Keywords: structural coupling, lightweight periodic structure, flanking noise transmission, diffuse-field excitation, finite element method.

Noise propagation in lightweight building structures is an issue that needs attention because of the current building trend. Direct sound transmission through various couplings in periodically connected lightweight panels and flanking noise transmission through varying periodic stiffeners in lightweight panels have been predicted using the finite-element method in previous work by the authors. It was deduced that, loose couplings do not necessarily transmit less sound, which is a result of a higher mode count in the investigated frequency range. Furthermore, the overall vibro-acoustic behaviour of a lightweight panel structure can be significantly changed by altering the positions of the internal stiffening ribs.

In this paper, a finite-element model is utilised for the analysis of flanking noise transmission in a lightweight panel structure consisting of two plates with internal ribs using various joints between the studs and plates. Four different coupling configurations are considered: 1) all structural contact points are completely tied; 2) only nodes on the centre lines of the structure are tied; 3) a narrow strip of 5 mm, 1 cm and 2 cm tied elements connect the frame to the plates; 4) evenly spaced discrete elements are tied. In all cases the interaction between non-tied elements is neglected.

The structural behaviour has been analysed for diffuse-field excitation on the surface of the source wall in the frequency range below 500 Hz. The first structural modes occur at 67 Hz, 68 Hz, 76 Hz, 81 Hz, 86 Hz and 89 Hz, respectively, for modes in which the stud and plates are coupled through line coupling, discrete coupling, 5 mm glued coupling, 1 cm glued coupling, 2 cm glued coupling and total surface coupling, respectively. It is illustrated that a panel structure with total surface coupling between stud and plates behaves stiffer than other panel structures. An increase of the modal density in all the panel configurations is seen at the higher range of frequencies.

With diffuse-field excitation, there are higher peaks at low frequencies which are the result of exciting the first structural modes in the respective panels. Similar behaviour is observed in all of the panels in the frequency range from 100 Hz to 200 Hz. A sudden increase of vibration transmission is seen above 300 Hz for all of the panels, which the result of a higher modal density. The panel structure with surface coupling between the studs and the plates is transmitting a higher amount of vibration than other panel structures at higher frequencies. An interesting observation is made in the cases of 1 cm glued coupling and surface coupling: A sudden decay and rise in vibration transmission is seen despite of modes occurring in the frequency range from 350 Hz to 470 Hz frequency range. This arises because several of the structural modes do not allow transmission of vibration in the direction towards the receiving wall surface.

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