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Does wind affect sound?

rcaz

In the shop I work in, only a few minutes away from croker, we couldn't hear anything from the U2 concerts. A few very unreliable sources said, "Ah, i must be 'cause of the way that wind's blowin'". I didn't think sound waves were affected by wind, which poses the question:

Are they?

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dudara

Sound travels through air or any medium by making the molecules vibrate. Wind could also make the molecules vibrate, thus disrupting the "flow" of the sound wave.

Or so I'd imagine.

rcaz

Gah, that was basic :eek: I should've known that.
Thanks for clearing that up. Nice one.

quarryman

wind has a significant effect on sound.

astec123

Dudara is correct but it is also that the wind moves the vibrating molecules so that the sound wont be heard by you ie the molecules travel in a more arc shaped path than a straight line due to the wind pushing them.

Sapien

El Pr0n wrote:

In the shop I work in, only a few minutes away from croker, we couldn't hear anything from the U2 concerts. A few very unreliable sources said, "Ah, i must be 'cause of the way that wind's blowin'". I didn't think sound waves were affected by wind, which poses the question:

Are they?

That's actually a good question, and my intuition is that sound and wind are caused by two different kinds of motion of gas particles.

Sound is not so much the vibration of air molecules, as oscillations in air pressure caused by the vibration of solid surfaces. Whereas wind, I imagine, is a drift velocity in a body of air as a whole, though I'm not certain about that.

My conclusion would be, that if my assumption as to wind being drift velocity is correct, then in an ideal situation wind would have little effect on how sound propagates. However, in reality, wind causes sound itself, which of course would obscure other sound if it became intense enough.

nesf

Sapien wrote:

That's actually a good question, and my intuition is that sound and wind are caused by two different kinds of motion of gas particles.

Sound is not so much the vibration of air molecules, as oscillations in air pressure caused by the vibration of solid surfaces. Whereas wind, I imagine, is a drift velocity in a body of air as a whole, though I'm not certain about that.

Oscillations in air pressure are a result of molecular movement.

Wind is also a result of this.

They, strictly, are the same and will effect each other. Wind has a "sound" remember.

Edit: The above would be my intuition btw, rather than quoted fact.

Sapien

nesf wrote:

Oscillations in air pressure are a result of molecular movement.

Technically yes, but it is not a change in the speed or movement of the molecules that causes the difference in pressure - it is the number of particles. Effectively, all air molecules travel at about 240ms^(-1), and this is not changed by the vibrations of surfaces that cause sound. Rather, these vibrations cause the particles to "cluster" into wavefronts of higher pressure.

nesf wrote:

Wind is also a result of this.

Wind is distinct because there is no sinusoidal variation in pressure. It is a uniform drift velocity - in other words, the *average* velocity of all particles has a finite value. The only way wind could effect the propagation of a sound is if there existed a cross-section of interaction between different particles that was not negligible, which is generally not the case.

nesf wrote:

They, strictly, are the same and will effect each other. Wind has a "sound" remember.

Wind does not have a sound in itself - or to put it more rigorously, wind *is* not a sound. Wind causes sounds by interacting with other objects - by creating harmonics in open spaces, like the operation of wind instruments, or causing objects to vibrate by resonance, like a tuning fork, or by mechanically interacting with objects to cause audible vibration, like a string or percussion instrument.

nesf

(I'm going to be speaking conceptually here and try to stay away from jargon and technicalese if I can btw)

Right, lets debate this one properly

I've time to kill

Wind itself is quite a vague term. It colloquially means a flow of air from an area of high pressure to low, but this is a very general meaning. More accurately we can state that wind is a product of a "pressure gradient induced force" acting on air molecules as a whole. Bear in mind that this definition is not a micro scale one. The definition of sound is micro.

This is where we first run into difficulties in my opinion. The concept of wind is concerned with large scale movements of air while sound is concerned with very small pressure waves propigating through the air itself.

Essentially this is a fluid mechanics problem, not a gas problem, thus it's going to get very complicated very quickly if we get pedantic about it.

