Impact of geometric modifications of small axial fans on psychoacoustic metrics.

• The small axial fans were 3D printed and tested according to ISO 5136 and ISO 5801. • Psychoacoustic metrics were compared under design and free discharge conditions. • Loudness, roughness and tonality showed the greatest impact on noise annoyance. • Individual modifications are not sufficient to reduce broadband noise. • A holistic, noise reduction strategy in the design of small axial fans is needed. This paper presents a comprehensive experimental investigation of a basic axial fan and its various modifications. The fan blades were designed according to recommendations from the literature. The prototypes were 3D printed and tested under different operating conditions in accordance with the ISO 5136 and ISO 5801 standards. The aim of the study was to investigate the effects of the axial fan design on the characteristic curve and on the acoustic and psychoacoustic properties of the emitted sound. The characteristic curves were influenced by geometrical features that can lead to obstruction of the flow. The A-weighted sound power level spectra showed that low-frequency (subharmonic) noise humps can be effectively reduced by modifications near the fan hub, while tonal noise in the mid-frequency range can be mitigated by blade root enlargement, small wing tips and low-amplitude edge serrations. The tonal noise was greatly affected by the addition of tip flanges and rotating shrouds, but these modifications reduced the high frequency noise. The psychoacoustic evaluation showed a higher sensitivity to the characteristics of the emitted noise compared to conventional A-weighted one-third octave spectra. While all modifications led to changes in psychoacoustic metrics, loudness, roughness and tonality showed the greatest variability and influence on psychoacoustic annoyance, while sharpness and fluctuation strength were less affected by the different modifications. The results presented emphasize the complex interplay between geometry, aerodynamics, acoustics and psychoacoustics in small axial fans and highlight the need for holistic approaches to fan design in order to solve noise-related problems. The 3D printed machines had similar energy and acoustic characteristics as traditionally manufactured fans, so the use of additive manufacturing techniques proved to be a cost-effective and practical step in the fan design process that could be easily implemented. [ABSTRACT FROM AUTHOR]