# Acoustic Shell Optimization in Opera Houses for Concert Halls
## 1 Introduction
Many different types of wood and finishings can be chosen, including fire-retardant wooden materials (Figure 1). Acoustic shells also support rehearsal activities. For musicians' practice, in fact, many rehearsal rooms with different volumes, shapes and cladding materials are necessary. Rehearsal rooms differ not only because of their dimensions and geometrical properties, but also because of their acoustic response, according to the type of ensemble (number  
Figure 1: Acoustic shell: longitudinal (**a**) and transversal section (**b**) section. of musicians) and repertoire. In opera houses in particular, rehearsal rooms for choirs would be recommended [16; 17; 18], but in many baroque theatres, built on the pass, these spaces were not included in the design, and to add them nowadays is quite impossible. In these conditions, an acoustic shell on stage often represents a good compromise for rehearsal activities. As for spaces designed specifically only for one use, theatres are considered to have specific acoustic characteristics, which are defined by the absorption surfaces' properties, the volume, and the shape of the hall, and nowadays, like the concert halls, they must deal with changing circumstances [19] during their life cycle. For example, hall occupation changes happen at different time scales: there is rapid change through different activities throughout a single day (rehearsals activities and concerts), medium-term change that occurs as result of re-organizations (concert for chamber orchestras instead of for big orchestras), and longer-term changes that might impact environmental and acoustic conditions [20; 21]. For these reasons, it becomes mandatory to support the set of the acoustic shell with an acoustic analysis to predict its effects on the sound filed, according to its shape and dimensions. The set of an acoustic shell should always be supported by an acoustic analysis to optimize the results in other performing spaces as well, like open theatres [22], circuses [23], squares, and ancient (Figure 2) or modern churches [24] if used for concerts, and generally in spaces with multifunctional activities where the shell can be installed. The optimization of the acoustic shell, achieving the preferred reverberation time, increases the multipurpose capability of the space, as an evaluation of the single acoustic quantity index MI could confirm [25]; it could also help to better control the acoustic features after refurbishment [26]. An acoustic shell installation supported by an acoustic analysis could also be greatly appreciated in avoiding focalizations from boundary concave surfaces [27], increasing the uniformity of the sound field in general. In the second section, the paper explains both the acoustic field differences and continuities of a space which is transformed from an opera house into a concert hall. In the third section, the acoustic shell optimization for the \"Opera di Roma\" is presented, while in the fourth sections results are discussed and musicians' opinion are considered. Conclusions follow. The study is focused on results obtained for a single case study, for which the acoustic shell shape is a compromise between the musicians' and acousticians' evaluations, but it  
Figure 2: Acoustic shell installed in the Church of Santa Marta, Morbegno (Sondrio, Italy). could be useful in other theatres, for which, musicians' opinions should be collected to evaluate the optimization of the acoustic shell, to satisfy both the acoustic quality and the musicians' wishes.