## 5 Conclusions  
The measuring of the external acoustic field makes it possible to identify the dominant noise sources of aerodynamic origin and to quantify their influence at selected operating points. In the case of the centrifugal compressor presented here, the influence of integral multiples of the rotational frequency was dominant at all measured operating points and can therefore be considered the most significant contributor to the psychoacoustic impact on humans. The analysed acoustic spectrum also showed an increase in the number of tonal components with increasing rotor speed, which is due to shock waves--that is, buzz-saw noise--typical of the supersonic operating mode of a centrifugal compressor. However, the detailed characteristics of aerodynamic phenomena, such as the number of stall cells and their rotational frequency in the case of rotating stall, cannot in principle be reliably determined this way. Measurements of the external acoustic field can therefore be used for a more global assessment of the influence of individual noise types. It is also worth mentioning that Kelvin-Helmholtz instability can occur precisely at the BPF [48], and the presence of tonality can therefore to some extent be influenced by it. However, its detection based on external acoustic field measurements is uncertain; one needs to find local maxima at the BPF in the time domain and form an envelope from them. If these maxima have the same value, that is, the amplitude of the BPF is constant in time, one could conclude that no other aerodynamic phenomenon is present at the BPF. However, in the opposite case, where the BPF amplitude changes over time, it is not clear from the acoustic measurements what effects are responsible for these changes. For the sampling frequency of \(65,536\) Hz used and the lowest measured rotor speed \(u_{2}=250\) m\(\cdot\)s\({}^{-1}\), the number of points describing the MBPF in the time domain was 17. In the case of the maximum measured rotor speed \(u_{2}=550\) m\(\cdot\)s\({}^{-1}\), there were only seven of these points. Thus, the number of measured points describing the MBPF in this case was insufficient to correctly predict the possible time history of the pressure fluctuations in the amplitude. The acoustic spectrum presented here does not show broadband TCN. The absence of TCN, due to the occurrence of aerodynamic instabilities in the tip clearance region, may to some extent have been caused by the fact that the experimental compressor configuration consisted of an open loop with no inlet duct attached to the compressor impeller inlet. Therefore, it cannot be excluded that the rotor-rotating instabilities in this case were different from those in cases where an inlet duct is used. Some of the implications of this research are quite serious, as it demonstrated the unpredictability of the sound pressure level for individual operating points, as discussed in Section 4. The analysis of the measured sound pressure levels did not show a clear trend in the aerodynamic noise at points on a given speed line and indicated an ambiguous relationship between the \(L_{\mathrm{P}}(\mathrm{A})_{\mathrm{MBPF}}\) and the \(L_{\mathrm{P}}(\mathrm{A})_{\mathrm{MBPF}}\) at each operating point. This unpredictability probably stems from the complexity and non-linearity of the flow field itself. Therefore, the results presented here cannot be generalised and should be seen as somewhat limited to the rotating machine, although they do show a typical global trend of increasing total sound pressure levels at higher rotor speeds and surge. However, the elucidation of aerodynamic phenomena leading to a deeper understanding of the acoustic spectrum presented here requires additional measurement techniques. These include, for example, the use of high-frequency pressure probes to detect rotating stall characteristics. [PERSON] did signal processing and wrote the paper, [PERSON] developed tonal metrics and reviewed the paper. All authors have read and agreed to the published version of the manuscript.