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Name: David
Status: Other
Grade: Other
Location: CA
Country: United States
Date: April 2009


Question:
Does moving air make noise or just what is moving the air, say, a fan?



Replies:
Sound you hear comes from vibrations. Sound is a mechanical wave -- which means that it is created by some kind of vibration. The vibration is propagated mechanically through the air where it is sensed by your ear. The vibration (sound) that your ear detects could come from air hitting your ear, it could be a fan, it could be dry leaves, or tons of other things. Air can produce a sound by itself (it can cause vibrations by itself), but then the question is how would you 'hear' it without being in the air. Your ear plays a role in what sounds you hear. A pressure transducer (a kind of highly sensitive vibration detector) would record a different set of vibrations than your ear, and so the 'sound' of air is different depending on what is hearing it. In any case, typically the noise comes from the air hitting something (you, your coat, your ear, etc.) rather than just the air itself.

Hope this helps,
Burr Zimmerman


Both processes can make noise. Wind, especially at high speed can produce sound. Thunder, for example, is a sonic boom. Air moving against a fan blade can also produce sound.

Vince Calder


Air in laminar flow (smooth flow) does not make noise. This concept is used in designing quieter airplanes.

Air in turbulent flow makes noise so airplane designers try to eliminate the turbulence of the air as the airplane passes through the air. It also creates drag on the airplane which has to be overcome by the power of the engines.

What is moving the air? For airplanes (and objects like that), sometimes wind blows over the airplane while it is parked on the airfield, other times, the engines propel the airplanes through the air mass. Try riding your bicycle downwind so that there is no relative wind in your face. I did that next to the flower fields in Lompoc, California one day and when I came up to the wind's speed, all breezes on my faces ended and it got very quiet, until some bees flew by and I could hear their buzzing like I was listening to my Ipod.

In the atmosphere, air temperature pushes hot (less dense) air up and cold air (more dense) falls to the ground. This causes air turbulence (wind) in the atmosphere. In the summer at Lompoc, the land would heat up during the day heating the air over it such that the heated (less dense) air over the land at Lompoc would rise. When that hot air rose it drew in cool air (more dense) from the Pacific Ocean that heats as it dwells over the land (becomes less dense) and then that formerly sea air rises creating a cycle of cool sea air being sucked over the land by the hot land air rising. So every afternoon at about 3 PM the surface wind (channeled into a specific path by the San Bernardino mountains) gets up to 30 to 40 MPH, sometimes causing electric power lines to fall down and start fires.

Large fans are used to create pressure differentials (low pressure in the back, high pressure in front of the fan) in wind tunnels to test aerodynamic models.

Nozzles in Jet engines are used to convert high pressure gasses to high velocity gasses.

Sincere regards,
Mike Stewart



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