## 1 Introduction
[16] showed that the ISO 3382 objective parameters are mostly affected when the diffusive surfaces with a scattering coefficient higher than 0.70 are located on the ceiling, lateral walls and rear wall simultaneously. These effects are more evident in the smaller volume and are reduced when the rear wall only is treated independently of the volume. [PERSON] et al. [19] have suggested the use of another objective parameter, namely the number of reflection peaks (Np) in an impulse response, which describes the spatial and temporal variation of the sound field. They considered a scaled model of a shoebox hall with polygon- and hemisphere-type diffusive surfaces applied to the lateral walls and ceiling, as well as a real reverse fan-shaped recital hall with diffusive front halves lateral walls closest to the stage. Their measurements showed an increase in the Np at higher frequency bands and no significant differences for the other ISO 3382 parameters. In addition, [PERSON] et al. [12] showed differences below the just noticeable difference (JND) for the ISO acoustic parameters through simulations in 12 performance halls of various shapes (shoebox, fan-shape, and other complex shapes) and with increasing scattering coefficient of the walls and ceiling. In a second part of the study based on measurements in a scale model of a vineyard-shape hall, they noticed that the periodic diffusers installed over the sidewalls and balcony decrease _RT_ and \(G\) (strength), while increase \(C_{80}\). However, this was mainly attributed to the absorption added by the diffusers. Besides the objective investigations, also the perceptual differences between different surface treatments have been the object of continuous research. [PERSON] et al. [20] showed that changes in diffusion characteristics of the surfaces are audible in a wide frequency region and depend on the input signals, i.e., sustained signals make the perception of the differences easier than impulsive signals. [PERSON] and [PERSON] [21] and [PERSON] et al. [7] showed that perceptual differences between reflective and diffusive surfaces are related to the listening distance from the surface itself. Moreover, they are related to the difference of scattering coefficient between the compared surfaces [13; 15]. [PERSON] et al. [22] found that the perceived diffuseness is related to the interaural cross-correlation coefficient (_IACC_),which is an important parameter in the design process. Furthermore, [PERSON] et al. [19] showed that the perceived diffuseness could be quantified in terms of the number of reflected peaks (Np), which is correlated to the listener preference. In another study, [PERSON] and [PERSON] [4] showed that the preference of the diffusive surface presence highly correlates with the perceived loudness (\(SPL\)) and reverberance (\(EDT\)). Other studies reported that changes in diffusive surfaces characteristics are mainly perceived in terms of coloration and spaciousness variations [7; 20; 21; 23]. [PERSON] et al. [12] showed that despite small changes in the objective parameters, the presence of the diffusers made a clear and positive contribution to the overall impression of the listeners, which was mainly related to intimacy and envelopment. Although these results highlight the importance of the location of the diffusive surfaces and their configuration combined to the size and shape of the hall, there is still need for clear and generalized guidelines useful for acousticians and practitioners alike. Since the scattering properties of these surfaces can be easily assessed by using the ISO 17497-1, -2 [24; 25], the application of diffusive surfaces based on scientific investigations, and not only on the architectural and design preferences, should be a common practice for modern concert hall designers. Moreover, the subjective data, i.e., the listeners' sensitivity, would help to determine the measurement accuracy needed for the characterization of these surfaces [26].