Tech Focus: Acoustics, Part 2 — Is ETFE a Miracle Material or Sonic Speed Bump?

Sports-venue sound design has a new element in the narrative

The story of sports-venue acoustics is a tale of unintended consequences. The rise of the massive domed stadium in the 1970s unleashed reverberation issues that made Lou Gehrig’s 1939 farewell speech at Yankee Stadium sound like a model of intelligibility and plague them to this day. The NBA’s embrace of hip-hop in the early 2000’s introduced the challenges of low-frequency energy at high SPLs into large, hard-surfaced arenas. Backlash against publicly funded stadiums used for all of eight games a year compelled municipalities to demand multifunctional applications from their new civic venues; that meant that music would have to cohabit with sports and trade shows, each with disparate audio requirements.

Click here for Tech Focus: Acoustics, Part 1 — In-Venue Noise Remains an Issue.

Though providing an outdoor feel and protection from the weather for U.S. Bank Stadium, the ETFE in the roof spurred noise complaints.

The latest wrinkle to disrupt the sector is ETFE, more formally known as ethylene tetrafluoroethylene, a co-polymer resin that is extruded into a thin, translucent film. For architectural-engineering firms like Thornton Tomasetti, which was the lead structural engineer on the Minnesota Vikings’ U.S. Bank Stadium and did structural renovations for the Miami Dolphins’ Hard Rock Stadium, ETFE is a wonder drug. As venues seek more natural light and better connectivity to the environment around them, ETFE offers structural strength but design flexibility.

“ETFE transforms enclosed facilities into light-filled, dynamic spaces,” the firm proclaims on its website. “Enclosed facilities have four-season functionality, which expands programmability and increases revenue for owners and operators. … [D]ark, confined atmospheres of conventionally roofed venues are no longer in step with patron expectations. At the 66,200-seat U.S. Bank Stadium, which hosts NFL football, MLS soccer, NCAA basketball and baseball, high school sports, motocross races, concerts, conventions and other events, the ETFE roof admits abundant natural light, creating an outdoor feel while protecting occupants from the elements.”

Side Effects
However, like many miracle cures, ETFE has its side effects. Brian Elwell, senior consultant at design consultancy Idibri, which integrated A/V for all the premium and team spaces —clubs, suites, locker rooms, meeting rooms — at U.S. Bank Stadium, says that, while ETFE’s transparency is welcome, its foil-like, semi-rigid nature tends to reflect high-frequencies while letting LFE pass through.

With a roof made of conventional materials, absorptive acoustical treatment could be added to the underside to help control some of the HF and mid-range reflectivity. But ETFE doesn’t offer enough mass to allow that to be applied. As a result, says Elwell, venues that contain ETFE design elements may experience elevated noise levels by comparison, unless the acoustics are addressed by other mitigating treatments.

From an architectural perspective, ETFE was the perfect solution for U.S. Bank Stadium: it let the venue address team owners’ desire for a stadium with an outdoor feel that also offered year-round, all-weather use but without the high cost of a retractable roof. The result, designed by architecture firm HKS, is a cover that is 60% ETFE, the largest of its kind in North America, spanning 240,000 sq. ft. The translucent roof and large wall panels also give fans a view of downtown Minneapolis while providing protection from the snow, rain, and cold.

However, noise complaints there began as soon as the first notes of concerts by country-rocker Luke Bryan and Metallica rang out last August. And reports of the venue’s inaugural NFL match, between the Vikings and the Green Bay Packers were characterized as “ear-splitting.” Not everyone saw this as a drawback, including Vikings coach Mike Zimmer, who recognized it as part of the home-field advantage. But, with preseason game measurements topping out at 114 dB, above the average human pain threshold of 100 dB, U.S. Bank Stadium could end up on OSHA’s watch list for employees.

According to Kevin Day, associate principal at WJHW, the consultancy that designed the bowl sound for U.S. Bank Stadium, some of the confusion surrounding ETFE’s properties was caused by its marketing materials, which portrayed it as acoustically friendly. However, he points out, the fine print in the material’s documentation notes that it was tested as a limp mass. Once ETFE is stretched out in its typical stadium application, it becomes resonant and reflective, with its specific frequency-response characteristics dependent on the degrees of tension and dimension each application calls for.

“Yes, it has better absorptive qualities than plate glass,” Day deadpans, “but we found that, under tension, it behaves a lot like other materials used [in stadium construction].”

At U.S. Bank Stadium, the architectural design called for the ETFE to be positioned opposite the side that was treated acoustically. That did result in the reflections’ being aimed at the visiting team’s side of the venue, underscoring its appeal for coach Zimmer.

Day says that wasn’t necessarily intentional. “They also had HVAC noise and a ceiling with a low R factor to deal with,” he explains. “So they went 50-50 on where the ETFE and the acoustical treatment were placed.”

He sees ETFE as one more evolutionary step in a process in which sports venues have to accommodate changing economic and political exigencies in their design. It succeeds Birdair, the material used to cover such venues as the Arthur Ashe Stadium at the USTA Billie Jean King National Tennis Center, succeeding the steel domes of the early domed-venue era. But ETFE will require ancillary design elements when it comes to sound: for example, even greater emphasis on loudspeaker directivity and precise coverage patterns and more interaction between outside-production users, such as concert touring, and the house sound systems.

“Everyone has to meet each other halfway now,” Day says. “We learned a few more things about ETFE in regard to room acoustics on [the U.S. Bank Stadium project] and expect to get even more data/experience from venues currently under construction.” (The Miami Dolphins’ Hard Rock Stadium will be the second major sports venue to use ETFE.)

As with all new materials and techniques in venue design and construction, time and experience will eventually deliver new solutions. But, says Elwell, “ETFE is here to stay” as an element of stadium design. “In terms of sound, we’re going to have to learn to work with it.”