Psychoacoustic Evaluation of Fan Noise

E2 - Sound Quality

Besides aerodynamic properties the sound quality of fans gains increasingly in importance. In contrast to technical parameters like efficiency, fan noise is directly perceived and evaluated by human beings. Hence differences in the sound character which are not always captured by the commonly used physical parameters like sound pressure levels and even more detailed spectral analysis can lead to very different product assessment.
It should therefore be an aim to develop a measure to characterize the sound quality of fan noise which is more oriented towards the subjective felt noise impressions and which is able to represent the sound emissions of fans in different applications more aurally-equivalent, i.e. in a manner that corresponds better to our hearing. The concerned special field of acoustics is the science of psychoacoustics.
For that reason the company ebm-papst built up a psychoacoustic lab with one moderator and eight listening work stations where hearing comparisons of recorded fan noise signals can be conducted by listening to the sound examples with loudspeakers and headphones.
In these listening tests the annoyance level and other acoustic relevant attributes are queried from a number of test persons by means of two methods: the semantic differential and the paired comparison.
Several examples of axial and radial fans in undisturbed laboratory setup and in different applications are investigated. One measurement series addresses the comparison between the acoustic (physical) and psychoacoustic (subjective) behaviour of different fans under different installation situations. In another investigation effects of varying operating point and rotational speed are studied.
The influence of dedicated acoustic phenomena like for example tonal components is investigated by means of signal manipulation.
In summary one can say that the annoyance level is decisively influenced by the objective psychoacoustic parameter loudness. Furthermore the tonality and especially in fan operating points near stall the roughness and the fluctuating strength can play an important role.