## 1 Introduction  
Attenuating sound in various industries, especially in aerospace and automotive engineering, is a critical concern as regulations on noise emissions become increasingly stringent (as identified for example by Airbus [1] in the early 2000s for various operating conditions and recently updated by [PERSON] et al. [2], theoretically described by [PERSON] [3] and [PERSON] [4] in the 1970s and 1980s and updated by various authors, such as [PERSON] and [PERSON] [5] or [PERSON] and [PERSON] [6], correlated with environmental impact by [PERSON] [7]). One of the key approaches to reduce noise is the use of passive devices, which are designed to mitigate sound without requiring external power sources (aerodynamic detuning by the replacement of alternate stator vanes with short chord splitter vanes [8], blade-tip modifications [9], entire leading edge [10; 11; 12] or trailing edge [13; 14; 15] modification using various serrations or cutouts). These devices, similar to passive blades, can be derived in elements like ejectors [16; 17; 18; 19] and chevrons [20; 21; 22; 23; 24], which play a crucial role in noise control, especially in jet engines, where subsonic speed exhaust flows generate significant sound levels. Previous studies have examined both configurations separately [25; 26], and a more in-depth statistical and acoustic analysis of both configurations would provide valuable insights. The objective of this paper is to assess the impact of two passive devices used for sound reduction in jet engines: the baseline nozzle paired with an ejector and the same baseline nozzle equipped with the chevron nozzle featuring 16 chevrons. The noise signature typically consists of a broadband noise, along with prominent spikes or tones at multiples of the blade-passing frequency. Although these discrete frequency tones may not significantly affect the overall noise level, they are the primary source of the irritating screech noise commonly associated with turbojets. Both studied configurations were found to be effective in reducing the jet noise signature [27]. Having no moving parts, minimizing jet noise and increasing thrust in an aircraft, the two above-mentioned passive devices need little maintenance and have low operational costs, coupled with no restriction on the working fluids and a long service life [28; 29]. The chevron nozzles increase the turbulent shear layer's rate of mixing [30; 31; 32], which is the layer of air between the hot, moving exhaust gas stream from the engine's inner flux and the cold secondary air flux around the engine core. Chevron nozzles contribute to a significant reduction in jet noise and pressure variations by enhancing streamwise vortices that increase mixing within the plume to speed up the decay of the jet potential core. Enhancing mixing zones typically intensifies motion at smaller scales, which amplifies high frequency noise (e.g., the effects of different nozzle heads on turbulent jet noise [33]; triangular chevrons' influence on acoustic spectra for several gas turbine engine regimes [20]; high bypass ratio engine gas turbine noise [34], the chevron-induced axial vorticity, which enhances mixing in the jet shear layer [35]; axial vortices induced by chevron serrations [23]; flow visualization for a fairly simple tooth nozzle [24] or flow mixing in a supersonic regime [16]). The interaction between a plasma jet and targets with different physical properties was investigated by means of Schlieren optics [36]. Schlieren academic applications on acoustics allow visualizing sound itself with the help of this visualization technique [37; 38; 39].