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The building codes set certain standards for insulation to affect the way sound carries through walls and floor-ceiling assemblies. It’s helpful for contractors to not only understand the sound transmission code requirements for floor-ceiling assemblies but also the methods available to demonstrate compliance in these provisions.

Building codes stipulate minimum performance requirements for noise transmission through certain interior walls, partitions and floor-ceiling assemblies. The International Building Code, the model code uniformly enforced in the United States, requires floor assemblies separating dwellings from each other or from public areas to meet minimum acoustical requirements for two parameters: Sound Transmission Class (STC) and Impact Insulation Class (IIC):

  • 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.
  • 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 include sounds created by footfall (stomping, heavy walking, etc.) or an object hitting the floor.

Compliance with these minimum requirements may be demonstrated either through expensive physical testing (costing several thousand dollars for each assembly configuration and potentially delaying projects) or through engineering analysis. The American Wood Council's (AWC) new report on sound transmission helps in the estimation of these sound transmission parameters, thereby facilitating the code-allowed engineering analysis alternative to time-consuming and expensive physical testing.

The “Technical Report 15 (TR15), Calculation of Sound Transmission Parameters for Wood-Framed Assemblies” summarizes how code provisions apply to floor-ceiling assemblies and demonstrates how wood-framed floor-ceiling assemblies can comply with a calculation-based analysis approach.

Since research and testing has shown inter-laboratory 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. AWC compiled the data and test results to provide an empirical sound transmission model as a tool that can be used to demonstrate compliance with code regulations, thereby eliminating the need to test every single assembly variation.

Comparing the test results with model estimates, 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.

Moving forward, AWC hopes that future work will build on this initial report to include focused testing on improved 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.

Along with release of the new model, a free eCourse is available online that provides an understanding of the calculation-based analysis approach. The new AWC course is accredited by the International Code Council for continuing education credit and is available here.

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