Sound reduction index measurements were performed inside a 1:10 scale model,
reproducing two adjoining rooms of 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 entirely built in 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. A good 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 constituted by two leaves,
connected each other only by a border frame running along the whole wall perimeter. It has been
found that the effect of the border frame is a reduction of the performance of the double wall
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 has been found that the presence of the studs causes a general decrease of the sound reduction
index values and a decomposition of the critical frequency.
A fourth 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).