Empirical Model for Sound Transmission of Wood-Framed Assemblies
When designing residential buildings, it is crucial that architects and engineers have an understanding of not only the code requirements for floor-ceiling assemblies, but also the methods available to demonstrate compliance within these provisions. American Wood Council’s (AWC) goal is always to make the job of code officials and building designers easier – this time by providing an empirical sound transmission model.
The 2018 International Building Code (IBC) stipulates minimum performance requirements regarding sound transmission through common walls and floor-ceiling assemblies that separate a dwelling unit from either a public area or an adjacent dwelling unit. AWC has released a new report on sound transmission, “Technical Report 15 (TR15), Calculation of Sound Transmission Parameters for Wood- Framed Assemblies,” to provide another tool that architects and engineers can use to demonstrate compliance with these code provisions.
Compliance with code-regulated sound transmission parameters may be demonstrated either through laboratory testing, field testing or through engineering analysis based on empirical laboratory test data from other similar assemblies. This initial version of TR15 summarizes an empirical method of engineering analysis that can be used to estimate the code-regulated sound transmission parameters for wood-frame floor-ceiling assemblies. This method can be invaluable to building designers as it can help make performance more predictable, thus eliminating the necessity of running independent acoustic tests and ultimately saving both time and money that would otherwise be needed to conduct laboratory testing.
Specifically, the method described in TR15 can be used to estimate the following two sound transmission parameters, which are regulated by the IBC to establish minimum acoustical performance requirements:
- Sound transmission class (STC) is a measure of the attenuation of sound waves initiated as an air-borne sound as they pass through a wall or floor-ceiling assembly. The classification indicates the assembly’s ability to reduce unwanted noise transmission from air-borne sounds, such as amplified sound systems, human voices, animal noises or musical instruments.
- Impact insulation class (IIC) is a measure of an assembly’s ability to insulate against structure- borne sound waves generated when an object induces sound waves directly into the assembly, such as when an object strikes the opposite surface of the assembly. Examples of this might include sounds created by footfall (stomping, heavy walking, etc.) or an object hitting the floor.
For applicable floor-ceiling assemblies, Section 1206 of the IBC requires minimum STC and IIC ratings of 50. The tests used to determine these ratings must be conducted in accordance with code-referenced ASTM test standards. However, this section also allows for sound transmission performance of assemblies to be evaluated through an engineering analysis based on empirical test data from other similar assemblies – which is exactly how AWC’s TR15 model works.
Since research and testing has shown interlaboratory differences, often limiting the ability to predict better acoustic performance measurements, AWC used data from a single laboratory source, the National Research Council of Canada (NRC). To fill gaps for data on assembly configurations where the available data was either sparse or non-representative, AWC performed additional ASTM E90 and E492 tests at NRC on 31 assemblies which were not previously addressed by NRC data.
The scope of the models presented within the report are limited to estimation of STC and IIC ratings of floor-ceiling assemblies comprised of components addressed within the model, and framed with one of the following:
- Sawn lumber
- Prefabricated wood I-joists
- Metal-plate-connected wood trusses
Because IIC ratings can be greatly impacted by the type and characteristics of floor coverings, AWC conducted additional testing and analysis of the influence of floor covering on various floor-ceiling assemblies. The TR15 model was developed and validated with test data from over 100 unique assemblies in total.
Comparing the test results with model estimates, both STC and IIC estimates were remarkably accurate. While all model-estimated values were within three decibels of the measured values, 83 percent of estimations were within one decibel for both STC and IIC, with a slight tendency to underestimate for assemblies having multiple component variations.
The TR15 provides architects, designers and engineers with an empirical model that will enable them to demonstrate compliance of wood-frame floor-ceiling assemblies with code regulations, without having to run additional time-consuming and expensive tests. Building on this initial report that covers the materials previously listed, AWC hopes that future work can include focused testing on improved acoustic performance design options and expansion of the model to include floor-ceiling assemblies constructed with cross-laminated timber (CLT), as well as wall assemblies constructed with sawn lumber and CLT.
For more information on the American Wood Council and TR15, visit www.awc.org.