## 1 Introduction
In order to enable a better acoustic design of existing buildings, the simulations first need to replicate the real acoustical conditions of the examined environment through three important steps: (1) appropriate geometry modelling; (2) material properties; and (3) simulation settings. This procedure, namely, the calibration of the model, is even more complicated for open-air theatres as the acoustic scattering and diffraction phenomena are more relevant than in closed theatres [14]. An appropriate calculation method and a geometrically detailed model are of fundamental importance to achieve accurate predicted results [15]. The reliability of simulations is an on-going matter of discussion and interest, as testified by the Round Robin comparisons of room acoustic modelling tools [16, 17, 18], and the more recent overview on the uncertainties of input data in simulations [13]. In the latter overview, it was reported that the specific uncertainties that characterise the absorption coefficient (\(\alpha_{\mathrm{w}}\)) and scattering coefficient (s) of materials [19, 20] could affect the estimation accuracy of room acoustic parameters in the end. Such parameters are derived from simulated Impulse Responses (IR) or from energy reflectograms, depending on which analysis algorithm of the room acoustics software is being used. In situ and scale measurements [4] have revealed that the IRs of ancient theatres are composed of the direct sound and of two major reflections, which come from the orchestra floor and the scenae froms (the ancient stage building), respectively, when these parts of the theatres still exist. Therefore, in the case of open-air theatres, the IR should be modelled with a limited number of specular reflections and a high number of scattered reflections, because of the irregularities in the steps of the cavea [21]. This configuration is difficult to handle using geometrical acoustic-based software (GA), such as Odeon (Version) and CATT-Acoustic [22; 23]. Yet, most of researchers still rely on such tools also for open-air theatres in everyday practice; thus, special attention should be given to properly controlling the boundary conditions. In fact, open-air theatres represent a special case, which creates a challenge for these prediction algorithms. The absence of a roof, and therefore of a reverberant field, urges to have a high reliability in the prediction of the early reflections. Moreover, the concave shape of these theatres is responsible for the creation of \"shadow zones\" of the mirroring surfaces in great lateral areas of the cavea [14]. This affects the deterministic method of the Image Source, which is used by the GA software to build the early part of the IRs. The aim of this work is to assess the performance of predictive software in calculating a set of acoustic parameters for ancient theatres, a particular type of open-air spaces, taking the case study of the ancient theatre of Syracusae (SR). The objective is to give more insight about the relation of the sensitivity of the simulated results to the input parameters. It is mainly referred to raise awareness on the use of this kind of software for outdoor unconventional environments. The theatre is located in Sicily, an island in the South of Italy, a region where ancient Greek culture had historically a lot of influence. The simulation accuracy of two kinds of software, Odeon and CATT-Acoustic, is considered. This theatre was selected because it was relatively easy to model due to the lack of contemporary additional elements. In this manner, the virtual model of SR could be considered as a valid archetype model. The paper is organised as follows:  
* Section 2 (Case Study) includes a brief description of the state of conservation of the theatre chosen for this research. * Section 3 (In Situ Measurements) includes a description of the acoustical measurement campaign carried out in the investigated theatre.