The objective frequency of the motor was decided by selecting
57.5 Hz as the basic rotation frequency, the multipliers of 808.72 Hz and 1731.9 Hz, and the BPF of 3000 Hz. The axis of rotation vertically penetrates the suction housing. Therefore, it is seen that the basic rotation frequency of 57.5 Hz and the multiplier
generate noise throughout the suction housing, whereas the frequency of 3000 Hz causes noise in the impeller area only. To identify the exact noise source for the main frequencies of 57.5 Hz, 808.72 Hz, 1731.9 Hz, and 3000 Hz, the results of sound
intensity were visualized; these visualizations are given as 2D color contours in Figs. 6 and 9. As seen in Fig. 6, noise at 57.5 Hz is observed throughout the suction housing. This noise is particularly strong in the separator at the bottom of the suction housing, where air and water are separated; that is to say, in the direction of the silencer outlet. Fig. 7 shows that the noise level at 808.72 Hz is greater on the top of the motor.
This might be explained by the influence of the cooler attached on the top of the motor. The impeller area was identified as the next greatest noise source at this frequency. This might be attributed to the outlet end of the silencer. The impeller area had the highest noise level at the 3000 Hz BPF; the open space in front of the impeller area had a higher noise level than the side hidden by the silencer, pipe, and relay board (Fig. 9). Therefore, in the suction housing, the noise of the basic rotation in the motor and impeller area and the noise occurring in the outlet end of the silencer make the greatest contributions to the overall noise level (see Fig. 8).