March 1, 2016
By Gary Sturgeon, B.Eng., MSc., P.Eng.
The effects of noise have been well-documented in studies by the World Health Organization (WHO). (For more on these issues, see Birgitta Berglund and Thomas Lindvall’s “Community Noise” document, prepared for WHO in 1995. The organization defines ‘health’ as “a state of complete physical, mental, and social well-being, and not merely the absence of disease.” See also WHO’s 2004 Large Analysis and Review of European Housing and Health Status [LARES] report, “Noise Effects and Morbidity,” authored by Hildegard Niemann and Christian Maschke of the Interdisciplinary Research Network in Noise and Health [EUR/04/504777]).
Noise can increase blood pressure or be a risk factor for coronary heart disease, as shown in M. McCarthy’s article, “A Health Impact Assessment Model for Environmental Changes Attributable to Development Projects,” in Journal of Epidemiology and Community Health (56 ). It is known to cause stress and hostility, interfere with sleep, speech, and tasks, and to affect the body’s physical reactions and our relations with other people. It is far more than a simple annoyance—a British study attributes up to 10 deaths annually in the United Kingdom to noisy neighbours (these being suicides or as a result of assaults). (This comes from “Building Regulations−Regulatory Impact Assessment [Final],” issued by the Office of the Deputy Prime Minister, Great Britain, in November 2002.)
Further, according to research by the National Research Council of Canada (NRC), “Noise from neighbours in multi-unit buildings is a serious problem that degrades the quality of life of the residents.”(For more, see John S. Bradley’s “Sound Insulation Issues,” published by the Institute for Research in Construction of the National Research Council in 2004 [NRCC-47054]).
Urban environments are inherently noisy and are likely to become more so. Cities and living spaces are becoming more densely populated as increasing numbers of people look to converted living spaces, and to apartments and condos. Effective sound control between spaces has become a critical aspect of urban quality of life, human comfort, and health. While noise can be controlled at its source, this is usually a very complicated undertaking. The most effective solution in building construction is by sound insulation—that is, by reducing noise along its paths from its source to the listener by blocking, breaking, or absorbing the sound.
Hearing the need for code changes
In 2006, the Standing Committee for Part 5 of the National Building Code of Canada (NBC), “Environmental Separation,” began its work on performance-based requirements for noise control of airborne sound in buildings. A review of the updated ASTM standards on sound transmission revealed ASTM E336, Standard Test Method for Measurement of Airborne Sound Attenuation Between Rooms in Buildings, had been recently revised.
Since the measuring and reporting of Field Sound Transmission Class (FSTC) ratings had been made more stringent, those requirements were relegated to an annex because of the impracticality of its measure. (FSTC is an ASTM field test used to measure the airborne sound insulation of an acoustically isolated separator [wall or floor]. The results should theoretically approach the sound isolation of the same partition constructed in the laboratory and tested for STC. FSTC is a very difficult test to perform because of the necessary shielding procedures required to reduce flanking transmission to a negligible level). Reportedly, the plan was to eliminate the metric altogether. A new metric was introduced in ASTM E336—the Apparent Sound Transmission Class (ASTC) rating—which was easier to measure and better representative of a ‘systems approach,’ rather than the ‘elemental approach’ to sound attenuation used by the STC rating. (STC and ASTC are explained in more detail below.)
These realizations compelled the Part 5 Standing Committee to fully re-examine the existing requirements for airborne sound control in the NBC. It was understood the work would affect requirements within Part 9 and notably Tables A-188.8.131.52.A and B. It was agreed that:
Traditional approach to controlling airborne sound in buildings
For decades, the basic approach for controlling airborne sound in buildings in the NBC has been to use the Sound Transmission Class rating. With some exceptions, the dwelling unit was required to be separated from every other building space by a separation that provided an STC rating of not less than 50.
The STC rating is a laboratory measure of direct transmission of airborne noise through the nominal separating assembly (i.e. the wall when the dwelling units are side by side, or the floor when the units are one above the other) acting in isolation, without any interconnection to other elements in the assembly. The higher the STC value, the better sound attenuation being provided.
However, the STC rating does not include transmission through flanking paths that inevitably exist. These flanking paths bypass the separating element and travel through other building elements that are coupled to the separating element such as the floor, ceiling, or abutting sidewalls.
