# Concert Halls as Nearly Adaptive Spaces
## 1 Introduction  
Adaptive buildings refer to spaces designed to adapt to their environments, their inhabitants or users, and their intended activities, as well as to buildings that are entirely driven by internal data. Adaptive buildings bring together several different concerns from a wide variety of disciplines, spanning architecture, music, computer science and engineering, among others. Buildings in this context are described as flexible, interactive, and dynamic, and they embrace the notion of architecture being adaptive rather than a static artefact, often with an emphasis on computer-supported adaptation [1]. Adaptive spaces for cultural production have a long history. Concert halls have long incorporated technologies that allow them to be used in events [2]. Complex technologies, such as stage systems, lighting systems, and more modern multimedia systems, help a concert to happen, while other equipment, such as air heating and ventilation systems (HVACs), allow for the audience to participate in good environmental conditions. Adaptive concert halls (ACHs) are considered to have specific acoustic characteristics, which are defined by the absorption surfaces' properties, the volume, and the shape of the hall in the design phase, and they must deal with changing circumstances [3] 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 instead of 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 on their environmental and acoustic conditions. Concert halls are designed and managed to be adapted for concerts. Their natural acoustic quality is a passive property of the hall that can vary only if some variable volumes or variable acoustic surfaces are modified to control the hall setup before the beginning of the concert. In most cases, when the hall is not designed as a multipurpose space [4], only curtains are introduced to act as variable acoustic elements. Variable acoustic elements are thought of as adaptation components that directly influence the natural room acoustic response, as well as the number of spectators and their percentage of occupancy [5], but the intercorrelation between the acoustic qualities and the commonly overlooked thermo-hygrometric parameters also plays an important role [6]. In particular, to optimize the acoustic response, it is necessary to consider the influence of temperature, humidity, and air velocity on acoustic parameters [7]. This means that temperature- and air humidity-related factors, such as occupancy and overheating of lighting equipment, can also indirectly influence the natural acoustic response (Figure 1). Considering the digital transformation era operating in the architecture, engineering, and construction (AEC) industry [8], this paper introduces adaptive concert halls through digital twins (DTs) as part of the automation and control process towards Construction 4.0 in order to optimize their acoustic quality through the main factors that directly and indirectly have influence, transforming the room's acoustic response into an active property of the hall. Great advantages are achieved when developments of the different interconnections converge to create new and exciting acoustic fields such as experiences and lived-in sound, which, as a further step, transform the hall acoustics into an instrument that acts during a concert without the use of any electroacoustic equipment. This possibility represents a novelty in the musical field since, although the variability in the reverberation time can be included during the performance of a piece of contemporary music, it is nowadays still left only to the electroacoustic system.