Interference effects in field measurements of airborne sound insulation of building facades

The limits of the procedure for the measurement of the sound insulation of building facades represent the main subject of this paper. To calculate the facade insulation, standard ISO 140-5 requires that the difference between outdoor and indoor sound pressure levels is determined. However, the measurement of outdoor pressure levels presents several critical challenges, not fully considered in the standard. In fact, the placement of an external source and receiver according to ISO 140-5 is flexible to permit adapting the measurement configuration to specific building characteristics. Unfortunately, according to the relative external source and receiver positions, destructive interferences among waves may occur in different frequency bands. Comparisons between different source-receiver combinations are hence investigated theoretically, before being compared with field measurements. The paper investigates circumstances which might lead to destructive interferences in the external final sound field. The increasing attention of reproducibility in building acoustics suggests an investigation of possible measurement errors that may occur. The influence of interference effects on the single number rating of the sound insulation is finally given.