## 2 Method
### Acoustic Maps and Immersive Visualization  
Once the IRs are recorded by the multichannel microphone, they can be processed to create the acoustic maps [28]. Specifically, the measured IRs are processed using convolution, which is a mathematical operation that digitally filters one signal with another. In this particular case, the convolution can be performed between the inverse filter of the sound signal and the multichannel IR recorded by the spherical array microphone [29]. The convolution is applied here for each source-receiver position combination, based on the position indicated in Figure 2.  
Both audio and video have specific settings. In particular, after the convolution operations, audio parameters can be adjusted based on microphone gain, frequency passband filters (i.e., high and low passband), and weighting filters (e.g., A-weighted, linear). After adjusting the audio settings, the 32 signals extracted from the 122 highly directional virtual microphones can be elaborated [30]. The capture window of the samples, as highlighted with two blue lines, is adjusted such to cover all the soundwave, from the small amplitude to the wide curve amplitude at the peak and going down again to the small amplitude; in this way this way, all the energy levels can be represented in the color map [31]. If the capture window is too narrow and collects only 1-2 samples, the color map would be more monochromatic and would only red if focused on a spike or only blue-violet if focused on a small wave amplitude [32].  
Previous research studies have already used a similar technique for regular shoebox room volume [33], but the innovation of these acoustic maps consists of the visualization of the IRs in a 360\({}^{\circ}\) view applied to a 19 th century opera theatre with a particular volume shape [13]. On the image/video perspective, the parameter settings include the step size and frame rate, which are involved in the realization of the video flow. An additional option of horizontally tilting the 360\({}^{\circ}\) image allows for the alignment of the microphone front (identified with channel No. 1) with the loudspeaker to increase the accuracy of the video overlay.  
Overlaying the panoramic image with the IR creates a series of contour levels. The output-rendered video is identified with a color scale ranging from red-orange, representing a high sound energy level, to blue-purple, where the sound pressure level is lower. Specifically, the color scale ranges from 75 dB (A) to 105 dB (A). The reflectograms at the bottom of each acoustic map represent the time flow of the impulse response, which is given in the unit of millisecond (ms). The source simulated for the acoustic maps has omnidirectional directivity for all the octaves.  
Combining audio and video for impulse response analysis is important for visualizing the reflections from the surfaces directed towards the receiver, with their direction of origin indicating the last surfaces implicated. Both early and late reflections can be visualized as they impinge on boundary surfaces. This is useful for acousticians when corrections are to be made in the form of installing new panels or similarly to eliminate undesirable reflections and/or flutter echoes. Accurate predictions, as can be conducted with this audio-video technique, are very useful for acousticians when mitigation solutions are very expensive; this method can be adopted to save stakeholders' money. The synthesis of arbitrary directional patterns that are available without having to move the equipment also reduces the need for several required site surveys. For non-technical people, the acoustic maps can also be used by music lovers when they reserve a seat for an artistic performance; this paper provides more information about the sound envelopment in the stalls and in the balconies and about the clear difference between the two.