# Impact of Using Chevrons Nozzle on the Acoustics and Performances of a Micro Turbojet Engine
## 4 Acoustic Evaluation Results  
In the present study, it is investigated how the noise reduction, produced by the hot jet gases through 2 nozzles with chevrons, compares with the reference nozzle. It must be mentioned that, while the micro engine is functioning, besides the jet noise, there are many other noise sources generated by: air circulation at intake level, turbulences generated by the air passage through compressor stage, mechanical noise generated by the speed of the shaft, rotor-stator interaction, known as Blade Pass Frequency BPF, combustion noise, etc. One of these types of noise, identified in this study, is the combustion noise, which is composed from two components: direct noise and indirect noise. Regarding the direct combustion noise, this is produced from the fluctuations of the heat release inside the combustion chamber in the flame region. Indirect noise is produced by the entropy waves, induced by the unsteady combustion process and vorticity. These two different noise sources will propagate through (potentially) multiple turbine blade stages to finally radiate in a far field. The noise of interest, evaluated in this study, is the jet noise, which is produced due to the turbulent mixing of high-speed gases with the ambient air [29]. In order to compare the noise reduction, produced only by the jet gases, with the chevrons nozzle, it is necessary to identify the acoustic spectral components of the acoustic sources, which were previously mentioned, followed by the filtering of those that are not in our interest. The data of the engine parameters, and the acoustic and vibration signals, were recorded and post processed, and their centralization is presented further on. The acoustic and vibration raw signals were processed using the Fast Fourier Transform (FFT) function in frequency domain and by averaging the entire time duration. For a better comparison and graphical representation, the noise spectra obtained for the three microphones were averaged according to the Equation (1), thus, for each regime and tested configuration, a spectrum resulted. \[L_{p}=10 lg\frac{1}{n}\sum_{i=1}^{n}10^{L_{i}/10} \tag{1}\]  
where \(L_{p}\) is the averaged sound pressure level, \(n\) is the microphone number and \(L_{i}\) are the sound pressure level in each microphone in dB. First stage of noise signal filtering was made by applying a band stop filter on the spectral components, which corresponds to the shaft speed, from which it results the compressor, turbine BDF, and its harmonics. In Figure 4, the unfiltered and filtered averaged sound spectra for each regime and nozzle configuration are presented. Figure 3: Nozzles tested configurations (**a**) references nozzle; (**b**) N8 L10 I0 nozzle (**c**) N16 L10 I0 nozzle. Figure 4: _Cont_. From the unfiltered noise spectra, it is observed that the tonal components corresponding to the engine speed, BPF components, and its harmonics have amplitudes that reach almost 110 dB (especially at high speeds Regime 3 and 4), and applying the band stop filter leads to a broadband acoustic spectrum. Comparing all the spectra from all regimes it identified a broadband component with the peak at a frequency of around 200 Hz, which regardless of engine speed, its maximum peak frequency does not change significantly (max 80 Hz frequency shift), instead its amplitude increases from 83 dB at Regime 1 to 95 dB at Regime 4. It is assumed that this component is produced by the combustion, and this will not be taken into account in overall sound pressure level calculation. The overall vibration and sound pressure levels (OASPL) in each microphone, and for each regime and nozzle, are presented in Table 1. The acoustic OASPL were computed based on the filtered acoustic spectra presented in Figure 4 in frequency domain of 500-20,000 Hz.