# Tunable Perforated Panel Sound Absorbers for Variable Acoustics Room Design
## 1 Introduction  
The acoustic design of a room is a task of great interest in building engineering that involves considering several aspects such as its dimensions, capacity, geometry, and, foremost, its acoustic elements (i.e., sound absorbers, diffusers, and surface-exposed materials). The choice of these elements may depend on the required intelligibility or reverberation conditions, these being mainly determined by the room's purpose: speech and/or music [1]. In this context, the use of sound absorbers is very important to improve the acoustic quality of a room as it reduces the reverberant sound pressure level (i.e., minimizes the number of reflections). A complete guide for the modeling and design of passive sound absorbers can be found in the books by [PERSON] and [PERSON] [2] and [PERSON] and [PERSON] [3]. Among these, micro-perforated absorbers are of great interest because of their higher durability and enhanced structural features when compared to traditional porous media such as foams and fibers. These resonant systems were suggested in a pioneering work by [PERSON] [4] and much research has been carried out ever since on their application to room acoustics [5]. Even though these devices have been successfully used through the years, when it comesto the design of multi-purpose rooms, there exists a growing interest in the development of sound absorbers with variable tuning capabilities [6]. Several approaches for variable acoustics room design exist, ranging from architectural solutions [7] to the use of active sound systems [8] or adjustable passive sound absorbers [9]. As for the latter, recent advances in the field of acoustic metamaterials have given rise to many innovative designs yielding reconfigurable structures. In a recent work by [PERSON] et al. [10], it was demonstrated that quiet zones and hotspots can be created in a reverberating environment by using an actively reconfigurable acoustic metasurface. Some other examples are metasurfaces with nesting helical tracks [11], the introduction of a rotatable plate inside a semi-cylindrical cavity [12], or the use of a manual slider [13]. Regarding tunable absorbers involving perforated panels, several research works can be also found in the literature, such as the development of a tunable absorber/diffuser using micro-perforated panels [14] or the study of an origami-based foldable sound absorber based on micro-perforated resonators [15]. Alternative but more complex solutions are those based on the use of hybrid passive-active systems consisting of a micro-perforated panel and a loudspeaker [16], micro-perforated dielectric elastomer actuators [17], or compact piezoelectric micro-perforated panel absorbers [18]. In this work, two different tunable perforated panel sound absorbers for variable acoustics room design are proposed. More specifically, two rotating perforated panels and a panel with an iris-type aperture make up variable open area ratio systems, hereafter referred to as Tunable Solution A (TS-A) and Tunable Solution B (TS-B), respectively. The sound absorption performance of both solutions was experimentally assessed using an impedance tube set up according to the ISO 10534-2 standard [19]. For this purpose, several specimens were fabricated using laser cutting technology over Poly (methyl methacrylate) (PMMA) plastic. A parametric study for different configurations was carried out, highlighting their tuning capabilities. Additionally, the influence of employing these solutions in diffuse field conditions was analyzed for a case study room by following a prediction scheme based on the ray tracing method. In doing so, the Reverberation Time (RT), the Definition (D50), and the Clarity (C80) were obtained at several room positions for different occupancy levels (i.e., number of people in the room), thus confirming the potential of using tunable perforated panel sound absorbers for variable acoustics room design.