Comparison of Sound Quality Metrics for Axial Flow Fans with Straight and Forward Swept Blades

2.4 Psychoacoustics

The design of industrial axial flow fans is driven by regulations regarding the efficiency as well as the noise generated during operation. A number of aerodynamic design improvements exist that succeed in reducing fan noise. For example, it has been shown that forward swept fan blades exhibit reduced noise levels during operation when compared to a straight bladed fan. Even though the primary driver for reduced noise generation is the concern for hearing loss in humans, it has recently become important to consider customer perception of one's product in an increasingly competitive marketplace. As such, the purpose of this paper is to compare subjective sound quality metrics (loudness, sharpness, roughness, fluctuation strength and annoyance) of a straight bladed fan and one with swept blades.
Two 630 mm diameter fans with similar blade designs were tested in a ducted facility for a variety of flow rates where their performance as well as sound quality metrics could be determined. As expected, it was found that even though both fans performed similarly, the fan with swept blades exhibited a significantly lower level of tonal noise at the blade pass frequency as well as reduced broadband noise around 2 kHz when compared to the straight bladed fan. Upon investigation of the sound quality metrics it was found that, in addition to a reduction in loudness, the swept bladed fan also exhibited lower sharpness, roughness and fluctuation strength values for throttled flow rates, which are commonly associated with positive consumer impressions. However, with increased flow rates the difference between the two fans generally decreased, thereby indicating that the benefit of the improved sound quality exhibited by a swept bladed fan is dependent on its operating point.