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Author Topic: Does Sound Make Wind ?  (Read 6430 times)

Offline neilep

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Does Sound Make Wind ?
« on: 20/02/2008 13:46:28 »
Hi All,

See Dr Mark Oni,





He's making sound waves !...nice aren't they ? Sound is my all time favourite thing to listen to.


Does sound move the air it travels through ?...if so....if the sound was loud enough...would I ' feel ' it against my face as a breeze ?...can it actually be measured ?

I'm not referring to deep bass vibrations that travel through the ground and walls and stuff !



 

another_someone

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Does Sound Make Wind ?
« Reply #1 on: 20/02/2008 14:25:36 »
The whole point about a wave is that it has a zero average deviation - i'e' you average out all the peaks and it comes to zero.

The only possible scenario where I can see that changing is if you start generating large amounts of heat (e.g. in liquids this could be through cavitation), which may cause the overall body fluid to expand outwards.  Gasses do not cavitate as such, but could nonetheless be heated by sound, and this may cause a slight breeze as it expands.
 

Offline JimBob

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Does Sound Make Wind ?
« Reply #2 on: 20/02/2008 16:49:51 »
Sound does move air and thus makes wind. We "feel" the movement with our ear. The tympanic membrane of the ear is moved back and forth in response to air movement, causing the bones of the inner ear to move and thus transfer the "sound" (air movement) into the cochlea which turns this into what our brain interprets as sound. We DO feel the wind.
 

lyner

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Does Sound Make Wind ?
« Reply #3 on: 20/02/2008 22:10:38 »
If you hold a candle in front of a large loudspeaker the flame is blown away from the cone. This must be because the movement of the cone, back and forth manages to generate a current of air at the same time as producing the sound, which is just longitudinal vibrations. This may be because of the flexing of the cone.
BUT they are two separate and distinct effects.
 

another_someone

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Does Sound Make Wind ?
« Reply #4 on: 21/02/2008 00:49:23 »
Sound does move air and thus makes wind. We "feel" the movement with our ear. The tympanic membrane of the ear is moved back and forth in response to air movement, causing the bones of the inner ear to move and thus transfer the "sound" (air movement) into the cochlea which turns this into what our brain interprets as sound. We DO feel the wind.

Yes, it is a movement of air, but no nett movement of air over the period of a cycle (the loudspeakers push air out as the cone goes out, and pull air in as the cone comes back - otherwise, the cones would go walkabout - ofcourse, wind instruments are different in that respect, but the wind they blow is not a consequence of the sound, but incidental to it).
 

another_someone

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Does Sound Make Wind ?
« Reply #5 on: 21/02/2008 00:52:38 »
If you hold a candle in front of a large loudspeaker the flame is blown away from the cone. This must be because the movement of the cone, back and forth manages to generate a current of air at the same time as producing the sound, which is just longitudinal vibrations. This may be because of the flexing of the cone.
BUT they are two separate and distinct effects.

I cannot say why the effect should be, but it need not be because of a wind at all - it could be that the sound disrupts the chemical reaction that is the flame, and the side away from the sound source is shielded from some of the sound, so is less disrupted by the effects of the sound.

Another possibility is whether it might be something analogous to the Casimir effect in quantum physics, which is supposed to effect ships on the high seas - but usually effects two bodies with a sheltered region between them, so I am not sure if one object with a lea behind it could have the same effect.
« Last Edit: 21/02/2008 01:07:08 by another_someone »
 

Offline lightarrow

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Does Sound Make Wind ?
« Reply #6 on: 21/02/2008 07:58:36 »
I'm not so sure that sound cannot be thought as a sort of "wind" because, yes, air moves back and fort, but in that case it's not a stationary wave, air does really propagate along a specific direction, it has a phase velocity. So, if you have a very low frequency sound wave, say 0.05 Hz, then for 10 seconds you have sound "wind" in a specific direction and for the next 10 seconds sound "wind" in the opposite direction, and so on.
You cannot however call it properly "wind" because this implies an average non zero movement along a direction, while in the case of sound the average (made at least in one period of the wave) is zero, as George wrote.
« Last Edit: 21/02/2008 08:50:24 by lightarrow »
 

lyner

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Does Sound Make Wind ?
« Reply #7 on: 21/02/2008 09:07:41 »
It's nothing fancy. There is just a current of air caused by the fact that the loudspeaker cone is of finite size and you are in the 'near field' of the vibrations. The flame just leans away from the cone (and the smoke drifts away too). This effect only occurs quite close to the speaker (10cm or so) and for very loud sound levels; the cone acts like a fan. When the cone moves forwards it pushes air forwards (momentum transfer to the air on its surface); when it moves backwared, air flows from all around into the vacant space because the pressure all around is higher.  This is a circulation effect which dies rapidly with distance (within less than a wavelength, probably) ; a near field phenomenon. Afaik, it is quite a well known effect.
Just had a thought. This is because we are dealing with a longitudinal wave; it wouldn't happen when generating a transverse wave.
« Last Edit: 21/02/2008 09:41:59 by sophiecentaur »
 

Offline lightarrow

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Does Sound Make Wind ?
« Reply #8 on: 21/02/2008 12:05:32 »
It's nothing fancy. There is just a current of air caused by the fact that the loudspeaker cone is of finite size and you are in the 'near field' of the vibrations. The flame just leans away from the cone (and the smoke drifts away too). This effect only occurs quite close to the speaker (10cm or so) and for very loud sound levels; the cone acts like a fan. When the cone moves forwards it pushes air forwards (momentum transfer to the air on its surface); when it moves backwared, air flows from all around into the vacant space because the pressure all around is higher.  This is a circulation effect which dies rapidly with distance (within less than a wavelength, probably) ; a near field phenomenon.

That's very reasonable, interesting! I didn't think about it.
 

lyner

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Does Sound Make Wind ?
« Reply #9 on: 21/02/2008 17:17:14 »
Well,  you know me. If there's a simple, classical, explanation for a phenomenon then I'll go for it. There are enough neat explanations without running off into Quantum Mechanics in the first instance.
I'd like to know how they got the picture of the diffraction pattern in the first post; is it smoke or precipitation?
Judging from the fashion, it was taken a long time ago. (Neat hair-do plus earnest expression).
« Last Edit: 21/02/2008 17:20:30 by sophiecentaur »
 

Offline lightarrow

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Does Sound Make Wind ?
« Reply #10 on: 21/02/2008 19:33:34 »
Well,  you know me. If there's a simple, classical, explanation for a phenomenon then I'll go for it. There are enough neat explanations without running off into Quantum Mechanics in the first instance.
Agree.
Quote
I'd like to know how they got the picture of the diffraction pattern in the first post; is it smoke or precipitation?
Probably smoke and a high intensity light source under a black background to have a high contrast.
 

Offline Sorcerer

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Does Sound Make Wind ?
« Reply #11 on: 24/02/2008 19:47:07 »
Does sound move the air it travels through ?...if so....if the sound was loud enough...would I ' feel ' it against my face as a breeze ?


It does. The ordinary wave that keeps its form can’t move the air. But it is approximation only. Real wave warping its form it propagates through. It follows to movement of air.
 

lyner

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Does Sound Make Wind ?
« Reply #12 on: 24/02/2008 22:12:43 »
Perhaps it should refer only to spherical and plane waves. This is the limit of 'far field' conditions. It also assumes a linear medium - dry air (an 'ideal gas') with low amplitude waves, for instance.
 

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Does Sound Make Wind ?
« Reply #12 on: 24/02/2008 22:12:43 »

 

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