STC – Sound Transmission Class
This measures the sound that transfers through a wall, from space to space. The scale of measure for this is not as simple as NRC, an STC <35 is considered low while an STC >55 is high. STC is the result of an assembly and not based on a single component.
When deciding if you need a high STC wall look at the design, the highest STC rated wall in the world will not help if you have an open plenum where sound goes over the top of the wall.
Things such as off-set stud walls and resilient channel have been around a long time and certainly do work when installed correctly. Sound board has been around for a long time as well. It is basically mineral fiber in a large sheet, think 4’x8’ ceiling tile, and installed behind the drywall. See manufacturer’s literature for ratings by assembly.
Of course an extra layer of drywall often times works; don’t let the seams line up.
Another product that has been around awhile and is very effective is vinyl loaded noise barrier. The material is approximately 3/16” thick and comes in rolls and it weighs one pound per square foot. This can be applied to the studs behind one side of the drywall or for very severe cases woven between the studs in an off-set stud construction.
One of the newer products on the market these days are drywalls that have a layer of material in the middle that will help stop sound transfer. These products are more expensive than regular drywall of course but do a good job. These can be added to an existing wall, the board can be finished just like standard drywall. See the manufacturer’s literature for assemblies
Important things to know when designing a wall to stop sound: back to back electrical boxes are a super highway for sound – if you have them in an existing wall there are pads that can be put between them to help stop the transmission. Sound caulk should be used where the wall meets the floor, ceiling and corners. A room designed with all the best sound control in the world won’t make up for a door that doesn’t stop sound, or an open window. Sound through a small opening fans out into the opposite space.
One of the things I have just learned a bit about is something called the Triple Leaf Effect. This tends to go against our idea of just adding a bit of air space to a wall to help with sound transmission. It is a complicated subject but I will try to give you a brief idea of what it is.
A triple leaf wall is a wall with two air cavities rather than the typical one air cavity. Similarly, a quadruple leaf wall would be a wall with three air cavities. This applies to both walls and ceilings alike and is also referred to as a retrofit installation when discussing the installation of resilient sound clips in an assembly that will create a triple leaf. A leaf in a wall is a solid layer; like drywall or plywood, and a triple leaf wall has three leafs.
It is not illogical to presume that the quadruple leaf wall would have the best sound isolation. After all, the sound has to go through drywall, air space, drywall, air space, drywall, air space, and then again drywall often with insulation within each air space. In a double leaf wall it has to make it through only one air space, and in a single leaf wall there is no air space at all. While it makes sense that the quadruple or triple leaf assemblies are better than double leaf assemblies, it is unfortunately not the case. To understand why this is, we have to take a look at how decoupling works.
So I would suggest you look up triple leaf effect and see what it is all about.
Evaluating a project for STC needs in the design phase is certainly the best way to go, too often acoustical issues get cut back to save money. As with all these things it will cost so much more to fix the problem after the fact.
As always – please feel free to contact me for more information or product samples.
