# Acoustic Impact of Hybrid-Electric DEP Aircraft Configuration at Airport Level
## 2 The Numerical Solvers
### NPD Calculation Methodology  
The numerical methodology for the calculation of the NPD data of a given aircraft, simply considers simulations of flyovers of the given aircraft, properly acoustically characterized, in compliance with the regulation proposed by Society of Automotive Engineers SAE [3] for the calculation of airplane noise in the vicinity of airports. Each of the numerical simulations considers the acoustic source flying along a straight trajectory above a hypothetical runaway at a fixed altitude and with a given engine-power setting. By varying the altitude and engine-power settings, it is possible to fill a database that can correlate the value of sound exposure levels (L\({}_{\text{AE}}\)) or effective perceived noise level (EPNL), in the centre of the hypothetical runaway, to each couple (engine-power setting, altitude) [14; 15]. Also, the peak value of A-weighted instantaneous sound level, L\({}_{\text{AMax}}\), and the tone-correctedperceived noise level maximum value, PNLTM, are desirable in order to have a complete acoustic data set. This very fast and analytical methodology is able to simulate an expansive experimental campaign and provides the NPD curves for the Integrated Noise Model INM and/or Aviation Environmental Design Tool AEDT [2].  
The process of populating a table similar to Table 1 is carried out by means of the RALPH code described in [4] and, in Figure 1, the process of the NPD evaluation is graphically represented.  
To better reproduce a real measurement campaign for building the NPD database, some constraints were given for trajectories and all consequent calculations. First, the acoustic source, representing the given aircraft, is assumed to be precisely flying at a constant speed of 160 kts (82.31 m/s). Moreover, the timestamp for the trajectory evaluation is no longer than 0.5 s, to allow the acoustic parameters to be evaluated with a time-window of exactly 0.5 s, fulfilling the corresponding standard [3].  
The RALPH code was configured to take into account the following phenomena and signal-processing options [14]:  
* Ground reflection was active, and considered a perfect reflector;
* The Doppler effect ws active, to better consider the consequent atmospheric absorption;
* Atmospheric absorption was active, in fulfillment of the SAE-ARP 866B standard [16];
* A-weighting for the source spectra was active, in fulfillment of SAE AIR 1845 [3], for the NPD calculation.  
Finally, it is worthy to remember that RALPH code does not consider obstacles, to date, and supposes perfectly flat terrain. These hypotheses are perfectly compliant with the actual scenario of an airport in the area of a runaway.  
As already assessed, NPD databases were built both in terms of L\({}_{\text{AMax}}\) and L\({}_{\text{AE}}\) for all the considered configurations of the aircraft. Further data important to remark upon relates to the acoustic source. It is represented by a hemisphere (see [13] for more details) of radius 100 m. The importance of this data will be clear in the next section, as the acoustic effect produced by the source is only valid outside the hemisphere.