# Investigation on Characterization of Typical Characteristic in Compressor Based on Flat Plate Model
## 3 Noise Experiments and Results
### Measurement System  
The high-pressure compressor of the aeroengine always works in a complex environment with a pressure and temperature which are as high as 3 MPa and 150 \({}^{\circ}\)C, respectively, and with the high-intensity noise (the sound pressure level of it reaches 170 dB or more and the frequency range covers 50 Hz-12 kHz). It is very difficult to meet the test requirements with conventional acoustic microphones in such an ambient environment. Therefore, a derived noise measurement method that is based on rigid-wall acoustic waveguide technology was adopted in the paper [34]. The features of this measurement system based on the principle of pipe sound transmission are that the sensing part of the microphone does not directly contact with the high-temperature environment, but leads the sound wave with a specially designed sound waveguide tube. Based on the derived noise measurement method of rigid-wall acoustic waveguide technology, a measurement system used to monitor the compressor internal noise is established, which consists of the acoustic waveguide, the microphone holder, the semi-infinite attenuator tube, the data acquisition, and analysis instrument. The connection diagram of the measurement system is shown in Figure 1. The sound wave guide is connected to the engine compressor casing, the end of which is installed flush with the inner surface of the compressor casing, so the sound waves inside the compressor flow channel can be led out through the sound wave guide. A quarter-inch condenser microphone is installed in the microphone holder. One end of the microphone holder is connected to the acoustic waveguide and the other end is connected to the semi-infinite attenuator tube, which is applied to avoid the reflection in the integral pipe.  
The experimental test of the noise between the stages of the high-pressure compressor of the turbofan engine is implemented with this measurement system. In addition, the non-contact blade vibration test system is taken to monitor the vibration of the first-stage rotor blades of the high-pressure compressor. In order to obtain axial sound pressure distribution of the high-pressure compressor of the engine, during the experiment, a total of four testing  
Figure 1: Schematic diagram of measurement system.  
points are arranged along the axial direction of the engine to measure the sound pressure of the inner wall of the compressor casing. The positions of four measuring points from one to four are the inlet guide vane (IGV), the clearance between the IGV and the first stage of the rotor blade (Rotor1), and the directly above Rotor1 and the clearance of the first stage of stators (Sator1), respectively. The schematic diagram of the positions of noise measurement points during the experiment is shown in Figure 2. Additionally, a total of three points used to monitor the rotor blade vibration are arranged at different circumferential positions along the high-pressure first-stage rotor blade, which is present in Figure 3. The fiber optic sensor probe is mounted at the designed position of the casing, and the positioning reference sensor is installed on the surface of the rotor blade. The time interval between the blade tip amplitude pulse and the rotational speed pulse is obtained through measurement and calculation. The blade tip deviates along the direction of rotation in the process of blade vibration, then the time to reach the sensor changes. Thereby, the vibration parameter information such as blade amplitude, phase, and frequency are obtained.