Investigation of sound transmission in lightweight structures using a waveguide finite element/boundary element approach

Abstract

The use of lightweight construction in building applications offers flexibility in use and ease of construction but often goes hand in hand with reduced sound insulation. Regarding this issue, this thesis investigates sound transmission behaviour of such structures. A numerical model is developed using a coupled waveguide finite element-boundary element (WFBE) method to predict the transmission loss (TL) of more complex structures and is applied to double panel systems. Initially, analytical waveguide models for a plate strip are developed. These models are used to gain insight into the vibro-acoustic behaviour of such a structure, particularly compared with an infinite system, as well as for validating the WFBE method.

Compared with results for an infinite double panel system, the finite extent in one direction of the waveguide system introduces some features in its TL. One of them is the presence of lateral cavity modes. These introduce additional stiffness to the air in the cavity so that the mass-air-mass resonance frequency of the waveguide structure shifts to higher frequency. Such additional stiffness reduces the overall transmission loss. This tendency is confirmed by measurement results. Another aspect related with the finite width is the presence of internal coincidence phenomena which cause dips that are not related with cavity resonance and are also independent of incidence angles. Moreover, a higher TL is found for the waveguide double panel partition at low frequencies as the finite width system radiates less efficiently than the infinite plate model. The results obtained also confirm that the dissipative mechanism behaviour found in the structure originates from the cavity rather than from the panel as postulated by London.

The effect of studs connecting the two leaves of the double panel system is also investigated. The effect of the air in the cavity becomes less significant with increasing frequency for the case of stiff studs so that the stud behaviour is predominant at high frequency. However, for more flexible studs lateral cavity modes and the internal coincidence effect become more significant and reduce the sound transmission loss. Therefore, for the case of elastic steel studs where no sound absorbent material in the cavity, both the transmission paths need to be handled carefully in order to achieve a good prediction of TL.

Comparisons of the numerical model results and measurements suggest that inclusion of an appropriate cavity loss factor is important to achieve accurate results particularly when sound absorbing material is absent from the cavity. A reduced air stiffness also needs to be considered to account for practical considerations. Moreover, it is of importance to include the detail in terms of elastic stud geometry in order to have a more representative stiffness. The comparison results also indicate that numerical models based on the WFBE method are able to produce good prediction results.

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Identifiers

ORCID for D.J. Thompson: orcid.org/0000-0002-7964-5906

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