# Improved FULMS Algorithm for Multi-Modal Active Control of Compressor Vibration and Noise Reduction
## 3 Test and Analysis of Compressor Excitation Signal  
### Full Band Noise Analysis  
In the anechoic chamber, the near-field noise of the compressor under working conditions is tested. Take points at the center line of the main shell and liquid storage tank, respectively, arrange microphones, measure the sound pressure data, and then pass through  
\[L_{p}\ =\ 20\lg\left(\frac{p}{p_{0}}\right) \tag{8}\]  
Realize the conversion of sound pressure level, where \(P_{0}\) = 2 \(\times\) 10\({}^{-5}\) pa is the reference sound pressure. The location of the measuring points and the near-field sound pressure level of each measuring point are shown in Figures 1 and 2, respectively.  
According to Figure 2, the sound pressure level frequency response increases first and then decreases with the frequency. The sound pressure level is large in the range of low intermediate frequency (2000-5000 Hz). After 5000 Hz, the sound pressure level at each measuring point decreases significantly. Compared with the measuring point of the reservoir, the sound pressure level measured at the measuring point of the main shell is higher. In the low and intermediate frequency region, the peak difference of the two sound pressure levels is between 5-10 dB, which is particularly obvious. From the average sound energy density formula [21]:  
\[\overline{\epsilon}\ =\ \frac{\Delta\overline{E}}{V_{0}}\ =\ \frac{p^{2}}{2\rho_{0}c_{0}^{2}}\ =\ \frac{p_{\epsilon}^{2}}{\rho_{0}c_{0}^{2}} \tag{9}\]  
Figure 1: Microphone arrangement. The numbers 1-12 indicate the positions of the 12 survey points. Measuring points 1, 4, 7, and 10 are 50 mm away from the upper surface. Measuring points 3, 6, 9, and 12 are 50 mm away from the lower surface, and measuring points 2, 5, 8, and 11 are 140 mm away from the lower surface.  
Figure 2: Sound pressure level frequency response of each measuring point: (**a**) measuring point of the main shell; (**b**) measuring point of the reservoir.  
where \(p\) is the sound pressure at the measuring point, \(p_{e}=p/\sqrt{2}\), is the effective sound pressure, and the sound power is directly proportional to the quadratic power of the sound pressure. Through the conversion of sound pressure and sound pressure level, the sound energy radiated per unit time at the measuring point of the main shell is calculated, which is about 3\(\sim\)9 times that of the corresponding measuring point of the liquid reservoir.  
By comparing the noise analysis results, it can be seen that under working conditions, the compressor noise is mainly distributed in the low and medium frequency section, and the sound energy at the main shell is dense. That is, the main shell and its connectors are the main noise source, and the energy distribution of the accumulator shell is sparse and has little impact on the sound radiation of the whole machine. The main characteristics of each excitation source are the shell radiation caused by irregular excitation inside the compressor and the harmonic component of the electromagnetic force with the same modal frequency as the whole machine. Therefore, the active control method for low-frequency vibration and sound signal is used to suppress vibration and sound radiation, and the main frequency band of active control is 1000-5000 Hz. However, it is still necessary to analyze the characteristics of the excitation source and determine the main excitation source.