Beware the Gypsum Association Manual STC Ratings

Beware the Gypsum Association Manual STC Ratings

Do you select Sound Transmission Class ratings from the Gypsum Association Fire Resistance Design Manual? The GA manual contains a lot of good information on fire and sound ratings. The sound ratings, however, must be used carefully. Knowledgeable acoustic consultants do not rely on the GA manual Sound Transmission Class ratings, but use laboratory test reports (multiple reports when available). We may also supplement the lab test reports with field test reports and calculations. Our goal is to determine the most likely average rating for a wall or floor-ceiling. STC tests, even in a laboratory, are not perfectly repeatable. The same wall can be tested in five different labs with five different results. The same wall can be tested multiple times in the same lab and yield different results. Wall construction isn’t perfect and the test method isn’t perfect. Multiple tests of the same wall usually follow more or less a bell curve distribution.     The Gypsum Association categorizes a partition based on one lab test result. The GA categories are typically STC 45-49, 50-54, 55-59, etc. The STC range given by GA is not the range of STC test results for a particular wall. It is simply the category that GA has assigned that wall to. GA gives no indication whether a particular wall falls at the low or high end of that category and a 5-point difference in STC ratings is significant. Unless you research and find the specific lab test referenced by Gypsum Association, you won’t know if your wall’s test is at the high end or the low end of the GA category. Partitions...