## 1 Introduction  
The employment of multichannel devices for acoustic measurements has become very popular in the last decade, especially among researchers interested in developing auralization, which can be characterized also by dynamic sound sources and receivers [1]. The goal of combining a 360\({}^{\circ}\) video with a 3D sound field is one of the ways that enables a virtual navigation and an immersive listening experience. This technique is becoming the basis of various application areas, including gaming, cinema/entertainment, and the automotive industry, with surrounding effects that make the products more attractive and competitive in the global market [2].  
In this paper, the 3D soundscape in the National Theatre of Zagreb is investigated, based on the use of a multichannel spherical array microphone capable of detecting the spatial sound waves characterized with a specific directivity. In order to understand the architectural development of this theatre, a brief description of its history is given, together with the geometric and dimensional details [3].  
Some knowledge regarding the functionalities of a multichannel spherical array microphone is presented [4], as well as the principles of a beamforming process and the filtering process of virtual microphones [5]. Specifically, the elaboration of these maps does not follow any commercial package, but it has been improved at the University of Parma over the years since 2008-2010, when the outputs detected with simple circles the virtual microphones overlapped onto the panoramic photo. Based on the needs of recording artistic performances within theatres and concert halls during the pandemic, the system was implemented by representing the contour levels of the sound energy arriving at the microphone from a direct path or after the latest reflections. The detection of the directivity over the image helps the listener to understand the sound behavior within that specific space. This tool was broadly used with the Sipario project during the pandemic, when the demand for assistance to virtual artistic performances from a remote position was significant. The Sipario project was held by the University of Parma and Bologna and was focused on virtually reconstructing both 3D audio and panoramic video of live artistic performances executed within the most historical theatres and concert halls spread all over Europe. For this occasion, both artistic experience and acoustic information of historical architecture were digitalized to provide broader access to the public, especially for those heritage buildings whose integrity is preserved by international organizations like UNESCO, therefore limiting their access. During the Sipario project, also a demo session was created to give the listener the possibility of virtually exploring the immersive experience by moving in different points across the space, as much as the measuring points were selected for the acoustic measurements within the theatre. One of the goals achieved with the Sipario project was the capturing of the acoustic characteristics of the main Italian and European opera theatres based on spherical array microphones, in order to be thereafter convolved with the anechoic signal related to music, acting, and prose.  
After a general assessment of the acoustic response carried out using traditional equipment [6], an in-depth study of the interaction between sound energy and architectural elements of the main hall was carried out and identified with the evaluation of 3D acoustic maps [7]. The authors of this paper would like to persuade the researchers in acoustics to present technical data by following these two methodologies in order to achieve a complete acoustic description of performance arts spaces.