NRC research reports
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The NRC research report RR-331, “Guide to Calculating Airborne Sound Transmission in Buildings,” provides an introduction to the design procedure. It contains detailed instructions on how to estimate ASTC ratings for a variety of constructions. The details of the design procedure are demonstrated in more than 50 worked examples presented in spreadsheet form, one of which is shown in Figure 6.
The data required as input for the design procedure is provided in a series of companion reports:
- RR-334, “Apparent Sound Insulation in Concrete Block Buildings;”
- RR-335, “Apparent Sound Insulation in Cross-Laminated Timber Buildings;”
- RR-336, “Apparent Sound Insulation in Wood-Framed Buildings;” and
- RR-337, “Apparent Sound Insulation in Cold-Formed Steel-Framed Buildings.”
These and other NRC reports provide extensive data sets measured in the acoustic laboratories of NRC. Tables such as in Figure 7 list the data necessary to calculate ASTC ratings of many construction configurations.
soundPATHS
A number of commercially available software packages allow designers to estimate the expected sound transmission for various building constructions. For the Canadian construction sector, NRC has developed the web application soundPATHS (Figure 8). The calculation performed by soundPATHS conforms to the design procedure described in 2015 NBC.
The application currently provides data for many building elements, including:
- concrete walls and floors;
- masonry walls;
- precast concrete hollowcore floors;
- CLT walls and floors;
- wood-framed walls and floors;
- cold-formed, steel-framed walls and floors; and
- optional linings for masonry walls, CLT assemblies, and precast concrete hollowcore floors.
The library of available data continues to grow as additional building elements and junctions are evaluated at NRC.
Design professionals can pick and choose the desired building elements and room dimensions (Figure 9). If data is available, the soundPATHS software calculates the ASTC rating. Users are also able to compare the sound transmission through the separating assembly and the sound transmitted via the adjoining constructions. The comparison can help to identify those transmission paths limiting the apparent sound insulation, and those building elements which may have been over-designed. Once the design has been finalized, the user can generate and export a PDF report.
What does the change mean in practice?
The transition to an ASTC from a STC rating requires a change in mindset when considering the sound insulation between dwellings. The building design must now be thought of as a system—it is important to consider the junctions between elements in addition to the sound insulation of the elements themselves. The apparent sound transmission is always dominated by the weakest path, and the sound transmitted through this weak path has the biggest effect on the ASTC rating.
Based on the data presented in NRC research reports and soundPATHS, it can be observed, in many cases, the separating partition provides the dominant transmission path, with some notable exceptions. For example, the ASTC rating of lightweight wood- or steel-framed constructions with a subfloor and floor joists that are continuous under the separating wall is often dominated by the flanking sound transmission through the floor. Attempts to improve the sound insulation by changing the separating partition will be ineffective. The floor must either be redesigned to break the continuous path between the dwellings, or floor toppings will be needed to attenuate the sound transmitted via the flanking path. Similarly, wall or ceiling gypsum board continuous at the junction can decrease the ASTC value significantly and generally should be avoided. For mass-timber constructions such as CLT, flanking sound transmission poses a particular challenge that should be considered carefully at the design stage (For more information, consult NRC research report RR-335, “Apparent Sound Insulation in Cross-Laminated Timber Buildings.”). Even for massive constructions (concrete or masonry) the transmission via flanking elements can limit the ASTC value, albeit usually at values above ASTC 47.
Besides the structure-borne flanking issue, there is also a need to identify and control other practical issues of sound transmission, such as air leakage through electrical boxes or degraded performance due to the quality of workmanship. Some of these issues are addressed by fire-resistance requirements, but practical experience shows that it is often beneficial to consult with acoustical experts to identify potential problems. The NRC publication, “Best Practice Guide on Fire Stops and Fire Blocks and their Impact on Sound Transmission,” provides a useful overview of such issues.Further, some practical issues affecting sound insulation in multifamily dwellings are explored in the article, “Good acoustic design is half the battle,” by Nick Walters, M.A.Sc., P.Eng., and Brad Pridham, PhD, P.Eng.
Conclusion
The new acoustic requirements in NBC are already being employed in some provincial building codes (e.g. Nova Scotia), and more provinces are expected to adopt them in the near future. Looking ahead, it is expected other areas of sound insulation such as protection against impact or environmental noise will be considered for inclusion in NBC. Irrespective of regulations, a successful building design always needs to consider all noise sources and transmission paths.
Christoph Hoeller is a research officer in the acoustics group at the National Research Council Canada (NRC). The current focus of his work is on supporting the transition to apparent sound transmission class (ASTC) metrics in building regulations by developing tools and guidelines in collaboration with industry partners. Hoeller serves on the ASTM and the International Organization for Standardization (ISO) committees on building and environmental acoustics and on task groups for the Canadian Commission on Building and Fire Codes (CCBFC). Hoeller can be 
reached via e-mail at christoph.hoeller@nrc-cnrc.gc.ca.
Jeffrey Mahn is a research officer in the acoustics group at the National Research Council Canada (NRC), specializing in flanking sound transmission. He can be reached via e-mail at jeffrey.mahn@nrc-cnrc.gc.ca.
Markus Mueller-Trapet is a research officer in the acoustics group at the National Research Council Canada (NRC), specializing in room acoustics, measurement methods, and the perception of sound in buildings. He can be reached at markus.mueller-trapet@nrc-cnrc.gc.ca.
