Problems and possible solutions in the evaluation of laboratory airborne sound insulation at low frequencies

Airborne sound insulation laboratories with typical volume between 50 m3 and 80 m3 are characterized by a non-diffuse field in the low-frequency range (50-100 Hz). New approach and measurement methodology are necessary in the evaluation of sound insulation with respect to ISO 10140:2010 standard. In the coupled system room-partition-room, in addition to natural modes of each system component, modal transmission from source to receiving room through the partition is observed. A first characterization of spatial distribution of natural and transmitted modes shows large spatial variation and maximum sound pressure levels in the 3-D corners of the rooms. A modal sound insulation, different from sound reduction index for diffuse field, is defined as the point-by-point difference between the maximum sound pressure levels of source room modes that occur in both source and receiving rooms. Different methods, measurement procedures, normalization terms and related problems are investigated to get a correct evaluation of modal sound insulation and a discussion on the meaning of such index is developed. Furthermore a new method to extend sound insulation measurement to all frequencies between 44 Hz and 112 Hz (the lower and upper bounds of the 50 Hz and 100 Hz third octave bands respectively) is proposed.