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 with an STC 50 are placed in the same category with a partition having an STC 54. There is no distinction.
What makes using the Gypsum Association ratings dangerous? Let's say the available lab tests for your wall range from STC 45 to STC 50, with an average of STC 47. If the GA manual references the STC 50 test and places it into the STC 50-54 category, you now have a wall with an average STC rating of 47 (based on multiple tests) that is represented in the GA manual as STC 50-54.
What happens when your wall is built in your new hotel, apartment, or condominium project? Field STC ratings are typically lower than lab ratings. This is due to many factors including imperfect construction and many variables that are controlled in a lab but not in the field. While it is somewhat arbitrary, 5 STC points has become the defacto standard for the allowable tolerance between a lab test and a field test. This is what the International Building Code allows with criteria of STC 50, or 45 if field tested, to separate multi-family dwelling units in hotels, apartments, and condominiums.
Now let’s say your wall must meet the International Building Code (IBC) requirement of STC 50 (45 if field tested). You may think that you have a bit of a safety margin since you thought you selected a wall rated at STC 50-54. We’ve shown that while your wall is STC 50-54 per the GA manual, it could actually be an STC 47 wall if you consider all of the available test data. It should not be surprising if your wall is tested in the field with a result of less than STC 45, which fails to meet the IBC requirement. Your wall is performing significantly worse than expected based on the STC rating you selected from the Gypsum Association manual. If poor sound isolation becomes a source of complaints, it will likely be more expensive to fix after the fact.
Whenever possible, research the actual lab test report for your partition. Ideally, find more than one test report. The consultants at Wave Engineering have access to hundreds of test reports for common walls and floor-ceiling assemblies. For less common partitions, we use calculations and assessment of similar assemblies to predict reliable STC ratings for your wall types and floor-ceilings. If necessary, we can provide an opinion letter that you can provide to a building official to justify your ratings.
Let us know if we can help!