In addition to airborne sounds from within the room, floors are also subject to impact sounds from people walking or jumping, objects being dropped or machinery. Impact and airborne sounds are transmitted through the floor/ceiling system in the same way as they are through wall systems. If there are rigid mechanical connections between the floor and the wall then these impact sounds can also travel into the wall and throughout a building. The transmission of impact sound through a floor can significantly reduce its sound performance even though the STC rating of the floor may be acceptable. The impact sound transmission of a floor can be rated using the Impact Insulation Class (IIC) number.
The impact sound performance of floor/ceiling systems can be improved by:
- Dividing a floor into two isolated or resiliently connected layers;
- The addition of carpet and carpet underlay to the floor surface;
- Isolating bare upper surfaces from the structural flooring;
- Having no direct fixing between floor surface and the lower ceiling and
- Increasing the floor's mass.
There are a number of certified fire-rated floor/ceiling systems that provide excellent IIC and STC ratings. A complete list can be found in MRTFC manual 2.
STC, IIC and fire ratings are improved by several means, including cavity insulation, resilient attaching channels for the ceiling, multiple layers of plasterboard, floor coverings, and floating floors made by sandwiching a 12mm layer of Caneite between layers of structural plywood flooring. A simple bare floor/ceiling system with 50mm of insulation and a single layer of 13mm plasterboard attached with resilient channels has STC and IIC ratings of 53 and 48 respectively. Addition of carpet and underlay, a floating floor system and an extra layer of plasterboard increases the STC and IIC ratings to 58 and 69.
References: Timber Datafile P6 – Timber – Sound Control of the NAFI Timber Manual.
National Association of Forest Industries: Multi Residential Timber Framed Construction Manual 2.