Paradigm shift in acoustic regulations

Photo © BigStockPhoto.com
Photo © BigStockPhoto.com

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,” and IRC-IR-811, “Detailed Report for Consortium on Fire Resistance and Sound Insulation of Floors.”  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.

Figure 1: The sound transmission class (STC) rating quantifies the sound transmitted directly via the separating wall assembly, as shown on the left. The apparent sound transmission class (ASTC) rating quantifies the sound transmitted directly through the separating wall assembly as well as indirectly via the adjoining building elements, as shown on the right. Images courtesy National Research Council Canada
Figure 1: The sound transmission class (STC) rating quantifies the sound transmitted directly via the separating wall assembly, as shown on the left. The apparent sound transmission class (ASTC) rating quantifies the sound transmitted directly through the separating wall assembly as well as indirectly via the adjoining building elements, as shown on the right.
Images courtesy National Research Council Canada

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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