40

fan and the pump impeller are identified in a separate experiment using a BK condenser microphone.

    The blade passage frequencies of the fan and pump impeller are determined for the conditions listed in Table 3-1. To determine the blade passage frequency of the fan, the pump is switched off and acoustic pressure measurements are obtained very close to the entrance of the blower with the microphone. A similar exercise is repeated for the pump impeller. The specifications of the microphone are given in below. The results are compared to Gyy in Figure 3-15.

    A strong tone at 364 Hz is noticed in the spectrum of the microphone data near the blower. A second tone of lesser intensity is noticed at 728 Hz in this spectrum. The second tone is inferred as the harmonic of the 364 Hz tone. In addition, a range of smaller tones is noticed at 40 Hz, 81 Hz, 121 Hz, 162 Hz and 202 Hz. This is explained as follows.

    The fan in the blower is known to have nine blades. The blade passage frequency is calculated as the number of blades times the frequency of revolution of the fan. The result of dividing the frequency of the first tone, i.e. 364, by the number of blades is very close to 40 Hz. Therefore, there is a high probability that the first peak of the range is a result of eccentricity in the mounting of the fan because it is likely to have the effect of producing tones at the frequency of revolution. The subsequent peaks at multiples of 40 Hz are most likely to be the harmonics of the tone at 40 Hz.

    The output spectrum Gyy has a strong tone at 364 Hz and 728 Hz, which is mostly due to the strong vibration exhibited by the blower at the blade passage frequency and its first harmonic. Also, discernible peaks are noticed at 40 Hz, 81 Hz and 202 Hz that arise