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:
- “Masonry and Concrete Constructions with Rigid Junctions” (i.e. loadbearing masonry walls with cast-in-place [CIP] concrete slabs);
- “Non-rigid Junctions in Masonry and Concrete Buildings” (i.e. non-loadbearing masonry walls with CIP concrete slabs);
- “Adding Linings to Walls, Floors, Ceilings in Concrete/Masonry Buildings;”
- “Buildings with Lightweight Framed Wall and Floor Assemblies;” and
- “Buildings with Hybrid Construction,” including concrete masonry walls with lightweight framed floors and walls.
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.