Acoustic characterisation of ETFE cushions and their impact on the acoustic comfort of building users

Nowadays, many buildings are equipped with ETFE membranes and cushions. ETFE is a fluoropolymer that can be extruded to form membranes. These membranes are commonly used to construct visually transparent building claddings. Their use for covering large indoor spaces is relatively common. The design benefits include design flexibility, designing with less support material due to the reduced weight and an increased lifespan of the cladding system. Despite this, their acoustic characteristics, and their impact on the perceived acoustic comfort in indoor spaces are poorly understood. It is known that these membranes exhibit some acoustic transparency. This transparency can be regarded as an equivalent to absorption from the point of view of room acoustics, hence the use of the term "effective absorption" in this research. The more transparent to sound these membranes are, the worse the isolation performance but the lower the reverberation time in the observed room. A lower reverberation time is often well correlated with an increase in acoustic comfort; therefore, it is worth examining the impact of this transparency on the wellbeing of building users. The theoretical approximations, laboratory tests and in-situ measurements made in this PhD research, show good agreement. ETFE membranes exhibit a high acoustic transparency at low frequencies that gradually decreases with increasing frequency. This frequency dependent transparency or effective absorption (from the point of view of room acoustics), is influenced mainly by the membrane thickness in the case of ETFE membrane structures. The thinner the membrane, the more transparent to sound it becomes. The performed tests have shown that the thinner membranes used in practice today are substantially more transparent to sound than the thicker ones. The focus of this research was on ETFE cushions, which are multi-layered inflated structures. According to tests in both a reverberant room and transmission room configuration the internal cushion overpressure did influence the effective absorption coefficient of the system, in so far that it changed the cavity depth (distance between membranes). This is consistent with several sources in literature. The tension in the membranes, which increases with increasing internal overpressure, did not seem to have any influence on the transparency of either membrane or cushion ETFE systems, despite its influence on the speed of bending waves and on the structural reverberation ti