# Acoustic Impact of Hybrid-Electric DEP Aircraft Configuration at Airport Level
## 4 Analysis of the Results
### The NPD Database Generation  
In this section, the NPD database is reported for each of the configurations of the vehicle here considered.  
By adopting the numerical process described in the previous section, acoustic databases of noise-power-distance were built. The considered engine power setting for the two configurations of the aircraft, were 40% and 100%, while the altitude was set as per the requirements of the AEDT software, starting at 200 ft and ending at 25,000 ft. It is worth noting that 200 ft is 60.96 m; while the radius of hemisphere for the acoustic source modelling is longer (100 m) than this altitude, validity problems may occur while performing the numerical simulations at this altitude. Indeed, the simulation finds a distance between  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **SPL Average [dB]** & **SPL-A Average [dB]** \\ \hline Configuration A, 40\% ETR & 85.117 & 66.967 \\ \hline Motorization 2, 40\% ETR & 84.269 & 72.189 \\ \hline Configuration A, 100\% ETR & 98.675 & 80.036 \\ \hline Motorization 2, 100\% ETR & 93.759 & 85.197 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Average SPL values.  
Figure 28: Comparison between Configuration A and Motorization 2; OASPL values on the transverse YZ plane (**a**), A-weighted values on (**b**). Flight condition Descent 40% ETR.  
Figure 29: Comparison between Configuration A and Motorization 2; OASPL values on the transverse YZ plane (**a**), A-weighted values on (**b**). Flight condition Climb 100% ETR.  
the source and microphone less than 100 m, and so performs a non-valid calculation of the real acoustic effect on the observation point. For this reason, the data obtained for 200 ft altitude are shown but considered merely an appraisal of reliable data.  
Figures 30-33 report the NPD database for the ATR aircraft configuration, respectively in terms of sound exposure level (\(L_{AE}\)), the peak value of the A-weighted instantaneous sound level (\(L_{AMax}\)), the effective perceived noise level (EPNL), and the maximum value of the perceived noise level with tone correction (PNLTM).  
Figures 34-37 report the NPD database for the DEP aircraft configuration, with the same variables as in the ATR configuration.  
Figure 34: NPD curves, in terms of L\({}_{\rm AE}\), for the DEP configuration.  
Figure 32: NPD curves, in terms of EPNL, for the ATR configuration.  
Figure 37: NPD curves, in terms of PNLSTM, for the DEP configuration.  
Figure 35: NPD curves, in terms of L\({}_{\text{AMax}}\), for the DEP configuration.  
The results show the DEP configuration was significantly louder than the ATR one. The higher the altitude, the lower the difference between the two configurations; this is related to the atmospheric absorption and the characteristic spectral contents of the two sources. Concerning the greater loudness of the DEP configuration, Figure 38 compares the A-weighted instantaneous sound levels (\(L_{A}\)) for the two configurations at 40% engine power and an altitude of 400 ft. The solid red line represents the time variation of \(L_{A}\) for the DEP configuration along the numerical simulation of the acoustic experiment, while the blue solid line is the same for the ATR configuration. It is evident that the DEP configuration always had greater sound levels, mainly due the acoustic effect of the mid-wing propellers (perfectly equal and reported as dash-dotted green lines), while the two wingtip propellers (perfectly equal and reported as dash-dotted black lines) had negligible acoustic contributions.