2.2. Sound intensity measurement
The sound intensity is a vector quantity indicating both the magnitude and direction of the acoustic energy. The sound intensity is influenced fairly little by background noise, and thus
is useful for finding the sources of noise. In this study, sound intensity was measured in the front and the rear; on both sides; and on the top of the suction housing during its normal operation (Fig. 2). The sound intensity probe kit (B&K Type 3595) was used to measure
intensity. The probe kit includes the 1/2 in. sound intensity microphone pair enabling 1/3-octave centre frequency measurements between 20 Hz and 6.3 kHz. The two microphone signals were analyzed using a frequency analyzer, and then only the values of the imaginary part were stored among the cross spectrum values of the two signals in the monitoring PC. Sound intensities were calculated using a computer and the results were visualized as 2-D contours. Sound intensity was measured by applying 3200 Hz as fmax to identify the noise level at 3000 Hz, the blade passage frequency. In general, the size of microphone and spacer