Vince Bailey / November 2017
No one dared disturb the sound of silence.—Paul Simon
We all make a lot of noise. It seems inherent in all of our post-modern endeavors—our collective fabrications, our pursuits, our transportation, our communication, even our leisure activities are all great producers of sound and fury. Our urban centers are concentrated islands of noise, and our construction activities are perhaps the most robust contributors to that commercial din. While noise is the natural byproduct of positive human activity, I do note that a vigorous resistance to the cacophony that assaults our ears also seems to be the natural response. We seek peace and quiet when the racket becomes too much to bear and seems inescapable. We slip on a pair of noise-cancelling headphones, or search for silence in an acoustically conditioned room.
However, there is a certain cost for the stillness of that quiet room—a significant cost that wall and ceiling contractors are well aware of. Specific levels of silence are achieved through special assemblies designed for that purpose. Sound transmission coefficient is an integer system that rates how well a partition, ceiling or floor attenuates airborne sound. The higher the number, the greater the reduction of sound transfer through the assembly. For example, partitions with an STC rating in the 30s allow ordinary speech to be audible from one side to the other, while partitions with an STC rating in the high 50s render loud sounds virtually mute.
Higher STC assemblies are demanded in many types of projects, depending on their intended purpose. These include schools, hospitals, hotels and office buildings where sound transfer between rooms can be distracting, or where privacy is required. Sound attenuation can be achieved through a number of supplements to a standard partition including fiberglass sound batts, rockwool or blown insulation added to the cavity. Multiple layers of board, acoustical caulk and resilient channel are also common elements in noise-reduction design.
Frequently, these elements will be designated on the “wall types” page of a typical plan set along with the STC rating the assembly is supposed to achieve. There are several possible pitfalls here that even a canny bidmeister might not see through the murk of ambiguity: 1) the elements designated for the partition do not achieve the noted STC rating; 2) the STC ratings are assigned in the specs, but the elements for achieving them are not designated on the plans; 3) the architect/owner wants suggestions for upgrading the STC ratings for their sound partitions.
Typically, STC ratings are noted with each wall type on the wall-types page, and the elements for achieving that rating are depicted. For instance, a standard 3 5/8” full-height partition with single layer drywall on each side, fiberglass sound batts and acoustical caulk (top and bottom track) is shown and designated as rated STC 44. Now, a good wall-and-ceiling bidmeister wears a lot of hats, but acoustical engineer is not one of them. That assembly may or may not achieve the designated STC. The silver bullet for this crouching tiger is to always include a disclaimer in the proposal stating assemblies are bid strictly per plan and we are not responsible for outcomes of STC testing, or words to that effect. This way, any shortfalls in post-construction testing are the responsibility of the design team, assuming the completed assemblies follow the plan design accurately.
The second and third scenarios from above are similar in that the design team is requesting that the drywall sub create an assembly that meets their requirements in both instances. Again, wall-and-ceiling exactimators are not acoustical engineers, and making design suggestions puts them on dangerous ground, especially in conditions where extreme sound reduction is critical. But when cooperation might enhance the chances of an award, a hungry estimator will cave and accommodate. When he does, there are a number of sources he may refer to for guidance. The principal manufacturers of gypsum products have published charts and diagrams that indicate which assemblies achieve what STC rating. For instance, a wall utilizing 6-inch, 20-gauge studs at 24-inch on center spacing with resilient channel on one side, two layers of half-inch drywall on the RC1 side, one layer on the opposite side, and 5 inches of rock wool in the cavity merits an STC rating of 60, according to the USG Acoustical Assemblies brochure. Citing such a source promotes confidence that the assembly will perform as expected and helps to alleviate the sub’s responsibility. Even better protection can be had by obtaining an engineered judgment from a manufacturer that offers such a service. (I recently obtained an STC-rated EJ from a rock wool manufacturer.)
But even though rated assemblies are rigorously tested to perform, acoustical tests performed in the field, post-construction can produce skewed results. That’s why even with the protection of cited sources, the best defense against having to demo and rebuild a wall (or a number of walls) is for estimators to always include the aforementioned disclaimer in the qualifications section of every proposal. Because demolition is a remedy that a design team is not likely to invoke if the responsibility rests with them.
Vince Bailey is an estimator/project manager working in the Phoenix area