Author:

Abstract:

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 time. The research yielded spectra for the effective absorption coefficient that can be used by practitioners to accurately model the acoustic field in rooms covered by ETFE cushions and membranes, this is information that was previously not publicly accessible. With this information, the impact of ETFE cladding systems on the indoor acoustic comfort perception was investigated. First, a limited on-site survey in an ETFE covered building was executed. It revealed that besides very common thermal comfort complaints in one ETFE building, noise complaints were more common in the ETFE covered areas than in the non-ETFE covered areas of the building. Indeed, using ETFE cushions, despite their high transparency, is not a guarantee to avoid noise complaints in every building. In the past, little conclusive work has been done on the influencing factors that determine acoustic comfort in large indoor spaces. The current paradigm in room acoustics, which is to provide room acoustic parameter target values depending on the function of the room is difficult to apply in rooms with a high variety of functions and with a large volume. Following the survey, subjective listening tests were executed using auralised sounds generated using room acoustic models. These room acoustic models were varied in such a way that the absorption coefficient of the cladding material was varied, leading to different sounds that were used in a comparative test. The effective absorption coefficients obtained during the characterisation phase of the research were used here. To provide a reference point, the absorption coefficient of glass panes was also applied to one of the virtual models and its resulting sound was included in the test. Glass panes (generally not acoustically transparent), as a visually transparent surface, are a design alternative to ETFE membranes and cushions. This comparison is most useful for architects wishing to observe the impact of their choice of cladding material. A parametric set consisting of combinations of room geometries and a change in absorption coefficient of the transparent surfaces (the cladding) was created (involving multiple types of stimuli at two distances). The results of this discrimination listening test (ABX) revealed that there is a substantial difference in perception of sound between rooms covered in ETFE and rooms covered in glass. This is true, to a lesser extent, for rooms covered in ETFE cushions with various (slightly) altered effective absorption coefficients. The difference is more noticeable in rooms with either more cladding surface area (proportional) or with smaller volumes. Statistical analysis showed that the difference of traditional room acoustic parameter (T20, C50, C80, G, Glate) values were well correlated with the scores of participants. To target the perception of acoustic comfort specifically, a two-part test, using two approaches, was also devised. There was a specific emphasis on examining the difference in comfort perception between rooms covered in ETFE cushions and rooms covered by glass panes, hence only two absorption coefficient values were employed in the making of the models. The set of stimuli presented to the listeners were auralised in two virtual room geometries with two different synthetic soundscapes (one consisting of a multi-talker environment and the other being one with intermittent realistic noises). A set of 10 soundscapes was selected for this two-part test. The first part of the test consisted of another discrimination test (ABX), in which participants were actively listening to the soundscapes to try to discriminate between them. The scores of this test were very high, indicating that even in the presence of noise, the difference in sound between rooms where ETFE cushions or glass is applied to the transparent surfaces is very noticeable. The second part of the test was designed such that participants were subjected the same noise/soundscape (but for longer) while they were asked to hold a conversation with the test examiner. The results of this test led to the conclusion that the difference in the perception, observed in the first part, was not observed in the speech comfort test (was not statically significant anymore). It seems that the context of a conversation between two people, diverts attention away from the room acoustic effect of a cladding material on the soundscape. This distraction that caused a perceivable difference while listening actively (first part), under these circumstances, did not lead to a measurable change in the self-reported acoustic comfort while listening passively (second part).