## 2 Materials and Methods
### Acoustic Solutions for Banasa Airport
#### 2.2.1 CFM 56 Acoustic Directivity  
The first step to improvement is to identify the acoustic directivity of a CFM 56-7D engine, used in the B737 aircraft, during aircraft operation on the ground. This was performed for two operating modes, namely, idle and maximum. For this study, measurements were made with a multi-channel acquisition system, the 01 dB Metravib-Orchestra.  
The measurements were taken at 12 different points, as depicted in Figure 7, at intervals of 12.85 degrees. A set of 12 GRAS 40 AE microphones was utilized, with a frequency range of 0-25.6 kHz, and an accuracy of class 1. Sensitivities were between 43.9 and 44.3 mV/Pa. The microphones were coupled with 12 GRAS 26 CA preamplifiers, with a frequency range of 2-100 kHz and an input impedance of 20 GOhmi and 0.4 pF. The 12 measurement points were positioned 25 m from the source. The data are available from Ref. [39].  
After conducting the acoustic measurements, the results were obtained and are presented as a comparative analysis of the 1/3 octave spectra for the idle regime (Figure 8) and the maximum regime (Figure 9).  
Figure 7: Illustration of the measurement points utilized to determine the sound-pressure level of the CFM 56 engine.  
Based on the spectral analyses conducted at each measurement point, noise polar diagrams have been generated that depict the frequency-dependent directivity of the noise source.  
In order to create the noise polar diagrams, the \(1/1\) octave sound power spectra were used for all 12 measurement points for both engine modes. Table 1 presents the spectrum of sound power as a \(1/1\) octave for idle mode.  
Figure 8: Comparative analysis of the \(1/3\) octave spectra for the A-weighted pressure levels of the 12 measurement points (idle regime).  
Figure 9: Comparative analysis of the \(1/3\) octave spectra for the A-weighted pressure levels of the 12 measurement points (maximum regime).  
Figure 10 displays the noise polar diagrams, which illustrate the sound power levels corresponding to the central frequencies of the \(1/1\) octave, which are in the range of 16 Hz to 8 kHz while the engine is running in idle mode.  
Table 2 presents the \(1/1\) octave sound power spectrum for the maximum regime.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & **16 Hz** & **31.5 Hz** & **63 Hz** & **125 Hz** & **250 Hz** & **500 Hz** & **1 kHz** & **2 kHz** & **4 kHz** & **8 kHz** \\ \hline Lw1 & 87.6 & 96.6 & 93.7 & 96.5 & 93.5 & 92.2 & 89.8 & 90.1 & 89.9 & 94.9 \\ \hline Lw2 & 84.5 & 96.9 & 96.3 & 97.1 & 98.4 & 96.7 & 96.2 & 92.8 & 92.1 & 101.5 \\ \hline Lw3 & 84.2 & 95.8 & 96.1 & 96.4 & 98.9 & 96.6 & 96.1 & 96.3 & 94.8 & 105.8 \\ \hline Lw4 & 85.7 & 98.0 & 97.6 & 101.5 & 102.2 & 100.6 & 100.4 & 99.1 & 96.5 & 108.5 \\ \hline Lw5 & 85.9 & 99.0 & 98.7 & 100.1 & 103.4 & 101.2 & 101.4 & 98.6 & 97.6 & 109.0 \\ \hline Lw6 & 95.6 & 100.7 & 100.1 & 101.7 & 103.3 & 100.3 & 102.3 & 98.9 & 99.6 & 112.2 \\ \hline Lw7 & 89.5 & 98.5 & 99.3 & 102.4 & 104.2 & 101.4 & 101.0 & 100.8 & 101.8 & 115.0 \\ \hline Lw8 & 95.9 & 98.6 & 99.7 & 103.0 & 104.4 & 102.9 & 103.7 & 101.7 & 102.4 & 114.5 \\ \hline Lw9 & 94.8 & 98.1 & 101.2 & 105.8 & 107.2 & 105.3 & 106.8 & 104.8 & 105.9 & 116.6 \\ \hline Lw10 & 88.7 & 97.5 & 101.2 & 106.0 & 107.6 & 106.9 & 106.7 & 101.9 & 102.0 & 107.9 \\ \hline Lw11 & 96.8 & 99.4 & 102.7 & 108.2 & 107.0 & 107.0 & 110.3 & 110.6 & 113.5 & 118.4 \\ \hline Lw12 & 92.3 & 102.0 & 105.6 & 109.9 & 106.1 & 113.6 & 115.2 & 113.1 & 116.5 & 121.6 \\ \hline \end{tabular}
\end{table}
Table 1: Sound power spectrum of the \(1/1\) octave for idling mode in dB.  
Figure 10: Noise polar diagrams for idle mode.  
Figure 11 displays the noise polar diagrams, which illustrate the sound power levels corresponding to the central frequencies of the 1/1 octave in the range of 16 Hz to 8 kHz.