Several rotors of axial-flow fans and side channel blowers with optimal circumferential spacing have been tested in a hemi-anechoic chamber; such rotors have been designed by means of an existing method based on the minimization of the tonal noise peaks prominence from the broadband spectrum. The resulting SPL spectra contain different components: tonal ones, which are a well-known cause of annoyance (intended as an undesired feature of the received noise related to the short-term exposure), broadband ones, which could have a positive masking effect, and further components due to the electric motor. As expected, the blade spacing strongly affects the tonal noise part of the spectrum, which is mainly of aeroacoustic origin; in axial-flow fans, it also affects the low-frequency broadband part related to the leakage flow. The present study has also confirmed the good consistency between measured spectra and theoretically predicted ones even for side channel blowers. The characteristic curves of the tested machines are not significantly affected, but the quality of the radiated noise considerably changes as the spacing non-uniformity increases: tonalness decreases and roughness increases, fluctuation has not a systematic behavior, and no significant variation in other perceived characteristics may be noticed. In order to quantify such perception, the psychoacoustic parameters commonly employed for automotive cooling fans have been computed. The tonalness decrease leads to a significant annoyance reduction, while the increase in roughness seems of minor importance from a noise annoyance standpoint. Nevertheless, in strongly asymmetric rotors, the change in the perceived acoustic signature may be relevant and an unaware listener could interpret the roughness increase as a potential mechanical malfunction. As for generality of the present discussion, it should be considered that the tested rotors have been optimized in order to reduce the tonal noise peaks prominence, which obviously results in a reduction in the perceived tonalness. On the contrary, spacing obtained by means of different criteria could yield different variations in tonalness, and the same likely happens to loudness. Instead, the rise of peaks at shaft frequency harmonics is a direct consequence of the uneven spacing which should sistematically yield a roughness increase.
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