> How come molecules "push" and "pull"? How come the strength varies?

The "push" and "pull" Warp is referring to (I guess) are the forces caused 
by a pressure difference.  This causes the next "bit" of air to be moved, 
thus creating another pressure difference, and the next bit moved, etc, this 
is how the wave propagates - it's the same as a longitudinal wave in a 
slinky spring.

Of course at a molecular level "pressure" is caused by colliding molecules 
etc, but I don't think you need to work at this level of detail to model how 
sound waves propagate, you can just assume that each bit of air has a 
pressure, and pressure differences will generate forces.

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In article <4a80139b@news.povray.org>,
scott wrote:

>> How come molecules "push" and "pull"? How come the strength varies?
>
> The "push" and "pull" Warp is referring to (I guess) are the forces caused 
> by a pressure difference.  This causes the next "bit" of air to be moved, 
> thus creating another pressure difference, and the next bit moved, etc, this 
> is how the wave propagates - it's the same as a longitudinal wave in a 
> slinky spring.
>
> Of course at a molecular level "pressure" is caused by colliding molecules 
> etc, but I don't think you need to work at this level of detail to model how 
> sound waves propagate, you can just assume that each bit of air has a 
> pressure, and pressure differences will generate forces.

Alright. But I guess my question is how do I distinguish, for example,
a push from a pull. Should it all be just forces pushing one another? 

But I guess when we talk about pressure, then a context in lower
pressure can be seen as pulling a context in higher pressure. Is that
the sort of pull that we're talking about?

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On 10-8-2009 12:19, Stephen wrote:
> 
> Probably due to reasonable sound insulation of the walls and floor. I've lived
> in flats where you would think that your neighbours were sitting in the same
> room. 
The sitting would not be the real problem I guess.

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Daniel Bastos schrieb:
> Alright. But I guess my question is how do I distinguish, for example,
> a push from a pull. Should it all be just forces pushing one another? 

Strictly speaking, with pressure there's only "push", with "pull" being 
actually a push in the opposite direction:

(1) Pressure differential exerts a "pushing" force from high-pressure to 
low-pressure region

(2) Pushing force accelerates medium in the gradient zone, speeding it 
up until the pressure differential is compensated

(3) Inertia of the accelerated medium causes "overcompensation" of the 
pressure differential

(4) Pressure differential is now reversed -> back to (1), now with 
opposite direction


This is all that happens locally; at a larger scale, this will cause the 
pressure differential to propagate through the medium as a wave.

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On Mon, 10 Aug 2009 17:50:35 +0200, andrel <a_l### [at] hotmailcom> wrote:

>On 10-8-2009 12:19, Stephen wrote:
>> 
>> Probably due to reasonable sound insulation of the walls and floor. I've lived
>> in flats where you would think that your neighbours were sitting in the same
>> room. 
>The sitting would not be the real problem I guess.

Indeed not! Thanks to the walls and the lack of a blunt instrument, I'm still at
liberty :)
-- 

Regards
     Stephen

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clipka <ano### [at] anonymousorg> wrote:
> Strictly speaking, with pressure there's only "push", with "pull" being 
> actually a push in the opposite direction:

  The "push" and the "pull" are differentiated by the direction in which
the effect propagates. Also a "push" is a higher pressure wave which moves
towards some directin, while a "pull" is a lower pressure wave which moves
in that same direction.

  This can be visualized by imagining a long string of small spheres, each
sphere being connected to the next with a spring. If you push the first
sphere, it will push the next sphere (through their connecting spring),
which will push the next one, and so on. Overall this causes an advancing
phenomenon which traverses through the entire string.

  If you *pull* the first sphere, it will pull the next one, which will
pull the next one, and so on. Again, the overall effect advances in the
same direction and the pushing did.

  If you think what a loudspeaker does, it's exact that: By using air
pressure it "pushes" and "pulls" air alternatively. When it does so in
fast succession, our ears perceive these differential waves as sound.

-- 
                                                          - Warp

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Invisible wrote:
> It doesn't sound *that* bad. But there is a quite pronounced decrease in 
> bass as I walk through the doorway. You listen to the music and you 
> think the neighbors will be banking on the wall any minute now, and then 
> you step just outside the door and it suddenly seems radpically less 
> loud. I don't know if it's because you're out of line-of-sight with the 
> speakers or the width of the doorway or what...

It's because you're out of line-of-sight of the speakers.  They can't be 
bothered to sing to you if they can't *see* you.  They're even less 
intelligent than the Bugblatter Beast of Traal, so closing your eyes 
where they can see you doesn't work as a noise-silencer.

--
Tim Cook
http://empyrean.freesitespace.net

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Invisible schrieb:
> ...all of which is true. However, a simple experiment with a wavetank 
> quickly demonstrates that, for some reason that I don't comprehend, 
> large waves won't go through small gaps, and yet small waves will. This 
> is the precise opposite of what you would expect, but the results are 
> clear.

Hum... I hate to admit this, but it seems you're right: I just hacked 
together a POV-Ray script to simulate a wavetank-kind-of-apparatus (very 
idealized, but still...), initially to disprove your statement - only to 
find that indeed small gaps will not let low-frequency waves pass. 
Changes in gradient will pass through - but not oscillations.

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Warp schrieb:
>   This can be visualized by imagining a long string of small spheres, each
> sphere being connected to the next with a spring. If you push the first
> sphere, it will push the next sphere (through their connecting spring),
> which will push the next one, and so on. Overall this causes an advancing
> phenomenon which traverses through the entire string.
> 
>   If you *pull* the first sphere, it will pull the next one, which will
> pull the next one, and so on. Again, the overall effect advances in the
> same direction and the pushing did. 

Speaking of pressure, this visualization attempt is flawed. I repeat: 
Strictly speaking, there's no such thing as a "pulling" force in air.

A proper way of visualizing it would be an arrangement of *loaded* 
springs: If you displace the first sphere towards you, it will still 
"push" the next one - but less so than the opposing push from the third 
sphere, so the second sphere will give way towards you. Similarly, the 
third sphere will then be pushed towards you by the fourth one, and so on.

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clipka wrote:
> Strictly speaking, there's no such thing as a "pulling" force in air.

Sure there is.  Ever heard the phrase "nature abhors a vacuum"?  That's 
a pulling force.  Any decrease in pressure in an atmosphere will cause 
molecules to be drawn to the source of the pressure difference.

--
Tim Cook
http://empyrean.freesitespace.net

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