## 3 Methods Used for Identification of Tonal Noise Sources
### Experimental Setup
The rotor speed \(u_{2}=450\) m\(\cdot\)s\({}^{-1}\) showed initial indications of buzz-saw noise development, with the most obvious being observed at sweep B, depicted in Figure 6.  
In all sweeps (Figures 5-8), at maximum rotor speed \(u_{2}=550\) m\(\cdot\)s\({}^{-1}\), significant tonality was seen at low frequencies, but these were not integer multiples of the rotational frequency. It can also be noted that the BPF was significant at all measured points, and a distinction can be made between MBPF and MSBPF. The cause of the opposite trend with respect to all other operating points can probably be found in the rotating stall at operational points leftward from the maximum efficiency point in the compressor map at the given speed line. As for TCN, it can be seen from the depicted amplitude-frequency characteristics that there was no distinct broadband noise whose origin could be attributed to TCN in the range \(\epsilon_{\rm R}=(0.36\)-\(0.64)z_{\rm R}\)[27]. By evaluating the sound pressure level for the entire audible spectrum at the measured operating points of the compressor, a compressor map can be obtained that presents the total sound pressure level (Figure 9). The results presented in Figure 9 clearly show the strong expected increase in sound pressure level with compressor speed. Figure 9 therefore presents the typical characteristics of aerodynamic noise distribution in a compressor map. Figure 8: Sound pressure level \(L_{\rm P}({\rm A})\) vs. frequency in the range 0–20 kHz, sweep D.  
Figure 7: Sound pressure level \(L_{\rm P}({\rm A})\) vs. frequency in the range 0–20 kHz, sweep C.  
However, an accurate determination of the sound pressure level would require the test setup to be placed in an anechoic chamber. The presence of tonal noise was already evident in the amplitude-frequency characteristics. However, the tonal metrics were used to qualitatively evaluate the presence of tonal noises. The results of applying the tonal metrics in the form of sound pressure levels of prominent tones at sweeps A-D are presented in Figures 10-13. Figure 10: Sound pressure level of prominent tones for sweep A in the frequency spectrum as a function of \(\varepsilon_{\mathrm{R}}\). Green indicates sub-synchronous tones, black indicates synchronous tones, blue indicates super-synchronous tones and red indicates super-harmonic tones. Figure 9: Compressor map showing sound pressure levels in the range of 0–20 kHz. Figure 2:0 Sound pressure level of prominent tones for sweep B in the frequency spectrum as a function of \(\varepsilon_{\rm R}\). Green indicates sub-synchronous tones, black indicates synchronous tones, blue indicates super-synchronous tones and red indicates super-harmonic tones. Figure 2:1 Sound pressure level of prominent tones for sweep C in the frequency spectrum as a function of \(\varepsilon_{\rm R}\). Green indicates sub-synchronous tones, black indicates synchronous tones, blue indicates super-synchronous tones and red indicates super-harmonic tones. At all rotor speeds and at all sweeps, the tonal components of the blade-related noise can be identified. The metrics also show the presence of sub-synchronous frequency components that are very likely related to constant tone noise. This type of noise is due to instabilities in the inner lubrication layer of the hydrodynamic semi-floating ring bearings. In addition, the results show synchronous tones that are probably related to rotor unbalance and most likely to buzz-saw noise at higher speeds. At higher rotor speeds (\(u_{2}=500\) m\(\cdot\)s\({}^{-1}\) and \(u_{2}=550\) m\(\cdot\)s\({}^{-1}\)), mainly super-harmonic components appear, which confirms the presence of buzz-saw noise at all four sweeps. The occurrence of super-synchronous tonal components in the frequency spectrum is relatively limited.