Different Ratings Measure Sound Control, and Knowing the Distinction is Essential
By Thomas Renner
Just one year after it opened, the Great Lakes Center for the Arts scheduled a wondrous array of events at the height of the 2019 summer. The Center in Petoskey, Mich. scheduled 31 events for the months of July and August – jazz concerts, movies, the Grand Rapids Symphony, ballet, violin recitals, a leadership forum, and a closing performance from two of Broadway’s most endearing leading ladies, Andrea McArdle and Donna McKechnie.
The diverse lineup of performers aligned perfectly with the mission of the Center. “The mission of the Great Lakes Center for the Arts is to inspire, entertain, and educate with year-round, diverse, world-class programming at affordable ticket prices with robust educational initiatives,” said the Center’s Executive Director, Jill O’Neill. “Visiting artists, intellectual dialogue, movies and film, and educational programming accompany music, theater, dance, and comedy in bringing exceptional, world-class opportunities to the shores of Lake Michigan.”
The architectural design team at TowerPinkster included many unique architectural elements at the 525-seat, 40,000 square foot venue, which sits just steps away from Little Traverse Bay, an offshoot of Lake Michigan. Roots for the project reached back 20 years, when a Community Cultural Plan for Emmet and Charlevoix Counties prioritized development of additional cultural facilities and envisioned the addition of a unique performing arts center for the region. After a nine-month design process and 16 months of construction work, the Detroit Symphony Orchestra raised the curtain at the opening gala for the Center.
Architectural pieces unique to the region adorned the facility. They include aged copper, Petoskey stone, natural sedimentary rock. “And of course,” lead architect Jason Novotny said, “the beautiful blue waters of Lake Michigan.”
Atop the roof of the Center are five acoustical smoke vents that help protect against noise intrusion. The vents, manufactured by The BILCO Company, are life safety products that exhaust smoke, heat, and burning gases to improve visibility and protect the building structure. They also allow firefighters to enter the building and contain the fire, and provide safe egress for building occupants.
Acoustical smoke vents, however, take on the added quality of controlling noise. They are used in theaters, concert halls, and in projects near highways, airports, and railroad lines. In a high-performing acoustical environment, Novotny said the design team knew it needed reliable acoustical smoke vents “With the potential for more than 500 visitors for larger events, our team knew we would have a need for a dependable smoke ventilation system,” Novotny said.
Acoustical smoke vents, however, have different ratings that measure how effective they are at limiting noise intrusion. Understanding those ratings is an important part of building design.
Rating Acoustical Smoke Vents
Ratings for acoustical smoke vents are divided into two groups: Sound Transmission Class (STC) and Outdoor-Indoor Transmission Class (OITC). The difference in the ratings is essential for architects, designers, and construction teams to understand. And in the case of the Great Lakes Center and other performance venues, the key rating is OITC.
OITC rates the transmission sound between outdoor spaces and indoor spaces in a structure. The OITC rating was developed in 1990 and is typically used to measure sound transmission loss over a frequency range from 80 to 4000 hertz. It is most applicable for measuring the prevention of low frequency exterior sounds such as automotive traffic, construction, and low-flying airplanes through exterior building surfaces.
“OITC is the preferred rating when addressing sound insulation from exterior noise – especially when transportation noise sources are impacting a building facade with significant low-frequency (bass) sound,” says Harold Merck, principal and acoustician for Merck & Hill Consultants of Atlanta. “While STC ratings may be fine for typical interior noise sources such as voices, STC doesn’t adequately address the extended low-frequency noise contribution of aircraft, traffic or even large roof-top equipment. This also applies to large roof-top equipment noise sources as well. The OITC better addresses low-frequency noise impacts and is the more applicable sound rating for roof mounted automatic smoke vents.”
STC measures the extent to which sound is prevented from being transferred from one area to another. The higher the STC value, the less that sound can be transferred through a building product. STC is typically used to measure sound transmission loss over a frequency range from 125 to 4000 hertz and is most applicable for interior areas that experience mid to high frequency noises, such as conversation, television, telephones, and office equipment. A product with a high STC value, ranging from 50-60, indicates that loud speech is barely heard, if at all. A low STC rating, 20-25, indicates that loud speech is audible.
OITC Ratings: A Deeper Dive
Earlier this year, BILCO introduced a new automatic smoke vent that provides the highest level of protection against exterior noise intrusion. The new model carries an OITC-46 sound rating and an STC sound rating of 50. Most acoustical smoke vents have an OITC rating of 39 or less
The vent also has an ISO 140-18 Rainfall Sound Rating, which measures the impact sound insulation, roof, skylights and roof/ceiling systems incur when exposed to artificial rainfall. BILCO’s new ACDSV’s rating of 37.5 dB features a nearly nine percent improvement over its previous four-cover smoke vent models.
“While any noise can be disruptive to a concert-goer, lower-frequency sounds such as outdoor traffic, construction and airplane noises travel further and fall within the range where human hearing is most sensitive,” BILCO General Manager Michael Toohey said.
It’s critical for architects to understand the project so that they can adjust for the noise control. “Windows might have an excellent STC rating, but without a high OITC rating, low-frequency sounds can be intrusive,” Merck said. “Windows with a high STC rating typically have the same OITC rating as windows with a lesser STC rating, which can be a bit misleading. Ratings that include the OITC are more useful to assess how well a window will isolate environmental noise.”
Albert Maniscalco, a partner with Cerami & Associates in New York, said it’s important to know all aspects of the project to determine which rating might be more essential. “If I’m designing a school, a music practice room might need an acoustical door so that the sound doesn’t bleed out,” he said. “In New York City, there are regulations about OITC to mitigate noise from the exterior. It has become more of an issue to control exterior noise. OITC ratings were adopted because STC ratings didn’t paint the whole picture.”
Acoustical design can play a part in the design of any commercial structure. Cerami, for instance, has its acoustical imprint on such diverse projects as the Metropolitan Museum of Art, Grand Central Station and a library at Barnard College in New York. In those applications, the STC rating might be more essential than the OITC rating. The point is that in any project, acoustical engineers need to find the best materials to balance the delicate equilibrium for noise control.
Besides the Great Lakes Center for the Arts, BILCO acoustical smoke vents have been installed in projects big and small. The Hale Centre Theatre in Utah includes 20 acoustical smoke vents in a 133,000 square foot building. Middlesex School in Massachusetts used six acoustical smoke vents in the renovation of a 55,000 square foot auditorium. When it comes to acoustics, any building can be improved by choosing the correct building products.
“It’s the balance of creating design, and how does acoustics layer into that to be able to give those areas focus,” Victoria Cerami, CEO of Cerami & Associates said in a video where she discussed acoustics and the workplace. “Each space should have its signature, its acoustical signature. And it should correlate with the design intent.”
The Great Lakes Center for the Arts provides an important piece to the arts community in Northern Michigan. The acoustical smoke vents help keep the occupants safe, while also allowing them to enjoy the diverse experiences from its creative lineup of performers.
Novotny and his team faced a lot of architectural choices in the design of the Center. First and foremost, however, was creating a space that patrons of the arts would enjoy from the moment they set foot in the facility until the last note of every concert and final sentence of every reading. Aesthetically, visually, and acoustically, the final product hit all the right chords.
“This building adds a space for world-class performers that did not exist in northern Michigan,” Novotny said. “It adds to the valuation of the performing arts community that was intended by the clients.”
Thomas Renner writes on building, construction and other topics for U.S. trade publications.