# A Metawindow with Optimised Acoustic and
## 1 Introduction  
Facade insulation to reduce outdoor noise, provisions for ventilation, and overheating mitigation have been commonly considered disconnected in building design development. Strategies involving building features to control these different systems have been developed with entirely separated approaches [1]. Conventional windows, for example, allow visual connection with the outdoors, natural ventilation, and partial acoustics isolation when closed; however, the way these mechanisms work forces the users to choose one function and to exclude the other, and, of course, neither choice is conducive to indoor comfort [2, 3, 4]. So far, researchers have used several methodologies to overcome both problems by using, for example, mechanical ventilation [5] and active or passive noise control systems [6, 7, 8, 9, 10, 11, 12, 13, 14]. The latter consume less energy and can be built through several features within the window, such as perforated panels [7, 15, 16], porous materials [17, 18, 19], or acoustic metamaterials (AMMs) [20, 21, 22, 23, 24, 25].  
AMMs, in particular, are artificially engineered composite structures with unique acoustic properties derived from the use of modern engineering and physics as they are unlikely to be found in natural materials. Their range of application includes sound absorption [23], noise attenuation [26], acoustic wave manipulation [27], asymmetric acoustic wave propagation [28], acoustic energy harvesting [29], acoustic holography [30], and topological acoustics [31]. Among these, applications to combine noise control with air transmission have received considerable attention from acousticians. Recently they have focused on methods such as resonant-based acoustic meta-absorbers [15; 32], subwavelength coiled channel metamaterials [33; 34], and a combination of labyrinthine AMMs with porous materials, such as foam and cotton [22]. However, due to their bulky and visually invasive geometrical nature, complex geometries, or relatively narrow bandwidth, they sometimes limit architectural choices, such that the design of thin and visually pleasant metastructures embedded in window systems remains a challenge.  
In this study, an acoustic metawindow (AMW) unit for broadband noise control (50 to 5000 Hz) and natural ventilation is presented, and its performance is examined numerically and experimentally. Firstly, since our previous study numerically tested the feasibility of a metacage [35], the ergonomic adaptation of the window is investigated in order to determine the most regular window shape [3] with the closest noise control properties to the previous model [35]. This is not of immediate determination since the previous paper highlighted that acoustic properties are strictly related to the 3D geometry [35]. In this model, the AMM system is incorporated in the window frame space for noise reduction and ventilation, while the central area allows a visual connection between the outdoor and indoor environment. Secondly, the acoustic performance of the AMW unit is investigated experimentally to show the validity of the design. Through the experimental results, our developed numerical model is validated. Thirdly, the influence of geometrical components and parameters is investigated to determine the optimal design setup for the AMW unit in a wider frequency range. A combination of multiple perforations-cavities of different configurations further leads to realising a higher bandwidth noise control range. Finally, the ventilation performance of the AMW unit needs to be investigated, including the acoustically optimised models. The finite element method (FEM) is used for the acoustic and computational fluid dynamic (CFD) analysis, while experimental measurements are set to validate the 3D-printed unit's acoustic performance.