Canadian masonry for sound buildings

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Photo courtesy GA Masonry

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 [2002]). 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.)

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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.

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The traditional approach in the National Building Code of Canada (NBC) to controlling sound between spaces has been to use Sound Transmission Class (STC) rating—a metric that only accounts for direct noise transmission through the separating assembly.

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.)

In addition to direct sound transmission through the separating assembly into the receiving room, vibrational energy from the airborne source is transmitted as structure-borne sound through wall/floor junctions, and radiates as noise into the receiving space. This is called ‘flanking transmission.’ Apparent Sound Transmission Class (ASTC) rating includes for both direct transmission and flanking transmission.
In addition to direct sound transmission through the separating assembly into the receiving room, vibrational energy from the airborne source is transmitted as structure-borne sound through wall/floor junctions, and radiates as noise into the receiving space. This is called ‘flanking transmission.’ Apparent Sound Transmission Class (ASTC) rating includes for both direct transmission and flanking transmission.

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-9.10.3.1.A and B. It was agreed that:

  • noise protection in buildings could not be effective without addressing all the paths through which sound may travel;
  • the current STC and FSTC ratings did not relate sound transmission criteria to a parameter providing consistent performance levels or satisfying code objectives; and
  • changes to the sound requirements in the NBC were needed. (This information 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).

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.

Whether the rooms are side-by-side or one-above-the-other, there are three flanking paths along each junction for each of the four edges, giving a total of 12 flanking paths.
Whether the rooms are side-by-side or one-above-the-other, there are three flanking paths along each junction for each of the four edges, giving a total of 12 flanking paths.

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.

European studies and building code requirements suggest using a minimum ASTC of 50. The 2015 NBC requires ASTC 47, intended to provide a ‘status-quo’ with STC 50.
European studies and building code requirements suggest using a minimum ASTC of 50. The 2015 NBC requires ASTC 47, intended to provide a ‘status-quo’ with STC 50.

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.

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