## 4 About Performers and Music inside of Opera Houses  
### The Evolution of the Orchestra, and Orchestra Pit  
In the early 17 th Century, musicians were hidden behind the stage, as sentenced by [PERSON] (2018) \"Tutta questa moltitudine d'instrumenti [...] rende cosi poco suono, che appena si ode da' piu vicini alla scena\" (All this multitude of instruments [...] yields so little sound, that can only just be heard from closer to the scene). Some years later, still in the court-theatre context, [PERSON] placed the musicians in front of the stage (2018). In the Venetian public theatre with boxes, the orchestra was placed on the stalls floor (2018). This kind of performance needed few musicians (see the St. Moise theatre case in 1720, Table 5) (2018). With the first large-sized theatres, such as the S. Carlo in Naples in 1741 (Table 5), the orchestra increased. Its composition was characterised by a large number of violins and more double-basses than cellos. It should be noted that the sound power level of historical instruments might differ from the one today, and generally historical instruments sound lower. In case of strings it is due to the string materials, in case of woodwinds it depends on the keys. An accurate measurement of these differences was recently made by [PERSON] et al. [PERSON] et al. (2018). [PERSON] set the balance between the woodwind section, fixing the ratio between flutes, oboes, clarinets, bassons and horns as 2:2:2:2:4 (2018). This ratio was keep during the evolution of opera. Indeed, it was increased to base \"at-three\"--i.e., three flutes, three oboes, three clarinets and three bassoons--in the [PERSON] orchestra (2018; 2019) and, then, to base at-four\" by the German composers [PERSON] (2018) and [PERSON]. As noted in severals works (2018; 2019), the balance between voice and orchestra plays a predominant role in opera-house acoustics. This is due to the fact that vocal and orchestral signals need different boundary conditions of the sound field, and a well-designed opera house should satisfy both. In an opera, the voice needs more intelligibility and more \"focusing\" on the soloist (2019; 2020). This means a lower reverberance, and a lower Apparent Source Width, using ISO 3382 subjective categories. In terms of objective criteria, this corresponds to low \(EDT\) values and high Inter-Aural Crosscorrelation Coefficient (\(IACC\)) values. Instead, music needs more sustain. This means more reverberance and more envelopment, which corresponds to higher \(EDT\) values and low \(IACC\) values. The development of the opera house from an acoustic standpoint is essentially aimed at the fulfillment of these needs. In order to assure these conditions the orchestra was placed in the Italian opera house in front of the proscenium. This provides a lot of reflections from the side walls, and scattering from boxes. If the geometry is well-balanced, a very low value of \(IACC\) is achieved. Moreover, the energy decay is quite regular, meaning an \(EDT/T\) ratio of about one (2018). The singers were placed instead in the fly tower, which provides less reflections due to side curtains, achieving a high \(IACC\) value. Moreover, the fly tower is coupled with the main hall through the scenic arch. The effect of this coupling is often a double-slope decay, then an \(EDT/T\) ratio is lower than one (2018). The evolution of the opera during the 19 th Century needed more instruments. This was one of the reasons for the original Wagnerian idea of mystic gulf, conceived for the aforementioned Bayreuth Festspielhaus (1872) (Festspielhaus, 1872). However, that was not the only reason: the dramaturgy was increasing in complexity. More complex characters meant more focusing on the soloist. The presence of the orchestra pit increases the envelopment and increases the reverberance (\(EDT/T\) ratio higher than one). The pit provides the perceptual effect of moving the orchestra to the background, while the soloist are still well 'focused' on the stage.