Keywords: low frequency, floor, vibration, impact sound, natural mode analysis, modal coupling.

The methods provided in current standards for prediction of impact sound transmission fail at low frequencies because both construction elements and sound fields exhibit a modal behaviour. Thus, prediction of low frequency sound transmission generally requires numerical modelling. Although the Finite element method (FEM) is the most widely used method, analytical models are also available.

In the present paper, a comprehensive method based on natural mode analysis, which has been validated either by comparison with laboratory and in-situ tests or with FEM results, is provided for studying low frequency impact sound transmission in dwellings. The method is as much accurate as other methods, such as FEM, and has the advantage of being faster and more easily adaptable to the needs of an extensive parametric survey on the factors affecting low frequency impact sound transmission [1].

The parametric survey provided relevant information for the acoustic design of floors but also highlighted the strong modal behaviour of floors and sound fields at low frequencies. Thus, the thicker floor, in the case of homogeneous floors, does not necessarily corresponds to the best floor solution and therefore the optimal acoustic design must be found by testing a set of possible floor thicknesses for a specific room. Although this can be done quite easily with the proposed method, there are other parameters which have to be controlled and they must be chosen carefully. The factors identified by the parametric survey as less important for the floor acoustic performance can be simplified in the analysis and only the most important factors have to considered, such as the floor thickness or the dynamic stiffness of the resilient layer when floating floors are installed.

As the model can be used also to predict the normalised impact sound pressure level, L_n (dB), the best floor design can be obtained by comparing L_n spectra with the "room sound quality" rating curves proposed by Broner [2]. More research on other criteria for identifying the best floor performance at low frequencies is still required.

In order to illustrate the application of the model to the acoustic design of floors, a simply supported concrete floor was considered on the top of a room. A point force was applied to the floor. A floating floor was also considered as this type of floor covering is generally used to control impact sound transmission. The criteria described above were applied to this floor in order to select the optimal floor thickness and also the dynamic characteristics of the floating floor.

1

A. Neves e Sousa, "Low frequency impact sound transmission in dwellings", PhD Thesis, The University of Liverpool, Liverpool, UK, 2005.

2

N. Broner, "Low frequency sound quality and HVAC systems", Proceedings of Inter-Noise 94, Yokohama, Japan, 1101-1104, 1994.

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