# Simplified Fly Tower Modeling for Preliminary Acoustic Predictions in Opera Houses
## 2 Methodology
### The Fly Tower Acoustic Modeling Identification
There are numerous ones, mounted in a row, some meters deep, that allow a very small ratio of sound energy to return to the stage, even at low frequencies, according to the theater's dimensions and fly tower conformation (with or without lateral stages). As a preliminary condition for the prediction analysis in the simulation model, it is suggested to replace them with a vertical lateral surface, whose absorption trend, in octave bands, is similar to the fly tower ceiling surface discussed above, having the house curtain with high absorption properties [39]. The absorption coefficients of the stage floor are published in the literature, being usually made of wood studs [40]. Finally, the back surface of the fly tower is usually that which hides the brick wall or the backstage from the sight of the audience. Typically, it is a heavy plastic curtain characterized by low absorption coefficients, the so-called cyclorama. It comes down from the grid, and to avoid crashing against the back balcony, it is at least ca. 2.5 m away from the brick wall, at circa 2/3 of the stage Depth. Those preliminary considerations about a typical configuration of the fly tower active space and the free space for the opera set in empty conditions were put as a reference for the two presented scenography layouts, while projection screens, which are becoming a key component in creating immersive visual experiences in theater setups, were studied for the new digital Baroque scenography. It was found that they are characterized by a low impact on the sound field because most of them, tested by different producers to be transparent to the sound, have low acoustic impedance Rs [41]. When a new digital Baroque scenography is installed, real objects on stage are minimized and the space remains almost empty. The stage configuration becomes very similar to the ballet one. Because of the new digital Baroque scenography, representations are becoming numerous and because of its similarities with the ballet set, the empty stage configuration is becoming the most common real stage condition. For these reasons, in the preliminary acoustic analysis, the modeling of the fly tower active volume (Figure 4), according to the previous considerations referred to the identified most common configuration of the free stage space for the opera set, is summarized as follows:  
Active fly tower volume: height equals the distance from the stage floor to 1.5 m above the prosenium, Depth equals 2/3 of the stage dimension, and length equals the prosenium dimension plus 2 m ca. Active volume absorptive properties: the ceiling is considered highly absorptive, the stage surface is a wooden floor on studs, the back wall is a heavy plastic screen for spotlights (not for projections), and the cyclorama becomes part of the fixed stage absorption areas. This absorption distribution guarantees that the average absorption coefficient will exceed 0.5 at central frequencies of 500 Hz and 1 kHz to reduce the variable effects of the traditional scenography elements, as suggested by the literature, justifying their absence in the simplified fly tower modeling. A table summarizing the simplified fly tower modeling properties, according to the set designer regulations, follows (Table 1): \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Simplified Fly Tower Model Characteristics**} \\ \hline volume Height & Proscenium Height + 1.5 m \\ volume Depth & 2/3 stage Depth \\ Length & Proscenium Length + 1 m for each site \\ Ceiling absorption & Highly absorptive porous materials \\ Back wall absorption & low absorptive porous materials \\ lateral wall absorption & High absorptive materials \\ Stage floor & absorption for wooden floor on studs \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simplified fly tower model characteristics. Figure 4: Fly tower active acoustic volume (green area) to be included in the simulation model.