Noise sound pressure level was measured by using a microphone (B&K Type 4198) placed 1 m in front and 1 m above the suction housing during its normal operation. The time domain signal of sound pressure was transformed to frequency domain data by using fast Fourier transformation (FFT), carried out using a FFT analyzer(B&K 3560B-040). Experimental apparatuses and measurement methods are shown in Fig. 1. Time-series data were collected up to the maximum frequency of 10 kHz by using a FFT analyzer.