An Investigation of the Effect of Uneven Blade Spacing on the Tonal Noise Generated by a Mixed Flow Fan

C1a - Tonal Noise Reduction

The acoustic signature of a desk fan is one of the keys to its market success. In a Dyson Air Multiplier fan, the internal air flow is generated by a mixed-flow impeller with a low blade tip Mach number. The impeller produces an audible tone, known as the blade passing frequency tone. This sharp tone is inherent to all fans and compressor units and is problematic to desk fans since it could be perceived by the user against the fan broadband noise. The two main factors influencing the blade passing frequency are the rotational speed of the impeller and the number of blades. The approach discussed in this paper was to develop a modulated (uneven) blade spacing mixed-flow impeller (driven by a DC brushless motor) in order to spread the associated energy from a single tone to several frequencies. First, a theoretical analytical tool was developed to define a suitable uneven blade configuration and the predicted acoustic signatures with associated relative magnitudes were calculated. The unevenly spaced blade impeller was then prototyped to measure aerodynamic and acoustic performance against a symmetrically spaced blade impeller using an ISO5136 test rig. Also presented in the paper are the results of a perceptive test conducted to establish a sound quality model for desk fans. The sound quality model quantifies the level of subjective preference for desk fan sound and was used to assess the acoustic performance of both the uneven and symmetrical blade spacing impeller designs. Results show that the uneven blade spacing impeller does not improve the subjective preference compared to the symmetrically spaced blade impeller.

Keywords: Tonal noise, Modulated blade spacing, Mixed-flow impeller, Sound quality.