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Moving Air and Noise Generation
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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Update: June 2012
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