## 6 Conclusions  
Measurements were undertaken on the popular QRD in two specific configurations: horizontal and vertical disposition of the wells, placed with a rigidly backed smooth panel. Following part 2 of the reference standard (ISO 17497), the measurements consisted of placing the samples at the center of a virtual concentric arch of evenly spaced 25 microphones. The microphones' positions ranged from \(\theta=0^{\circ}\) to \(\theta=90^{\circ}\) with a spacing of \(7.5^{\circ}\) between each other and along a semicircle. This configuration has been employed in two rooms of different volume sizes and characteristics of wall finishes: a semi-anechoic chamber available at SCM Group in Rimini (Room A) and a reverberant laboratory available at the University of Parma (Room B).  
Other than the physical features of the two testing rooms, the measurements differed in the radius dimension of the virtual semicircle, where the samples were placed at the center: \(r=4\) m for Room A, \(r=5\) m for Room B. The choice of testing inside a semi-anechoic chamber, although considered too narrow for carrying this type of test, fell by keeping unwanted acoustical contributions from the vertical walls, while the concern of the floor reflections was solved by disposing of microphones on the floor. Although very minimal, the contribution of extraneous reflections inside Room A compromised the results of the scattered sound from the panels, which should be discarded because they cannot be compared with the affordable values obtained in Room B. This outcome would be considered a challenge when trying to undertake a data analysis where the conditions are not favorable. On this basis, the best solution was to discard the results not in line with the procedure, as outlined by the standard requirements.  
The results of panels tested inside Room B instead were compared with the reference curves of a theoretical model based on the theory of sound propagation in free field conditions, and data analysis consisted of a polar distribution of the scattered field through the use of the Fast Fourier Transform (FFT). A final calculation related to the diffusion coefficients of the selected panels in the specific configurations has been carried out for the frequency range comprised between 125 Hz and 8 kHz.  
This paper would like to highlight the attention given to the correct procedure outlined by the regulations to be followed for a successful data output, which, instead, would be compromised by extraneous factors that should not be in place.  
Conceptualization, L.T. and A.F.; methodology, L.T. and A.F.; software, F.M.; validation, A.B.; formal analysis, L.T. and A.F.; investigation, P.F.; resources, L.T.; data curation, F.M.  
Figure 14: Comparison of the scattering coefficient based on measurements undertaken inside Room A and B (“Coeff. Scattering Horizontal Plywood Panel–Room B” = “Horizontal plywood panel with back-room B”–“Refecting panel”).  
and P.F.; writing original draft preparation, A.B.; visualization, A.B. and P.F. All authors have read and agreed to the published version of the manuscript.  
This work was carried on within the project \"SIPARIO-II Suono: Arte Intangibile delle Performing Arts--Ricerca su teatri italiani per I'Opera POR-FESR 2014-20\", n. PG/2018/632038, funded by the Regione Emilia Romagna under EU Commission.  
Not applicable.  
The authors declare no conflict of interest.