Article: Cold-formed steel design for acoustic code compliance
Article originally published in Construction Canada Magazine and on their website.
Cold-formed steel design for acoustic code compliance
By Steven R. Fox, PhD, P.Eng. & CSSBI General Manager
The North American steel industry has completed a research program to develop the necessary tools to meet code requirements for acoustic separation in cold-formed steel frame construction.
For decades, Canada’s building codes for residential construction only took sound transmission through separating walls or floors into account when assessing acoustic separation and performance. Known as sound transmission class (STC) ratings, this system led to innumerable situations where inadequate acoustic separation between neighbours was blamed on design or workmanship of those assemblies, often focusing remediation efforts in the wrong place. This problem was especially evident in multifamily housing projects where privacy and sound transmission between units are a major concern for owners and residents.
The National Building Code of Canada (NBC) has evolved to consider additional paths for sound waves, including flanking transmissions through shared ceilings and floors. “Flanking transmissions” simply means sound waves in a room engaging with all of the room’s surfaces, including ceilings and floors. When those ceilings and floors are shared with adjacent rooms, the transmitted vibrations are expressed as sound next door regardless of the acoustic separation designed into the shared wall between the two rooms that was the focus of previous NBC editions.
As 2015 NBC is adopted throughout the country, architects and specifiers are designing to this more holistic understanding of sound transmission, referred to as apparent sound transmission class (ASTC) ratings, for their assemblies. This evolution in treatment of sound in residential construction can reduce sound transmission between rooms and units, but it also creates new challenges and requirements for architects and specifiers.
Those challenges are frankly why it has taken so long for building codes to catch up to what has been widely understood for decades. ASTM E336, Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings, providing an ASTC rating, was first published in 1997. However, while measuring acoustic performance is one thing, offering architects the tools to reliably predict the performance is another. While some European building codes have used the 2005 International Organization for Standardization (ISO) 15712-1, Building acoustics — Estimation of acoustic performance of buildings from the performance of elements — Part 1: Airborne sound insulation between rooms, these methods have not found broad adoption in North America. This is because they largely provide reliable estimates for buildings constructed from heavy, homogeneous building elements, and not for structures constructed from lightweight-framed elements widely used in mid-rise construction projects in North America. Additionally, from a practical perspective, ISO standards for building acoustics have many differences from the ASTM standards used by the North American construction industry—both in terminology and in specific technical requirements for measurement procedures and ratings.