Paradigm shift in acoustic regulations

by nithya_caleb | September 21, 2018 12:00 am

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By Christoph Hoeller, Jeffrey Mahn, and Markus Mueller-Trapet

Urban centres in North America are experiencing an increase in densification, leading to a high demand for mid- and high-rise residential buildings. The increase in multitenancy buildings has brought with it a greater awareness of the importance of good sound insulation.

Noise enters a living space in a variety of ways. For example:

• airborne or impact noise generated by neighbours enters via interior walls and floors;
• environmental noise (e.g. traffic noise) enters through the building façade; and
• noise from building service equipment or restroom fixtures enters via ducts and pipes.

Of these sources of unwanted noise, only the airborne sound insulation between dwelling units is regulated on a wide scale in this country. The National Building Code of Canada (NBC) and most provincial building codes have for a long time mandated the sound insulation of partition walls and floors. In recent years, the minimum requirement has been a sound transmission class (STC) rating of 50 for walls and floors between dwelling units. A significant body of literature and data collections exists for STC ratings of common walls and floors. Examples for collections of sound transmission class (STC) ratings include the National Research Council Canada (NRC) reports, IRC-IR-761, “Gypsum Board Walls: Transmission Loss Data,”[2] and IRC-IR-811, “Detailed Report for Consortium on Fire Resistance and Sound Insulation of Floors.”[3]  NBC also contains a substantial collection of walls and floors with fire and sound ratings. However, in 2015, the acoustic requirements in NBC underwent a major change.

From STC to ASTC

It has long been recognized by researchers and acoustical experts sound travels between rooms not only through the separating wall or floor, but also via the building elements connected to the partition. This is known as flanking sound transmission. In practice, flanking sound transmission is the reason noise complaint issues often lead to significant costs without a noticeable increase in the perceived sound insulation. For example, attempts to improve the sound insulation of the wall separating two dwellings by adding additional layers of gypsum board will be a waste of time and money if the primary transmission path is the common subfloor underneath the separating wall.

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In recognition of this reality, the sound insulation requirements in the 2015 edition of NBC were revised. The new requirements are given in terms of apparent sound transmission class (ASTC) rating rather than STC. While the STC rating only describes the sound transmission of the separating assembly, ASTC takes into account the sound travelling through adjoining constructions and common building elements (Figure 1). Therefore, ASTC is a better metric than STC for describing what the building occupants actually hear in practice.

NBC requires dwelling units be separated by a building configuration with an ASTC rating of not less than 47. Compared with equivalent requirements in other developed countries, this can be considered rather low. For example, most Scandinavian countries have requirements equivalent to ASTC 55, and other European countries mandate at least ASTC 50. Based on surveys and studies conducted by NRC, it can be expected many building occupants will be annoyed by noise in a building meeting a requirement of ASTC 47. It is, therefore, important to realize NBC provides minimum requirements designed to address health and safety issues, but not occupant comfort or satisfaction. In cases where a higher level of acoustic comfort is desired, a design target should be ASTC 55 (for satisfactory performance) or ASTC 60 (for ideal performance), and other unwanted sources of noise such as impact or environmental noise must also be addressed.

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How can one demonstrate compliance?

NBC prescribes three methods to demonstrate compliance with the new acoustic requirements (Figure 2). They are:

• field measurements;
• a prescriptive procedure; and
• a design procedure.

All three methods are valid for both Part 5 and Part 9 buildings.

Field measurement

The first method to demonstrate compliance is through measurements in the finished building. ASTM E336, Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings, describes a method to measure the ASTC rating directly. The advantages of this compliance path are one does not need to know all of the construction details of the building under evaluation and the measurements are straightforward. The results are a true representation of the sound insulation of the building under test, including the effects of any construction deficiencies. The disadvantage of this compliance path is it can only be used once the building has been built. Any subsequent changes to the construction as a result of unsatisfactory sound insulation will likely be costly.

Prescriptive procedure

The second method to demonstrate compliance is by following a set of solutions prescribed in NBC. The advantage of this compliance path is that it is easy to use. As long as the instructions in NBC are followed, the finished building is deemed to comply with the required sound insulation rating of ASTC 47.

The first part of the prescribed solutions considers the separating wall or floor. The assembly must have an STC rating of at least 50. This connects the new acoustic requirements with the mandates in previous editions of NBC, and allows designers and architects to continue to use established wall and floor assembly designs.

The second part of the prescribed solutions concerns the building elements connected to the separating element. For these adjoining constructions, NBC provides a simple set of requirements. For example, the requirement for adjoining constructions in heavy homogeneous structures (such as concrete) is given in terms of a minimum mass per unit area. For lightweight wood- or steel-framed constructions, one of the requirements for adjoining constructions is simply the gypsum board at a wall-wall junction is not continuous. The appendix of NBC also includes several tables with detailed examples of constructions that are deemed to comply. While the list of examples is limited, optional modifications—expected to yield similar or better ASTC ratings—are provided.

A disadvantage of this compliance path is it does not yield ASTC values, but instead provides a pass or fail mark. Building professionals who are interested in achieving better sound insulation than ASTC 47, or who want to use designs that are not part of the prescriptive procedure, cannot demonstrate the actual ASTC rating when employing this compliance path.

Design procedure

An advantage of this compliance path is it allows the estimation of the expected sound insulation for building configurations at the design stage, including a cost-benefit analysis. Building elements needing to be modified for better sound insulation and over-designed elements (where cost-savings are possible without affecting the sound insulation) can be identified. The disadvantage of the design procedure is it involves more effort than the prescriptive method and requires laboratory measurement data for flanking sound transmission. If data for the specific construction detail is not available, then the procedure cannot be used.

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Where can one find help?

To enable designers, builders, and architects to use the design procedure, NRC has teamed with industrial partners including:

• Canadian Concrete Masonry Producers Association (CCMPA);
• Canadian Institute of Steel Construction (CISC);
• Canadian Precast/Prestressed Concrete Institute (CPCI);
• Canadian Ready Mixed Concrete Association (CRMCA);
• Canadian Wood Council (CWC);
• Gypsum Association (GA);
• Owens Corning Canada; and
• Rockwool.

In co-operation with these and other industry partners, NRC has conducted a significant amount of research by testing and evaluating the sound transmission through different types of building elements and junctions. Figure 4 shows a laboratory setup at NRC designed to evaluate the flanking sound transmission in cold-formed, steel-framed constructions. Figure 5 shows a laboratory setup to determine the vibration transmission on a junction mockup constructed from cross-laminated timber (CLT) elements. Based on these research studies and the input from industry partners, the following tools and guidelines were developed to help with the transition from STC to ASTC.

NRC research reports

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The NRC research report RR-331, “Guide to Calculating Airborne Sound Transmission in Buildings[6],” 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[7];”
• RR-335, “Apparent Sound Insulation in Cross-Laminated Timber Buildings[8];”
• RR-336, “Apparent Sound Insulation in Wood-Framed Buildings[9];” and
• RR-337, “Apparent Sound Insulation in Cold-Formed Steel-Framed Buildings[10].”

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[11] (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[12],” 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[13],” 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.

[14]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 [15]reached via e-mail at christoph.hoeller@nrc-cnrc.gc.ca[16].

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[17].

[18]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[19].

Source URL: https://www.constructioncanada.net/paradigm-shift-in-acoustic-regulations/

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