## 1 Introduction  
The ability of a building facade to ensure appropriate sound insulation is of paramount importance when a high level of comfort is required in environments that are supposed to be as quiet as possible. This demands an accurate design, especially for buildings located in the close vicinity of transport infrastructure, industrial sites or commercial activities [1; 2]. To protect people's health, some national regulations, such as the Italian DPCM 05/12/97 [3], require a minimum acoustic insulation performance to be achieved for facades. Such performances must be experimentally verified according to ISO 16283-3 [4]. In other countries such as, for example, Denmark, Finland, Iceland, Norway and Sweden, a maximum indoor noise level must not be exceeded [5]. In this case, the prediction of the indoor sound pressure level depends on the acoustic insulation of the facade and on the characteristics of the noise source. Several studies have been published in the last few years on the correlation between indoor comfort and facade sound insulation. In [6], [PERSON] et al. showed a method based on a statistical approach to define the sound insulation of facades required to achieve indoor comfort. In the same paper, a ranking of the most common acoustic insulations attainable for the facades of existing buildings is specified. [PERSON] et al. [7] proved that the sound insulation of a building facade influenced indoor annoyance due to transportation noise and the frequency content of intrusive noise. In [8] [PERSON] et al. reportedinformation about the acoustic environment in schools considering the facade acoustic insulation as a key element for reaching a good level of indoor comfort. Sound insulation performances of facades are influenced not only by the characteristics of their elements, but also by their shape, as pointed out by [PERSON] et al. [9], who showed a significant effect of facade shape on the propagation of outdoor sound pressure levels. [PERSON] et al. [10] studied the influence of the facade layout on the acoustic characteristics of an urban space and on the subjective spatial perception of the users. [PERSON] et al. [11] studied the influence of the facade architecture on indoor acoustic quality. [PERSON] et al. [12] investigated the effect of sound scattering due to the facade shape on the sound propagation in urban canyons. Regarding other possible elements influencing the sound insulation properties of the facade, [PERSON] et al. [13] used a bi-dimensional pressure acoustics finite element model of a shading device to analyze the effects on indoor and outdoor acoustic environments. The performances of walls are usually determined by laboratory tests. When the sound insulation performances are not known, it is always possible to apply prediction methods (see, for instance, [PERSON] [14], who proposed a combined simulation approach using the Transfer Matrix Method and a 3D acoustic model to find the sound insulation of curtain walls). However, the acoustic performance of a facade is also determined by the junctions between the elements. [PERSON] et al. [15] analyzed the structural direct and flanking transmissions with reference to the joints connecting the million of the curtain wall with the surrounding lightweight plasterboard partitions. As concerns the experimental methods, [PERSON] et al. [16] tested different combinations of noise reduction methods as a function of the noise angles of incidence on an experimental setup featuring a window, in order to simulate the contribution due to different sections of a road. [PERSON] [17] found some low-frequency range limitations in the methods proposed by different standards to assess the sound insulation of a facade. In terms of measurement uncertainty, [PERSON] et al. [18] observed that the in situ reproducibility standard deviation of facade measurements is not adequately reflected in the ISO 12999-1 standard [19]. In particular, the values given by the standard underestimate the uncertainty of in situ measurements.