## 3 Experimental Activity and Results  
The experimental activities carried out in the framework of HYPROB-NEW project also allowed to perform acoustic measurements. In this context, acoustic data were collected at the military outdoor firing range located at the Grazzanise Air Force Base, recorded with a 1/2\" free-field prepolarized condenser microphone. Data acquisition was handled with a four-channel real-time sound level integrator and analyzer (SoundBook), from the Spectra Corp., with a sampling rate of 51.2 kHz per channel (Figure 6).  
The microphone, microphone preamplifier, extension cables, preamplifier power supply, and analyzer present characteristics in accordance with IEC 651 Type 1, IEC 804 Type 1, IEC61672 Class 1, IEC 1260 Class 0. More details on the specifications of the devices are given in Table 1:  
Figure 5: Rocket engine sound directivity properties. (**a**) OASPL directivity for different type of jet flow [11; 22]. (**b**) DI for standard chemical rockets and for the case study [11].  
The acoustic sensor was deployed at a distance of 22 m from the nozzle exit and at 35\({}^{\circ}\) to the plume exhaust axis. Instead, the test article, a 1000 N paraffin-based HRE, was attached to the mount and fixed parallel to the ground in a partially open facility (Figure 7).  
The beginning and ending of signal acquisition were performed manually before the countdown and at the end of the test. The 5-second jet noise signal was extracted from the whole recording and analyzed by performing a Fourier transform (Figure 8), with a sampling frequency of 51.2 kHz. According to [PERSON]'s theorem, the Nyquist frequency is equal to half the sampling frequency (25.6 kHz). This avoids aliasing in the audible spectrum. To reduce noise in the data set, a MATLAB signal toolbox was used for data smoothing, specifically a Savitzky-Golay filter.  
The occurrence of multiple tones at a fundamental frequency and its harmonics can be generated by a closed-loop feedback process, namely screech tone. When a supersonic jet is imperfectly expanded, as in the test case, a quasiperiodic shock-cell structure is formed in the jet plume, causing a radiation of additional noise to the turbulent mixing noise: the broadband shock-associated noise and the screech tone. However, the noise directivity [1; 2] suggests that the latter is mainly radiated to the rear of the nozzle, while the broadband shock noise and turbulent mixing noise are radiated almost normal to and downstream of the jet axis, respectively. Since the acoustic measurement was performed at approximately 35 degrees from the exhaust flow axis, the turbulent mixing noise is  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Equipment** & **Model** & **Frequency Range** & **Dynamic Range** \\ \hline Microphone & PCB 377B02 & \(\pm\)2 dB (3.15 Hz to 20 kHz) & 147 dB (3\% distortion limit) \\ Microphone preamplifier & PCB 426A30 & \(\pm\)0.1 dB (10 Hz to 126 kHz) & - \\ Preamplifier power supply & Larson Davis 2221 & \(\pm\)0.2 dB (10 Hz to 100 kHz) & - \\ \hline \hline \end{tabular}
\end{table}
Table 1: Equipment specifications.  
Figure 6: Experimental apparatus schematic diagram.  
Figure 7: Test facility and test set-up sketch.  
expected to be the most dominant noise component of the spectra. Therefore, the observed SPL peaks are presumably associated with resonance modes of the combustion chamber or post-combustion chamber (Figure 9). In fact, pressure fluctuations within the chambers, e.g., due to the combustion process, can propagate toward the boundary and then be reflected back toward the flame. These waves combine to produce acoustic pressure and velocity oscillations in the vicinity of the boundary; If these acoustic fluctuations are able to alter the combustion rate with the correct phase, they will be converted into higher amplitude acoustic disturbances [16].