite obvious that sound is much richer over the full musical frequency range after renovation of the Pavilion."

About 200 composite panels were molded for the Ravinia project. The majority of the panels are permanently installed. However, five panels on each side and a ceiling panel can be moved out to meet the floorplan requirements of dance and pop shows. Even though the panels are assembled to heavy steel framework, the total system weight is lower than it would be if the panels were wood. The lighter weight makes the panels easier to move.

Sanders appreciates the ruggedness of the composite panels when the stage is being set. "Sometimes, the panels take a tremendous beating when we move them," she says. "The composite panels are very forgiving when hit by a road case or other equipment. We have only experienced minor chips and scratches on the painted surface, which is easily repaired. The panels themselves show no signs of damage and are virtually maintenance-free."

West Coast Connection
Louise M. Davies Symphony Hall The composite panels were installed for the Ravinia Festival Pavilion renovation of 1995. Kreysler was called in by the project architect, the Chicago office of Skidmore, Owings and Merrill. The Chicago team found out about Kreysler through the San Francisco office of Skidmore, Owings and Merrill. The San Francisco office worked with Kreysler in 1992 to create new wall panels that contribute to the much-improved acoustics of the Louise M. Davies Symphony Hall in San Francisco.

The philosophical approach to the Davies Hall composite panels is different than that for the Ravinia project. For the San Francisco site, Kreysler molded a series of curved composite shells filled with sand and permanently bolted to a massive steel framework.

In another contract, the Davies Hall panels are molded with fiberglass chopped strand mat instead of biaxial fabric. What both jobs have in common are exceptional acoustic properties and the versatile use of isopolyester resin.

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Last updated: 09/21/06