Neves e Sousa A., Gibbs B.

The present paper reports on a study of low-frequency impact sound transmission in buildings that was sponsored by the Portuguese Foundation for Science and Technology (FCT). It is recognised that in general, building elements such as walls and floors fail to comply with performance standards at low frequencies. This problem is exacerbated by the modal behaviour of the pressure fields in small often rectangular rooms, and in the vibration fields of intervening walls and floors, which results in large spatial and spectral variations in impact sound level and airborne sound level difference. These variations are not taken into account in the present national and international standards and a measurement method has yet to be confirmed for very low frequency sound transmission. In this study of impact sound transmission, a parametric survey has been performed, using an experimentally validated analytical method, on the impact sound transmission between dwellings in the frequency range 40 – 200 Hz. Results are presented as narrow-band and one-third octave band values and conclusions refer to the relative importance of factors such as the type of excitation (footfall, standard impact sources), the location of excitation, the type of floor (solid or with an isolated layer), the edge conditions, the room dimensions, the absorption and the listener position. The expected variances in impact sound pressure level, due to the range of room volumes encountered in dwellings, are given for typical floor types and a proposal is given on how the low frequency behaviour can be incorporated into present standards.