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I used to live in a house that had ceiling fans, which were great for the summer and so wonderfully silent when running.

Now however I've moved to a house that has no ceiling fans so I've bought a couple of standing fans. However, they're too noisy to use at night without disrupting sleep.

So my question is: what is it about the design of a ceiling fan that allows it to be so quiet?

(I'm in Australia, hence thinking about fans at this time of year)

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  • $\begingroup$ mostly because they rotate in relatively low velocities $\endgroup$
    – user98038
    Commented Jan 3, 2017 at 22:40
  • $\begingroup$ This seems to be more of a design/engineering question than a question about physical principles. $\endgroup$
    – ACuriousMind ♦
    Commented Jan 3, 2017 at 22:43
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    $\begingroup$ You make a fan quiet, for moving the same amount of air, by making the blades as large and slow-moving as possible. $\endgroup$
    – Mike Dunlavey
    Commented Jan 3, 2017 at 22:56
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    $\begingroup$ Beyond the mentioned, ceiling fans are able to push air upwards and then down along the walls of the entire room.. portable fans typically are pushing air right onto you which can be loud as air moves across your ear canals. $\endgroup$
    – bleuofblue
    Commented Jan 4, 2017 at 0:54

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The noise is generated to a large degree by the fan blades. The portable fans you mention typically provide much higher flow velocities than ceiling fans, and their blades obviously rotate much, much faster. Two factors are then responsible for their significant noise level:

  1. Aeroacoustic sound pressure levels are proportional to the 6th to 8th power (depending on the type of interaction) of the flow velocity relative to the blades.
  2. The blades that are typically used in the kind of fans you mention are of very low aerodynamic quality, meaning the factor in front of that sixth (or eighth) power is (relatively) large.

In order to make the fans more quiet, one would want to both reduce the blade speed and install higher quality blades (with proper aerodynamic profiles, rather than just bent pieces of sheet metal).

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A box fan or a room fan has to be portable. I wouldn't expect to see one with a blade diameter larger than 50cm.

A ceiling fan is much less constrained on size. A fan sized for a bedroom would probably be 100cm on the small end.

The box or pole fan is going to have to run at a higher speed to move a similar amount of air. The higher speed is going to cause more turbulence and noise.

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