What an STC number is, and what it is not
Sound Transmission Class is a single number that describes how well a wall or floor blocks airborne sound. It is calculated under ASTM E413 from measurements made under ASTM E90, which places a sample in an opening between two rooms and measures how much sound gets through. A higher number means less sound passes.
An STC rating for drywall partitions is a lab rating. The test wall is built by the laboratory's crew, the perimeter is sealed, and there are no doors, outlets, ducts or pipes in it. The number belongs to that specimen. A finished wall in a building is a different object, and it can test lower for reasons the rating never included.
STC also says little about low frequency noise such as mechanical rooms, bass from a restaurant or truck traffic. If that is the problem on your project, ask the acoustical consultant whether a single number is enough.
What the tested assembly assumes
A listed acoustical assembly is a recipe, and every item in it is part of the result. Board type and thickness, number of layers, stud depth, stud gauge, spacing, insulation in the cavity and any resilient channel or isolation clip were all fixed when the wall was tested. Change one and you are no longer building the tested wall.
The published guidance from board manufacturers shows how much each item moves the number. In one National Gypsum example, adding cavity insulation to an uninsulated wall raised the rating by 8 points. The same source notes that 24 inch stud spacing tends to test higher than 16 inch, that lighter gauge studs tend to test higher than heavier ones, and that spray foam does not do the job of fibreglass or mineral wool in the cavity. Those are trends from one manufacturer, not rules. Read the listing for the wall you are actually building.
Field conditions that lower the number
Sound follows the easiest path. A wall with a good rating and one weak point performs like the weak point. These are the conditions that come up most often on commercial and multi unit work.
| Condition | What goes wrong | How it is prevented |
|---|---|---|
| Back to back electrical boxes | Two openings in the same stud bay give sound a direct route through the wall | Stagger boxes into separate bays and seal around each box |
| Wall stops at the ceiling | Sound travels over the wall through the plenum | Run the partition to the underside of the deck where the drawings call for it, and seal the head |
| Unsealed perimeter and penetrations | Small gaps at the floor track, the head and around pipes and conduit let sound leak | Acoustical sealant at the perimeter and around every penetration, before the second layer goes on |
| Rigid connection across a decoupled wall | One screw through a resilient channel into the stud ties the two faces together and cancels the decoupling | Correct fastener length and placement, and inspection before the board is covered |
| Substituted insulation | A thinner batt, a different material or a partly filled cavity removes the absorption the test relied on | Install the specified product, fitted snugly, and get any change approved in writing |
None of these needs unusual skill. They need someone to own them, and to check them before the wall is closed. Once board is on both sides, most of them cannot be seen or fixed.
Sealant, and which sealant
Acoustical sealant stays flexible, so it keeps its seal when the deck deflects or the wall moves a little. That is why it is used at the perimeter of acoustical partitions and at penetrations. It is not the same job as fire caulk, and one product should not be assumed to do both. On a rated and acoustical wall, the firestopping scope and the acoustical sealant scope should each be written down, so neither trade assumes the other has it.
Sealant does not replace mass or decoupling. It protects them. A wall with the right board and framing can still lose a lot of performance to a gap along the bottom track.
How to protect the rating on the job
Three habits do most of the work. First, put the acoustical walls on the wall schedule with their assembly references, and mark where each one ends. Second, coordinate the electrical and mechanical trades before rough in, so boxes and penetrations are placed with the wall type in mind. Third, inspect the framing, insulation and sealant before the second face is boarded, while a fix is still cheap.
We self perform steel stud framing, insulation and boarding on partitions, and we seal to the assembly the drawings show. Send the wall schedule and the acoustical assembly references, and we will price them as drawn. If a substitution would help the schedule, we will ask the designer before we make it.