# Simplified Fly Tower Modeling for Preliminary Acoustic Predictions in Opera Houses
## 2 Methodology
### The Fly Tower Acoustic Modeling Identification  
The fly tower volume varies from theater to theater, depending on both the historical period in which the theater was built, and the hall volume [32; 33; 34]. In the past, especially in provincial theaters, fly towers were not too high, and the stage volume was used to raise the scenography folded in two parts, while, in larger and more modern theaters, these were stored entirely in the fly towers' higher upper space, without being folded or partially removed. Nowadays, inside the fly tower, a grid floor is hung from the roof, for storing the machinery which moves the scenery (Figure 3), while lateral galleries manage both the scene movements and the lights aiming. Stage towers, therefore, achieve big dimensions, in which the upper space, above the proscenium, from the first balcony, is generally fully occupied by scenography. Nevertheless, the free volume for the opera set is usually only the one above the stage area, which is visible to the spectators. For this reason, the modeling parameters of the active fly tower acoustic volume require some specific considerations to be included in the theater acoustic model [35; 36]. As confirmed by many technical directors, the fly tower is fully occupied by scenography, by draperies and canvases, from the first gallery over the proscenium to the top, while rigid elements lie on wagons or revolvers of the lower stage machinery. Because of these and the previous considerations, as a preliminary condition for the prediction analysis in the paper, the sound rays from the stage are considered to impinge  
Figure 3: Fly tower storing. on the vertically stored surfaces and are supposed to be specularly reflected many times to the top of the fly tower. Low scattering effects are added only by the scenery frames. As for acoustic baffles, since most of the scenes are made of heavy fabrics or painted fabrics, only a very small ratio of sound energy returns to the stage, even at low frequencies, since the size of the suspended surfaces and of the volume above is compatible with the wavelengths of the low frequencies themselves. Resulting in stage absorption being an important factor in the acoustic design of opera houses, it can be estimated through a standard. To minimize the influence of various genres of performances and stage sets and curtains on the acoustic parameters of the auditorium in opera houses, it was in fact already recommended by the research that the side and back walls of the stage house could be treated with sound-absorbing materials [37]. For these reasons, in the proposed acoustic model, the upper volume of the fly tower is not represented from the first balcony, and is substituted by a flat ceiling characterized by high acoustic absorption properties; the empty fly tower active volume is reduced to a parallelepiped made of the fly tower ceiling surface, the lateral walls, the stage floor, and the frame around the proscenium opening to the hall, as will be better explained in the following paragraph. To find the fly tower acoustic model dimensions properly, a criterion is suggested for which the flat ceiling height must be equal to the distance from the first balcony level of the fly tower to the stage floor. The first balcony level is usually from 1 to 2 m higher than the proscenium top, according to the theater proscenium dimensions. On average, a distance of 1.5 m is suggested. From the proscenium, the surface is tilted at 7\({}^{\circ}\) down to the backstage to avoid a possible increase in specular reflections from parallel surfaces in the acoustic model [38]. Preliminary considerations about the other fly tower surfaces and their acoustic modeling follow. The lateral surfaces are usually built with heavy draperies, parallel to the proscenium, set at a constant distance. Their position allows the surrounding technical spaces and actors' entrances to be hidden from the audience, controlling the sight lines in the hall. Their texture is usually like that of the house curtain.