An Investigation on the Use of Porous Natural Stone for Sound Absorbing Treatments

The introduction of sound absorbing surface treatments in aesthetically valuable spaces aimed at acceptable acoustic conditions and low reverberation times often results in aesthetic problems of integration. In these cases using natural stones as sound absorbers could significantly contribute to control the acoustic conditions without affecting the aesthetic image of the space. An interesting case is represented by “tufa blocks”, a porous sandstone typically encountered in the Southern regions of Italy. A surface of about 10 m2 made of such blocks having a density of 1700 kg/m3 was analysed in a series of reverberant chamber tests. First the material was assembled providing a continuous surface in order to measure the absorption coefficient, which proved to grow as a function of frequency reaching αsab values of about 0.30 at 4 kHz. Later blocks were assembled in order to create an air space behind them. In addition they were mounted creating vertical slots of different dimensions which, combined with different air gaps, allowed the blocks to work as resonant absorbers showing low frequency αsab values up to 0.73 at 200 Hz. Comparisons with theoretical models were finally carried out in order to predict and design the wall layout as a function of specific acoustic needs.