Accuracy of geometrical formula for predicting the insertion loss of balconies in two dimensions

A geometrical formula which includes the effect of first order reflections, diffraction, and a diffuse reverberant field is presented to predict the acoustic insertion loss of rectangular balconies on a building facade. This method is compared to two-dimensional boundary element predictions for numerous geometrical configurations. Results are presented as a function of the height of the shadow zone on the facade due to varying source position, with various balcony depths investigated. With a reflective ceiling or solid balustrade present, the effect of the modes is shown to make the diffuse assumption inaccurate at low frequencies. The frequency of each mode is compared to the analytical solution for the quasinormal modes of a baffled rectangular cavity, finding that this approximation is effective to predict the frequencies of the balcony modes. The understanding of the contribution of the direct and reverberant components inside the balcony cavity offers an explanation to the possibility of decreasing insertion loss with elevation angle which has been observed in three-dimensional scale model experiments in the literature and contradicts the insertion loss values recommended by ISO 12354-3:2017.