The use of the STC rating approach places requirements on only the separating partition while ignoring the often dominant effect of other sound transmission paths. Consequently, the actual field performance of the constructed assemblies can vary greatly from the measured laboratory STC rating of the individual separating elements. To compensate for the difference between the measured STC rating and the in-situ performance, it was widely known that designers would specify an STC rating that was five to 10 points higher than required. (This information, along with the next paragraph, comes from the meeting minutes of the Part 5 and Part 9 Standing Committees of the National Research Council of Canada’s Canadian Codes Centre). However, when flanking transmission is the controlling factor for the total sound insulation, simply increasing the specified STC rating of individual elements does not account for the flanking transmission loss through the structure, and will not improve the overall sound insulation.
A focus by the NBC on the STC laboratory rating of the separate elements rather than performance of the entire holistic system, and placing limits on it, was an oversimplified design approach that encouraged over-design of the separating elements. Neither scientific nor cost-effective, it resulted in investments in the wrong building elements—it is akin to attempting to achieve thermal performance of a system by avoiding thermal bridging. Since the effects of this design approach are truly unknown, the outcomes in the field are not understood until subsequent in-situ testing or after occupancy. If the sound insulation is inadequate, the causes can be falsely attributed to poor design or construction, and it may be neither.
New approach: Using ASTC to control airborne sound in buildings
The noise heard by occupants in a space is the net result of all possible transmission paths from the airborne noise source, including what is transmitted directly through the separating assembly and what comes through the flanking paths.
From the noise ‘source room,’ it can be reasonably assumed that:
By considering all these paths, the sound performance of the complete system can be reasonably predicted, and meaningful airborne sound requirements can be codified. This systems rating is best described and determined by the aforementioned ASTC rating, which is a single number rating defined and measured in the laboratory using ASTM E336.
As with the STC rating, higher numbers mean better sound insulation. It is important to remember the ASTC rating is usually lower than the STC or FSTC rating because it includes the contribution from the flanking transmission. In practice, and as a means for compliance under the building code, using ASTC rather than STC will reduce ‘over-design’ and make it more efficient. It should also provide more cost-effective construction, help identify design errors before construction, and likely result in fewer complaints from building occupants.
New airborne sound control requirements in NBC 2015
The requirements for sound transmission in Section 5.9 and Section 9.11 of the 2015 edition of the NBC are markedly different from those in the 2010 edition. They are based on a systems approach to sound control using the ASTC rating.
Two compliance paths exist: one which is fundamentally a performance-based path of “measure or calculate” requiring an ASTC rating of not less than 47, and another that is largely prescriptive-based where separating assemblies must have an STC rating of not less than 50 and be combined with adjoining constructions that conform to certain new requirements placed within Part 9 Tables A.184.108.40.206A and A.220.127.116.11B intended to address flanking transmission.
A review of European sound studies and code requirements offers compelling reasons to choose an ASTC rating of 50 or greater to achieve suitable sound control. However, an ASTC rating of 47 was agreed on for this edition of the NBC because it was resolved to be somewhat ‘status quo’ with the existing requirement of an STC 50 rating. This ASTC rating will likely be increased in the successive editions of the code.
Whereas the prescriptive-path (with revised Tables A.18.104.22.168A and A.22.214.171.124B) may be enticing to use under both Parts 5 and 9 because of its apparent simplicity, there is an inherent complexity about this path, some uncertainty, and limitations in use. The tables are seen to be a little difficult to follow to ensure all the stated conditions of the solution are met for the separating and flanking assemblies. The tables only contain those separating and flanking assemblies that together achieve an ASTC rating of 47, and the ASTC rating provided by the assemblies is not specifically stated. Consequently, the tables cannot be used when designing for an ASTC rating higher than 47, and the number of assemblies identified in the tables is quite limited.
The code also cautions users that adding materials to the prescribed solution may actually reduce the ASTC rating rather than improve it. In some cases, not all the desired direct test data were available, and ASTC calculations for some assemblies were based on a conservative ‘best estimate.’ All issues considered, one might reasonably conclude the NBC encourages use of the performance path for design. Notwithstanding, masonry wall systems are well represented in these tables because of the depth of reliable information available about these assemblies at the time of creating the tables for the code’s inclusion.
The performance-based ‘measure or calculate’ path in both Parts 5 and 9 provides the most reliable and flexible design path, and speaks to all the encumbrances of the prescriptive path.
