Sound Transmission Class Rating for Masonry Walls

By Mark A. Nunn

The Masonry Society Codes and Standards Committee recently completed a new standard for determining the sound transmission class rating for masonry walls. The new standard is in response to current activities stemming from the development of the first single model code, the International Building Code (IBC) 2000, scheduled for completion in September 1999. For any standard to be referenced in the IBC 2000 it must be developed through a national consensus process. This could be the first TMS standard to be included in any model building code other than the Masonry Standards Joint Committee (MSJC) Code.

The scope of the standard includes both clay masonry and concrete masonry wall systems. It defines the acceptable methods of determining STC ratings of masonry wall assemblies by either field and laboratory testing in accordance with standard test methods or by calculation.

The standard covers:

applicable referenced standards for masonry units
notations and definitions
a materials section which specifies the ASTM standards for masonry units, mortar, grout, and joint sealants
construction requirements
quality assurance
STC rating calculation procedure

Many of these requirements have been addressed in current industry publications such as BIA Technical Notes on Brick Construction and the NCMA TEK Series. This venture is the first major step in providing all requirements for masonry STC ratings into a single standard.

The calculation method has been in use for some time for concrete masonry wall systems. Clay masonry wall systems, however, have relied on the results of actual tests conducted in conformance with ASTM E90. An investigation was performed by the TMS Codes and Standards Committee to determine if one equation for calculating the STC rating would be acceptable for all types of masonry wall construction.

The results of that study verified that one equation can be used for both clay and concrete masonry assemblies. The STC rating can be determined from the following equation:

STC = 0.18W + 40

where:

STC = Sound Transmission Class rating
W = wall weight based on the average density of the masonry units, the weight of mortar, grout, loose fill materials, and the weight contributed by surface treatments

A commentary, which is not a part of the standard, accompanies the standard and is provided to assist the user in the application and understanding of the specific requirements included. It contains background information including illustrations, example problems, and clarification of the requirements of the standard.

This standard is a major achievement in the masonry industry. It provides designers with comprehensive requirements for determining the STC ratings of both clay and concrete masonry wall assemblies and is a document that can be referenced in the model building codes which establish minimum regulations for building construction.

It is anticipated that this new TMS standard will be available in late spring of 1997. Inquiries should be directed to:

William D. Palmer, Jr.
The Masonry Society
3970 Broadway, Suite 201-D
Boulder, CO 80304-1135
Phone (303) 939-9700 or FAX (303) 541-9215
Email 102516.540@compuserve.com