How sound transmission class (STC) tests for wall joints are conducted

When Trim-Tex says all their code compliance solutions for wall joints produce the same or better soundproofing capabilities as acoustical caulk, with none of its drawbacks, how do they know that? Recently, a team of engineers at North Orbit Acoustic Laboratories in St. Paul, Minn., pitted two of Trim-Tex’s fire rated drywall accessories, Hotrod XL and Fire Bead, head-to-head against acoustical sealant in the ultimate soundproofing test. 

For many years, the common way to seal the gaps around drywall to dampen sound was with acoustical caulk. But as anyone who’s worked with this kind of sealant knows, it’s tough to apply evenly, and over time, it hardens and shrinks back. An effective soundproofing solution needs to remain flexible to remain effective.

The NOAL team performs acoustical tests by constructing two nearly identical rooms outfitted with cutting-edge audio equipment, with an opening between the two areas. In that opening, a field-accurate wall assembly was built using steel studs, R-13 batt insulation and ⅝” Type X drywall. Loud noises are blasted from one side of the testing facility, and the amount of sound that passes through to the other side is then measured as a Sound Transmission Class (STC) value.

For the first test, sound sealant was applied on all four sides of the wall, filling the half-inch gaps between the drywall and the test opening. Sealant like this takes weeks to fully cure, so in this test, it was left in a wet state, which would give it the best possible STC value. After the sealant hardens, its sound-dampening ability would decline.

The sound test commenced, with a result of 50 STC, the accepted standard for most building codes.

To prepare for the second test, the sealant within the sides and top of the wall assembly was painstakingly removed. The team then installed Trim-Tex’s Hotrod XL along the head-of-wall joint. Hotrod XL’s compressible foam center is designed to form a perfect seal upon installation, providing ¾” of structural movement and staying permanently flexible. Meanwhile, Fire Bead, with its flexible apex gasket, is perfect for finishing vertical gaps and protecting them from the passage of fire, smoke and, yes, sound; this solution was used along the sides of the test opening. The two different fire rated vinyl beads were stapled into place every two to four inches, and finished with five-minute hot mud to get it ready in time for their test. 

And once all was said and done, the team at NOAL had a result: a 51 STC, a full point higher than acoustical sealant. A single point may not mean everything in every field, but for acoustical testing, it’s a major statement. 

With third-party sound testing completed, it proved that Trim-Tex’s code compliance solutions provide the better STC values as acoustical caulk. Plus, with these fire rated drywall accessories, you also get the most advanced fire stopping proficiency you can get, due to intumescent tape that expands to 30 times its size when exposed to intense heat. And you get far longer-lasting flexibility for building movement, preventing cracks in your finish and other, more serious kinds of damage. And you’re getting a cleaner, faster, more foolproof installation. And you’re getting a more aesthetically crisp finish on your drywall. And you won’t have to rip it all out and do it again in five years; you’ve got an effective fire-resistant, soundproof setup that will last the life of your wall. When one adds up the true value they are receiving with Trim-Tex’s groundbreaking code compliance solutions, there’s really no comparison at all.

Learn more about these sound tests and their results, including photos, video and technical specs, here.

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