Two options are permitted:
Design tools will help
These alterations to sound control in Parts 5 and 9 of the 2015 code are a significant change for manufacturers, building designers, and regulators. Moreover, direct testing for the ASTC rating of all permutations and combinations of separating assemblies, flanking assemblies, and acoustical linings is cost- and time-prohibitive. To minimize the potential impact, consortium research projects involving industry partners (including the Canadian Concrete Masonry Producers Association [CCMPA], the National Research Council of Canada, and the Canadian Codes Centre) were launched or expanded early in the NBC development process.
Most notable are the development of Guideline RR-331, Guide to Calculating Airborne Sound Transmission in Buildings, and the commitment to expand the scope of NRC’s soundPATHS acoustic design software. The intent is to have designers use these to establish code compliance with the airborne sound requirements, and to design for specific ASTC ratings other than 47 to meet market expectations for buildings with better performance.
Referenced for use in the 2015 NBC, RR-331 is intended to support the needs of Canadian designers (and perhaps more so of acoustical experts) as they move through the acoustics transition and adapt ISO 15712-1. (RR-331 is available for free on the Internet. Visit doi.org/10.4224/21268575). It describes the technical concepts, terminologies, convention labelling, needed input data, effects of linings, and required step-by-step calculation processes with explanation, as well as numerous worked examples for both the detailed and simplified ASTC calculation paths identified for use in the code.
The guide identifies that the ISO 15712-1 calculation procedures provide very reliable estimates for some types of construction, notably isotropic heavier forms of construction like masonry and concrete, but do not for non-homogeneous anisotropic forms, and notably not for lightweight framed construction such as wood and steel stud. RR-331 specifically describes the strategies for dealing with each of these types of assemblies. Like the NBC requirements, it clearly distinguishes design procedures between heavy and lightweight construction.
Sections in the guide include:
The means to calculate the ASTC rating for rooms side-by-side and those one-above-the-other are clearly described and illustrated. The illustrative design examples are presented so each successive example builds on the knowledge offered by a previous example. Each section of the guide offers a discussion summary that provides some context to the results of the example calculations for the various assemblies analyzed. This offers the reader a better understanding of the acoustics fundamentals and the effects on the ASTC rating of changing performance of individual components in the overall assembly. The procedures described allow designers to change the various details of the constructed assembly, identify outcomes, and explore various design solutions with great flexibility.
To provide assurance to designers that the ASTC ratings calculated in the guide are rational and reasonable, RR-331 only contains sections and design examples for wall and floor assembly combinations where reliable lab-measured data have been used as input. Wall and floor assemblies that have not been suitably tested in the laboratory to generate input data needed for the calculation procedures, such as measuring the vibration reduction index, have not been included. The vibration index quantifies the structure-borne noise transmitted through the floor into the connected walls and floors through the junctions between them. Without lab measurement, it is otherwise difficult to predict with accuracy for non-homogeneous anisotropic elements. This requires full-scale mockups of the junctions, walls connected to the floors and the junctions between them, and measurement in accordance with ISO 10848, Laboratory Measurement of Flanking Transmission of Airborne and Impact Sound Between Adjoining Rooms.
RR-331 is a ‘living document.’ As test data become available in time, it will be revised and new sections will be included for other wall−floor assemblies. The databases of the flanking transmission data used in the guide (and in soundPATHS) will be consolidated in a series of NRC publications presenting data from studies in collaboration with industry partners such as CCMPA. (One such document is RR-334, Apparent Sound Insulation in Concrete Block Buildings, which will be discussed later in this article.)
Concrete masonry unit (CMU) wall systems of both normal weight and lightweight block constructed with various floor assemblies and junctions are well-represented in the guide.
soundPATHS is a software web-application tool developed by NRC. It calculates the ASTC rating for a variety of wall−floor assemblies with or without linings using the calculation methods required under the 2015 NBC. (It is available for use at www.nrc-cnrc.gc.ca/eng/solutions/advisory/soundpaths/index.html). The results and graphical outputs can be used to demonstrate compliance to the authority having jurisdiction (AHJ).
Walls and floors built with masonry, concrete, or steel assemblies are outside the scope of soundPATHS at this time. However, the software will be expanded to include these construction systems after measured test data become available through ongoing research by NRC with industry partners. It will only include construction systems/assemblies tested in the laboratory for direct and flanking sound transmission.
To facilitate high-performing, economical design solutions, the software identifies the weakest and strongest sound transmission links in the assembly, respectively, where overall sound transmission can be best improved, and where the assembly is over-designed.
