# Acoustic Shell Optimization in Opera Houses for Concert Halls
## 1 Introduction  
Acoustic shells are used to improve the sound early reflections of a performance on stage, supporting the acoustic field of the surrounding space. They can be installed in different contexts, outdoors and indoors. However, there is not much research about acoustic properties and the effects of acoustic shell forms and their modifications. Some advice for the design in practice was discussed in the past [1]. A minimum volume was suggested [2]; [PERSON] [3] presented room acoustic measurements of the effects of adding or modifying an orchestra shell in three multipurpose halls. It was also observed that an absorptive back wall of orchestra shells would be preferred by musicians [4]. More recent studies have focused on the acoustic shell design for outdoor applications [5, 6, 7], even if it is becoming more common, nowadays, to install them in historical theatres when concerts are being held on stage. Acoustic shells help to achieve longer reverberation time requirements compared to the theatre hall configuration. This solution brings about a significant alteration in the acoustic environment, effectively isolating the Fly Tower volume behind it, and defining a considerably larger active acoustic space. The outcoming acoustic field results in different average values of the main acoustic parameters. The necessity of using baroque theatres as concert halls or as multipurpose spaces is related to economic reasons, a lack of human resources and leadership, and strategy organization, for which it has become increasingly apparent that theatres, which are dedicated tojust one single use, are no longer sustainable. Especially in large cities, a degree of flexibility is required to transform the space from speech or music uses to drama, performances or fashion show configurations [8; 9; 10]. In this context, satisfactory acoustic properties must be achieved for concerts as well as for theatrical performances and congresses [11]. In a baroque theatre, the audience feels enveloped and surrounded by the sound. However, the reverberation time is shorter than in a concert hall, and, according to volume, construction time and location, it can achieve an optimal range between 1.3 and 1.8 s in occupied conditions [12; 13; 14]. Usually, the reverberation time rises with decreasing frequency, yielding a beneficial sense of musical warmth below 500 Hz. In concert halls, during rapid musical passages, the acoustic parameters clarity and definition can be appreciated in detail, thanks to the reflections coming from supporting surfaces, which are located close enough to the audience to ensure a short initial time delay gap. In addition, the sound has adequate loudness that is evenly distributed throughout the hall, and a wide bandwidth is supported, considering that musical instruments generate sounds from 30 Hz to 12,000 Hz, which is much broader than the spectrum of speech [15]. Acoustic shells are temporary constructions which can transform a theatre into a concert hall and can be installed in different configurations on stage. They vary in dimensions and shape according to the musicians needs, between symphonic music, with or without choruses, chamber music, and solo performances. They are usually designed and implemented with a modular system, which allows for different storage procedures, following the operational scene and acoustic needs. Usually, an acoustic shell is made up of two main types of elements, the ceiling and the wall elements, so-called \"towers\". The ceiling is made of modular elements, hung to the multiline hoists of the fly tower to be adapted in different tilted positions according to the requests. Towers are usually self-supporting walls complete with weights and stabilizers, that can be closed and moved on rubber wheels. Sound-diffusing convex surfaces of multi-layer curved wood structures are hung on the back frames of steel sections. The ceiling is composed of modular wooden panels, suspended and adjustable by means of motorized rigging to also reduce its dimensions for storage.