## 2 Noise Characteristics and Source Identification  
The research object of this paper is a two-stage centrifugal air compressor used in the air supply system for vehicle fuel cells. Figure 1 shows the schematic diagram of a typical hydrogen fuel cell system for vehicles. In order to increase the efficiency of the fuel cell, the air is compressed by a centrifugal compressor and then enters the fuel cell. Pressurized air reacts with hydrogen in the fuel cell to generate electric energy, which is supplied to the motor or battery. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**References** & **Number of Blades** & **Designed Rotation Speed [rev-min-1]** & **Designed Pressure Ratio** & **Designed Mass Flow Rate [kg-s-1]** \\ \hline
[1] & Main: 13; Splitter: 13 & 50,000 & 4.0 & 2.8 \\ \hline
[2] & 8 & 4800 & & \\ \hline
[3] & 20 & 14,000 & & 5.32 \\ \hline
[4] & 32 & 2900 & 1.02 & 1.25 \\ \hline
[5,6] & Main: 8; Splitter: 8 & 22,000 & 3.85 & \\ \hline
[9] & Main: 7; Splitter: 7 & 98,529 & & 0.211 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Operating conditions and structural parameters of the compressors in references. Figure 1: Schematic diagram of a typical hydrogen fuel cell system for vehicles. The cross-section of the centrifugal compressor prototype is shown in Figure 2. The air is sucked and accelerated by the first stage impeller. After being pressurized by the first stage vaneless diffuser and the first stage solute, it enters the inter-stage pipeline and is further accelerated by the second stage impeller. The pressurization and discharge are completed by the second stage vaneless diffuser and the second stage solute. The impellers of both stages have no splitter. The main structural parameters of the air compressor are shown in Table 2. The suction pressure is 101.3 kPa. The specific operating conditions are shown in Table 3. The three conditions are all near the design condition with different rotation speeds. The near-surface noise measurement can alleviate the distortion of an acoustic signal caused by self-attenuation and external interference in the transmission process and, thus, reflect the basic characteristics of the noise radiated from the air compressor as truly as possible. Table 4 shows the noise pressure levels of different measuring points under different working conditions. With the increase in impeller rotation speed, the sound pressure level at each measuring point increases. The sound pressure level of Measuring Point 3 near the motor is relatively high, but it is lower than that of Measuring Points 1, 2, and 5 near the gas flow channel under various working conditions, which indicates that the motor noise is one of the main noise sources of the air compressor, but that the aerodynamic noise is more prominent. Measuring Point 5 has the highest sound pressure level under different working conditions, which is the result of the joint action of electromagnetic noise and aerodynamic noise. Figure 4: Real images of the test rig and the sound level meter (**a**) Test rig. (**b**) Sound level meter. Figure 3: Schematic diagram of the test rig and the positions of the noise measuring points. Figure 5 shows the near-field noise spectrum characteristics of Measuring Point 5 on the surface of the compressor under different working conditions. The noise of the compressor has significant periodic harmonic characteristics. When the compressor is working under the common condition, the noise is prominent in the fundamental frequency (1000 Hz) and the second harmonic frequency (2000 Hz) of the rotation speed, with the noise peak at 76.9 dBA and 83.8 dBA, respectively. The noise peak of the high-order rotation frequency shows a downward trend with the increase in frequency. The discrete single-tone noise of the blade passing frequency (15,000 Hz) is not prominent, and its sound pressure level is only 30.6 dBA, which is far lower than the sound pressure level of the rotation fundamental frequency and its second harmonic frequency.