No, IIC and STC are not the same. While both are metrics used in acoustics to measure sound transmission properties, they address different types of noise and evaluate different elements of a building structure.
Understanding Acoustic Ratings: IIC vs. STC
When evaluating sound control in buildings, two common ratings you'll encounter are STC and IIC. These acronyms represent different standards for measuring how well building materials or assemblies block sound.
What is IIC?
IIC stands for Impact Insulation Class. This rating specifically measures how well a floor-ceiling assembly reduces impact sound. Impact sound is the noise generated when an object directly strikes a surface, such as footfalls, dropped items, or furniture moving.
The reference states: "IIC stands for Impact Insulation Class. This is the rating given to flooring assemblies and determines how well they mitigate impact noise (footfall mostly)."
A higher IIC rating indicates that the floor-ceiling assembly is better at preventing impact sounds from traveling to the space below. Factors influencing IIC include the flooring material itself, the underlayment used, the subfloor, the ceiling structure, and any insulation present between floors.
What is STC?
STC stands for Sound Transmission Class. This rating measures how well a building element or assembly blocks airborne sound. Airborne sound travels through the air and includes noises like voices, music, television, or traffic.
Common building elements rated with STC include:
- Walls
- Ceilings
- Windows
- Doors
A higher STC rating means the assembly is more effective at reducing the transmission of airborne sound.
Key Differences: IIC vs. STC
The fundamental difference between IIC and STC lies in the type of sound they measure and, crucially, what is being rated.
| Feature | IIC (Impact Insulation Class) | STC (Sound Transmission Class) |
|---|---|---|
| Type of Noise | Impact sound (footfalls, dropping objects, etc.) | Airborne sound (voices, music, etc.) |
| Measurement | How well a floor-ceiling assembly mitigates noise | How well an assembly blocks airborne sound |
| What is Rated | The flooring assembly as a whole | Typically rates materials or assemblies |
| Common Use | Floors and floor-ceiling assemblies | Walls, ceilings, windows, doors, assemblies |
As highlighted in the reference: "As opposed to STC, IIC rates the flooring assembly as a whole, not just the material on its own." This is a critical distinction. While an STC rating might apply to a wall material or an entire wall assembly, an IIC rating is specifically designed to evaluate the complete layered structure of a floor and ceiling system and its ability to absorb or block impact vibrations.
Practical Applications
Understanding the difference between IIC and STC is crucial in construction and renovation projects, especially in multi-story buildings like apartments, condominiums, or offices.
- For reducing noise from footsteps above: Focus on improving the IIC rating of the floor-ceiling assembly. This might involve installing a high-quality underlayment, choosing specific flooring materials (like carpet), or adding insulation and resilient channels to the ceiling below.
- For reducing noise from talking or music from a neighboring unit: Focus on improving the STC rating of the wall or floor/ceiling separating the units. This could involve adding mass (e.g., extra drywall), using resilient channels, sealing gaps, or adding insulation.
Both ratings are essential for achieving effective sound control and creating quieter, more comfortable living or working environments. However, they serve different purposes and address different noise sources.
