## 4 Results and Discussions
### Chevron Nozzle
#### 4.2.2 Time and Frequency Domain Analysis  
Similar to the last topic, where the baseline and the ejector configurations were examined, this section analyzes experimental data, in both the time and frequency domains, from the other two configurations--the baseline nozzle and the nozzle with 16 chevrons--at the same maximum operating regime of the microjet engine. Finally, the configurations are compared to identify the most effective noise reduction. Comparing the 16-chevron nozzle to the baseline, Table 6 demonstrates a considerable reduction in noise. OASPL (A-weighted) declines by 1.21%, and SPL(Z) drops by 1.28%, indicating less noise in the region that is most audible to humans. SPL(C), or low-frequency noise, drops by 1.15 percent. The chevrons effectively reduce sound levels in both the low-frequency range (C-weighted) and the human hearing range (A-weighted), by 1.21% and 1.15%, respectively, shown by both L\({}_{\text{Aeq}}\) and L\({}_{\text{Ceq}}\), which integrate sound levels over time with the fast time weighting (using a 125 ms time constant). L\({}_{\text{Zmax}}\), L\({}_{\text{Amax}}\) and L\({}_{\text{Cmax}}\) all decrease by 0.89%, 0.73%, and 0.70%, respectively, at peak noise levels, indicating that the chevrons aid in mitigating peak noise events. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Configuration**} & **SPL(Z)** & **SPL[dB]** & **SPL[dB]** & **SPL[dB]** & **SPL[dB]** \\  & **[dB]** & **Spectrogram Power** & **Spectrogram** & **Welch Periodogram** & **In-House** \\ \hline B & 118.49 & 118.49 & 118.75 & 118.49 & 118.49 \\ E & 117.90 & 117.90 & 118.15 & 117.90 & 117.90 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: SPL comparison obtained in time and frequency domain for baseline and ejector engine. Figures 10a,b and 11a,b present the time-variant sound pressure level \(L_{p}(t)\) and the three time-averaged sound pressure levels \(L_{\text{Aeq},T}\)\(L_{\text{Ceq},T}\) and \(L_{\text{Zeq},T}\) over a defined period to capture short-term variations in sound levels. The nozzle with 16 chevrons shows a clear reduction in SPL across all X-weighted levels (LXeq) compared to the baseline. The reductions range from approximately 1 dB to 1.8 dB, particularly in the A-weighted and C-weighted frequencies. The most noticeable frequency appears around 10,175 Hz, which is detected in both the periodogram from Figures 10c and 11c and in the frequency spectrum (Figures 10d-f and 11d-f). This can be explained similarly to the case of the ejector nozzle, where the signal was selected at the maximum operating regime of the turbo microjet, running at a rotational speed of 101,520 RPM (or 1692 revolutions per second), as the microjet engine is equipped with a centrifugal compressor with six blades, giving a theoretical frequency of 10,152 Hz. The fundamental harmonics of the centrifugal compressor are located at 1700 Hz (n.b. with the magnitude of the spectrum of 1.2 Pa) for the baseline configuration and 1685.21 Hz (n.b. with the magnitude of the spectrum of 0.99 Pa) for the chevron nozzle, further confirming the relationship between these frequencies and the operating characteristics of the compressor. Similar to the ejector, the baseline and the chevron nozzle configurations are compared by looking at the Sound Pressure Level (SPL) readings from the Baseline (B) and Chevron (C) setups using various techniques. Table 7 reveals small but measurable differences in the overall SPL values produced by these techniques. The SPL calculated from the pressure signal serves as the reference for comparing FFT-based techniques. The in-house method, Welch periodogram and spectrogram power methods produce nearly identical results, confirming their reliability for consistent SPL calculations. The STFT method, while close, shows slightly higher SPLs, especially for configuration (C), likely due to its sensitivity to transient features and short-duration variations. Figure 11: _Cont_.