Let’s Control Sound
Mark L. Johnson / July 2017
I live in Evanston, Ill., in a building erected in 1922. It’s a classic, wood framed, plaster and lath structure with a red brick exterior. My wife and I love our wood floors, high ceilings and crown moulding. We don’t, however, love hearing the footsteps of the four-year-old boy above us.
Our little friend’s frequent stomping is just shy of thunderous. I’d love to hire a contractor to come and control the sound reverberations overhead. But, I think it’s too late for my building. Someone should have added sound insulation 100 years ago.
Lately, the industry has learned a lot about sound transmission and how to attenuate it. Research from Canada provides new understanding and answers.
Canada Takes the Lead
In April, the National Research Council of Canada released an important study. It’s titled, “Apparent Sound Insulation in Cold-Formed Steel-Framed Buildings”—report number RR-337. It focuses on the problem of flanking sound.
Sound is vibration. It’s wave motion traveling through air, liquids and solids—such as the studs and plaster in my home. Sound travels through the building partitions and through the solid pathways connecting the partitions. The latter involves flanking sound transmission.
In 2015, Canada changed its requirements for flanking sound transmission. The National Building Code of Canada now mandates the use of apparent Sound Transmission Class ratings. A wall provides sound reduction, but sound can flank through the rafters above and below the wall. The STC rating of one wall assembly varies from the rating for the entire system of walls, rafters and joists.
So, Canada’s building code now directs engineers to take a system-wide approach to sound control. RR-337 shows how to calculate and predict the sound transmissions. Wall and ceiling contractors will not be able to implement every sound control suggestion in RR-337 without, perhaps, some change-order requests, but they can be informed on the topic and share new ideas with building owners, architects and general contractors. Here are two suggestions for controlling sound.
Subfloor mass matters more than anything in controlling building noise, says Christoph Hoeller, NRC research officer. The greater the mass of the partitions—the walls and floors, the better the sound insulation.
Hoeller suggests adding mass where possible. Install two sheets of OSB or plywood as a subfloor rather than one layer of material. Install two layers of drywall in the ceilings, rather than one layer of board. The extra layers take up room, but they’ll add mass and improve sound control.
Add Decoupling Factors
Decoupling is a way to add a sound buffer between partitions. How?
One way is by increasing the joist spacing within the flooring system. Wide joist cavities—joists placed 24 inches o.c. versus 16 inches o.c., for example—help to deaden sound transmission especially when fibrous insulation is used in the cavity, Hoeller says.
A second way to decouple sound transmission is through the use of resilient channels. Hoeller says it’s difficult to achieve good sound insulation without resilient channels, but he cautions contractors to be sure they’re installed correctly. Long screws may fasten the resilient channels too rigidly in place and defeat their purpose.
Third, Hoeller suggests adding breaks at partition junctions. It’s one of the biggest findings of RR-337, Hoeller says. “Cutting the joists at the junctions is much better for sound insulation than having a continuous joist,” he says.
Fourth, fasten the subfloor to the cold-formed steel joist as far from the joist web as you can. Direct your crews to place the screws toward the outer edge of the joist flange. This makes the flange act as a decoupling element and can reduce noise.
Download the Report
I recommend that you download the NRC’s RR-337 study, which represents the latest research. Why not email a copy of it to the structural engineering or GC of an upcoming project?
We’ve learned a lot about sound control in the past 100 years. And, I have learned to love my neighbor’s little boy. My wife and I want him to enjoy his childhood, and so we’ve decided to let him run about to his heart’s delight. We’ll just crank up the volume when we watch our action movies. This form of sound control works for us, but only us.
Mark L. Johnson writes about livability and building design. Reach Mark on Twitter, @markjohnsoncomm, and at linkedin.com/in/markjohnsoncommunications.