Understanding the influence of impedance transitions of acoustically reflective vertical surfaces on the frequency-dependent effectiveness of absorptive ceilings in large non-Sabinian empty spaces

An unexpected phenomenon was observed during measurements of reverberation time in empty spaces to be fitted out, in which the only sound absorbing treatment consists of mineral fiber tiles on the ceiling, a material typically highly absorbing at mid and high frequencies during laboratory measurements: this involves a systematic increase of the reverberation time in octave bands between 1 kHz and 4 kHz in the measured empty spaces. The physical understanding of this surprising observation has focused on the question of the influence of impedance transition on sound scattering, and the influence of the frequency-dependent scattering properties of acoustically reflective vertical surfaces of different natures (glazing, plasterboard lining, masonry) of these empty spaces on the efficiency of a sound absorbing ceiling. These scattering phenomena were studied and modeled in a computer model in order to verify the model's adjustment hypothesis, an essential initial step before the study of the acoustic parameters in future fitted spaces and the effectiveness of acoustic solutions for space planning of open plane offices. Good consistency between the measurements and the empty model was observed when the frequency-dependent scattering behaviour of the vertical reflecting surfaces of different nature was taken into account in the simulation.