Reliable sound transmission design using concrete block masonry wall assemblies
In 2012, the Canadian Concrete Masonry Producers Association partnered with the National Research Council to undertake a comprehensive laboratory test program to obtain reliable and accurate direct and flanking transmission test data. Data sets obtained by the test program are conservative and intended to bracket the lower system performance limit. These data will be used as input to the RR-331 guide and soundPATHS software.
The project consists of five ‘thrusts.’ The first included a measurement series in the direct transmission facility on CMU walls of normal weight and lightweight units, bare and with various linings added, to generate updated and new input data for the ISO 15712-1 prediction methods. Thrust 2 included a calculation series of ASTC values using the ISO 15712-1 prediction method for heavy monolithic, isotropic construction that includes CIP floors (and finishes) with concrete masonry walls (and linings).
The remaining three thrusts are each a measurement series in the flanking sound transmission facility on building systems consisting of concrete block masonry walls with wood flooring (Thrust 3), masonry with hollow precast concrete floors (Thrust 4), and masonry with steel joist flooring systems (Thrust 5) to generate input data for junction coupling needed for ASTC prediction using the calculation method. These latter thrusts consist of full-scale mockups (11 m long x 7 m high x 5 m wide [36 x 23 x 16 ½ ft]) of the loadbearing masonry wall, floor system, and junction (with separately constructing and measuring performance for each of X- and T-junctions). Full-scale test mockups are required with the steel or wood joists or hollow core slab running both parallel and perpendicular to the masonry wall because these flooring systems are anisotropic.
Canada is the only country in the Organization for Economic Co-operation and Development (OECD) without a requirement for impact sound in building codes. However, discussions have already taken place at the NBC Standing Committee level for possible inclusion. Additionally, it is known the code’s sound requirements—STC and ASTC—do not effectively address the effects of low-frequency noise. Looking in the longer term, relevant data are also being gathered in this test program to better understand impact sound insulation and low-frequency sound performance for these assemblies in preparation for code change.
Work on the first three thrusts is now completed, and testing on Thrust 4 (masonry/precast floors) will begin shortly. All work is expected to be completed this year.
NRC Report RR-334, Apparent Sound Insulation in Concrete Block Buildings, is in ongoing development as each of these thrusts is completed. (The current draft/edition can be downloaded at doi.org/10.4224/21275887). It is a comprehensive and detailed report, and serves as the basis for the development of the RR-331 Guide for those sections pertaining to concrete block masonry wall systems.
This masonry test program (and the development of design tools supported by it) helps ensure reliable and accurate data to facilitate the design of high-performing and economical assemblies using masonry wall systems with an assortment of linings, and a full selection of commonly used floor assemblies. Compliance with the sound requirements of the NBC can be demonstrated by the users easily and readily.
Substantive changes have come to the airborne sound control requirements in Parts 5 and 9 of the 2015 National Building Code of Canada. The focus now will be to use the ASTC rating rather than STC as the means to better measure and limit sound transmission between building spaces.
Unquestionably, designers, building officials, and construction industries will be heavily affected by these changes. The effects are not limited to the main structural industries such as masonry, concrete, steel, and wood—the industries providing finishes for wall linings, floors, and ceilings are also impacted. Design tools under development by the National Research Council of Canada and industry partners will help facilitate the transition. Development of these tools is essential, and their development is ongoing, requiring continual updates as data from laboratory testing become available.
In the near future, impact noise requirements and low-frequency noise requirements may be embedded in the NBC. Some construction industries will be working closely with key partners to help undertake the research and generate the data, and to develop the design tools needed by designers. Others may not, and may find themselves outside the market, looking in. (For more background, this author recommends the following NRC articles, which contain information communicated in this feature: [a] Construction Technology Update No. 66, by J.D. Quirt and T.R.T. Nightingale , [b] meeting minutes of the SIG, “Development of Canadian Framework and Guide regarding Apparent Sound Insulation in Construction,” [c] “Supporting Better Noise Control in Canadian Buildings,” by Sabourin et al , and [d] “Calculating the Apparent Sound Insulation,” by Mahn et al).
Gary Sturgeon, B.Eng., MSc., P.Eng., is a senior structural engineer with Calgary-based BBSTEK Design Ltd. He has 35 years of experience in masonry design, construction, and materials. Sturgeon is a member of the Part 5 Standing Committee of the National Building Code of Canada (NBC). He can be reached by e-mail at firstname.lastname@example.org.
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