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I'm currently designing my own centrifugal fan for one of my projects, everything excluding the motor.

A diagram of a generic centrifugal fan

My design is similar to the above just that the outlet is much shorter.

The problem I'm facing is that my fan produces a high pitch noise. Assuming I keep the blades design and the power parameters the same, how can I reduce the high pitch frequency?

To my understanding, the volute casing has a tiny gap at the start of the volute tongue (indicated as Rc), could this be the cause of the high frequency noise?

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Yes, you are probably hearing the noise generated when the blades on the fan pass by the start of the volute tongue. Modeling this process will be hard, so experiments are the best way to proceed. Try making that intersection more gradual by, for example, radiusing that lip or cutting shallow slots in it that run in the same direction as the fan movement.

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  • $\begingroup$ Thank you for your suggestion, I will try that! The shallow slots you suggested, are they vertical or horizontal? And could you explain how the slots help in reducing the pitch? $\endgroup$ – Wb16 Jan 14 '19 at 3:48
  • $\begingroup$ the shallow slots point in the direction of airflow. their purpose is to make the blade passings at the lip less sudden. a fan that is finessed in this way will make more of a hisss noise and less of a bzzzz noise. This trick is done in axial flow fans ("muffin fans") by making the trailing edges of the blades zig-zagged, to obtain the same effect. $\endgroup$ – niels nielsen Jan 14 '19 at 4:00
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Spin it faster or add more blades. The threshold of human hearing is about 22.5 kHz for 90% of people. You want to get the primary frequency over 25 kHz to be on the safe side, and it will still drive dogs, cats, and birds crazy. Also, some people have "poodle ears". I met a lady who could hear 26 kHz coming from a grease trap vent fan located across the street in at a restaurant. I shut their fan off for two days rather than comp her a hotel room.

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  • $\begingroup$ Thank you for your suggestion, I will try to play around with the blade count. I doubt I can run it any faster due to power limitations. $\endgroup$ – Wb16 Jan 14 '19 at 3:49

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