Footstep noise and other impact sounds are common sources of annoyance in multi-family dwellings. Research has shown that the impact sound transmission ratings provided by standard tests rank floors differently than people would, mostly due to low-frequency thumping noises. This is especially true for wood joist and similar lightweight constructions with carpeted floors; the carpet ensures a good rating in the standard tests, but does little to attenuate low-frequency impact sound.
The data in this report was taken from a research project that investigated a possible new test for evaluating the transmission of low-frequency impact sounds through lightweight floor constructions.
Two types of wood joist floors, three types of wood truss floors and one solid concrete floor were tested as shown in Table 1. As well as testing the basic floors with the top surface unfinished (bare), five different floor toppings were placed on the floors: a carpet, a carpet and underpad and three types of floating floors (Table 2).
Table 1 includes a summary of the Sound Transmission Class (STC), Impact Insulation Class (IIC) and Tapping Machine Rating (TMR) for the assemblies tested. The TMR is the new single number suggested by the researchers to replace the impact insulation class and the weighted impact sound index.
It was found that the addition of soft floor coverings or a floating slab does not always give the same improvement for all floor types. If, however, floors of the same general type are considered, the improvement is roughly the same. Therefore, while there are some deviations, the following are useful approximations for wood joist or truss structures: adding carpet improves IIC by about 8 and TMR by 2 to 5; a carpet and underpad improves IIC by about 25 and TMR by about 15; Float1 and Float2 improve IIC by about 12 points and TMR by about 15.
The only system to consistently show the capacity to reduce vibration transmission to a floor and the retransmission of the noise is that involving the installation of 14 mm thick neoprene pads under the appliances' support points. This system also eliminated the resonance tones in most cases.
As the 1990 National Building Code (NBC) does not recommend a minimum value for impact insulation class, the researchers recommend a minimum value of IIC 55, obtained when the floor is tested without carpet. They also recommend a minimum STC 55; this value is greater than the STC 50 recommended in the NBC to provide a margin of safety which allows for degradation due to construction and design errors that introduce flanking transmission. Another study has demonstrated that people living in buildings with STC 55 are satisfied and less likely to complain. Also recommended is the use of the TMR with a minimum value of 50. These recommended values and the results presented in Table 1 can be used to select construction for use in multi-family buildings.
Project Manager: Jacques Rousseau
Research Report: Sound Transmission Through Floors
Research Consultant: Institute for Research in Construction, National Research Council Canada
full report on this research project is available
from the Canadian Housing Information Centre.
The information in this publication represents the latest knowledge available to CMHC at the time of publication, and has been thoroughly reviewed by experts in the housing field. CMHC, however, assumes no liability for any damage, injury, expense or loss that may result from use of this information.
Table 1. Basic Floor Constructions Tested and Summary of STC, IIC and TMR Ratings for Main Series of Measurements
Table 2. Floating Floor Descriptions
©1999 CMHC-SCHL. All rights reserved.