A Theoretical Model to Consider the Influence of Absorbing Surfaces Inside the Cavity of Balconies

A theoretical model is developed to predict the sound field inside non-rigid cavities in the low and middle frequencies. The main idea is to use the Green's function for rigid walls and replace the non-rigid areas by monopole sources, which have to fulfil the prescribed boundary conditions. Only a locally reacting impedance is considered. The model is applied in the case of balconies on building facades where parts of the balcony walls are covered by absorber material. The model shows good agreement with measurements. The reduction of the sound pressure in front of the window in the balcony is studied for different absorber locations inside the cavity of the balcony. Best results are achieved when treating the ceiling and the rear wall of the balcony. With this absorber configuration, an insertion loss between 4 and 7 dB can be expected inside a room behind the balcony when the window to the balcony is open. The model can be helpful to improve the insertion loss of balconies below 1 kHz and to optimise the use of absorbing material. It can also be applied to the more general problems dealing with the sound field in cavities where parts of the boundaries are non-rigid.