Scale model investigation on the influence of boundary conditions on the airborne sound insulation of lightweight double walls

Sound reduction index measurements were performed inside a 1:10 scale model of two adjoining rooms of a test facility in an acoustic laboratory, with the aim of investigating the influence of the boundary conditions on the acoustic performance of lightweight double walls. The model was built entirely of polymethyl methacrylate (PMMA). The specimen double walls were built using the same acrylic material. At first, measurements were performed on a single homogeneous panel, in order to check the reliability of the whole procedure and the repeatability of the measurements. Agood correspondence was found between theoretically calculated values and measured values of the sound reduction index above the critical frequency. A second set of measurements was carried out on model double walls comprising two leaves, connected to each other only by a border frame running along the whole wall perimeter. It was found that the effect of the border frame is a reduction of the performance of the double wall, to under that of a single plate with the same total surface mass. A third set of measurements was carried out inserting vertical studs to connect the two leaves. It was found that the presence of the studs causes a general decrease of the sound reduction index values and a decomposition of the critical frequency. Afourth set of measurements was carried out on the same double wall using border frames made of different materials. The experimental results show that damped materials (wood, rubber) give a better performance than rigid materials (acrylic, aluminium).