Decoding STC, OITC, IIC, and other Ratings – No Perfect World

The Code of STC Ratings

Much like Fire Ratings, different building materials and different wall details provide different Sound Transmission Ratings (note: STC is the abbreviation for “Sound Transmission Class” and it is a single number descriptor that rates the airborne sound attenuation qualities of a building element (roof, wall, door).  A higher STC indicates a greater ability to reduce airborne sound through the assembly.

Understanding the required STC ratings can help determine what wall type is best suited to what situation (ANSI 12.60 has specific requirements based on what occupies the adjacent side of a wall or floor).

Different wall construction types yield very different STC Ratings. You may be surprised at the low ratings of typical stud wall construction. But there are subtle changes you can make to your wall types that will enhance their ability to contain sound and limit transmission. While some of these changes will impact construction cost, they will control sound and greatly enhance listening comfort.

Minimum STC ratings for a monolithic or composite wall, floor-ceiling, and roof-ceiling assemblies depend on the adjacent spaces. Example layouts representing core and ancillary learning spaces are shown in section 13, “Isolation- A Strategy to Limit Sound Transmission”. Tables 4 and B.1 of ANSI 12.60/Part 1 further clarify the requirements

Impact Insulation Class (IIC) rating: Impact noise from footfalls, rolling equipment, dropped items, or similar sources must also be addressed.  It is important to note that the IIC of an floor/ceiling assembly can be very different from its STC;  STC addresses airborne sound, and IIC addresses the  sound resulting from impacts on the other side of a construction*.

*walls are rated with STC while floor/ceiling assemblies are rated with both STC and IIC

IIC ratings of floor/ceiling assemblies shall have an un-carpeted IIC rating of at least 45 if they are located above core learning spaces and and IIC 40 if they are located above ancillary learning spaces.

Gymnasia, dance or other high floor impact activity shall not be located above classrooms or other core learning spaces for any new construction. If it is unavoidable in a retrofit/renovation situation the following IIC’s apply:

1. IIC 70 if located above a core learning space <20,000 cu. ft.

2. IIC 65 if located above a core learning space >20,000 cu. ft.

3. IIC 65 if located above an ancillary learning space

To achieve high IIC ratings it may be necessary to isolate the ceiling from the floor above. This can be accomplished by suspending the ceiling with resilient channels or isolation hangers.

For examples of both STC and IIC ratings of various constructions, see section x ” Simple Mitigation strategies”

Composite Construction and STC: While a wall or roof or floor has a STC rating, it is important to note that the rating is based on the construction being monolithic with no breaks, and sealed airtight at the edges.  If we put a door or a window in the wall it is considered composite construction and the STC rating of the wall changes.  The following table and example are an acceptable method to calculate the actual STC of sample composite constructions.

insert  table D.2 12.60-2006 and sample calculation

In addition, when making a rough estimate for STC of the design of outer shell of a building, you should employ the table below.

insert table D.1 12.60-2006

there is nothing here on OITC….hmmmn…cut it from the title or add more?

Figure M: Composite construction graphic representation. Note that the larger difference of the STC of the elements creates a significant effect on final STC. Better matched elements will save money and effort.

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