We see that a running fan produces some "humming" sound - "The sound of air". What kind of sound it is? Why does a running fan produce some sound?
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asked Apr 10, 2020 at 12:15
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$\begingroup$ Do you mean the "rushing air" sound of a fan? Or the hum itself? $\endgroup$– taciteloquenceCommented Apr 10, 2020 at 12:55
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1$\begingroup$ My questions is about all kinds of sounds. I do not know the reasons behind them. The "hum" word I used just to mention the sound. $\endgroup$– Devansh MittalCommented Apr 10, 2020 at 12:56
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$\begingroup$ Related post by the OP: physics.stackexchange.com/q/542853 $\endgroup$– user258881Commented Apr 10, 2020 at 13:05
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$\begingroup$ The "hum" could be the motor. When the fan blade hits the air or moves through it, it will cause the air to move, this can become acoustic sound. The same thing happens for other movement of objects through air. Not just fans. $\endgroup$– user196418Commented Apr 10, 2020 at 14:21
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1 Answer
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As a fan blade moves through the air, it pushes a pulse of air away from the fan and towards you. You perceive this as a single pulse of "whump" sound.
For a slow-turning three-bladed fan, three pulses of whump sound are produced for each motor shaft revolution, and you hear whump whump whump.
Now you speed the fan all the way up and the individual whumps blend into a continuous wave that is more or less a sine wave at a frequency equal to the number of blade passages per second.
Superimposed on the blade passage noise is a steady roar or hiss caused by turbulent motions of the air flow as it squeezes through the protective grill on the fan housing.
If you make the fan blades wide enough so the trailing edge of each one overlaps the leading edge of the next one, you can greatly reduce the blade passage noise and all you will hear will be the hiss.
Because of the greater surface area of an overlapping fan disc, it will experience more friction with the air and therefore exhibit lower efficiency than a (far noisier) fan with slender blades that have no overlap.
answered Apr 10, 2020 at 17:57