For the moment, ignore air's gaseous properties and focus on it's behaviour as a fluid. Sound is quite simply a wave travelling through this fluid. There is no "movement" of the fluid itself on a large scale but there is a transfer of energy (leaving cert physics here btw). It also, as we all should remember is a compression wave.

In essence, this wave operates as a pressure variation in the fluid. Think of the wave as baing a oscilliation of pressure if you wish. It's also dampened but we won't worry about that yet. These pressure "pulses" travel through the air via a compression wave.

This works fine as a model, so long as we assume the air is "stationary". It does not remain true if the air is moving at the same time.

Now this is where wind comes in. Wind is a "macroscopic" movement of air pressure. I'm talking in terms of air pressure rather than molecules here because I don't want to get into gas theory, I know that talking of a "movement of air pressure" is not strictly accurate but I'm trying to paint a picture here rather than trying to be rigorous.

Air will move along pressure gradients. Wind is a "correction" of these gradients so to speak so there is an ongoing pressure change in the air when wind is flowing.

Now stop for a second and think about fast moving pressure pulse in water. Now think of that pulse flowing across a very slow river. What will happen? Will the pulse be effected by the river's flow? Of course it will, but the question is by how much. (the answer here is, it depends on how wide the river is and the relative speed difference)

The slow moving river here is equivilant to large scale macroscopic winds.

If we reduce the scale we are in a whole new ball park. Microscopic "winds" or microbursts/gusts are much faster and more likely to influence sound. (Microbursts are defined as winds over a scale of 10 to a couple of hundred meters, not microscopic ones, we won't even begin to think about those).

I'll post more later, I've other stuff do right now, but I do wish to discuss this with people. It's an interesting topic.

Some questions you guys to ponder while I'm away:

Is it fair or accurate to assume the wind is going to act as a single movement of air, or should we think about "currents and eddies" within this movement caused by buildings etc?

Why does sound travel at 320m/s? Why do all sounds travel at the same speed?

More technical:

Conceptually do we need to define wind differently when we are talking about scales for sound waves?

What forces will effect the compression wave and what distortion to the wave will occur from a physical movement of the medium considering that the basic model assumes that it is motionless?

Compression waves are quite different to regular waves. Why is this?

Sapien

Yes, well it was clear from your initial post on this topic that you think about air, sound and wind in terms of fluidic motion. The purpose of my second post was to illustrate that this is not the correct way to understand the nature of sound in air. Rather, the kinetic gas model is appropriate.

The reason why the fluid corollary does not work is because in a fluid the intermolecular forces are dominant. This is not the case for a gas. In fact, as I said, simple kinetic gas models go so far as to neglect interactions of any kind, even collisions, and still work very well.

You are choosing to interpret wind as the macroscopic motion of the medium through which the waves propagate. It is more helpful to see it microscopically as the drift velocity, as I have described. It is important to understand that this does *not* mean that all of the particles are moving in a certain direction. It means that more will be moving in one direction than another. This is the effect of the pressure gradient. All particles, regardless of sound or wind, move at about the same speed. This is dependent on temperature alone.

To describe wind as the "correction" of pressure gradients is a teleological fallacy. Wind is *caused* by pressure gradients, because there are more particles coming from the high pressure area than from the low pressure area.

Saying that air is "stationary" is relatively meaningless. All that is "stationary" in an open body of air is the pressure - the component particles are whizzing through it at the speed of sound - in constant flux (to use the word very unscientifically).

Ultimately, in order to understand this problem, one has to cease thinking in terms of everyday macroscopic intuition, and work from microscopic theory.

nesf

My post deliberately used terms in a non-scientific fashion, I made this quite clear in the beginning. I'm not talking directly to you but to any interested person who chooses to read this thread.

Attacking my posts over their use of terms is thus pointless since I am purposefully not using technical or precise terminology but trying to draw a conceptual picture for non-techies.

I do appreciate your point however, but I was treating it as a fluids problem in that post rather than as a gas problem. I don't "think of air as a fluid". I chose to look at it from that perspective for that post, you should be able appreciate such an approach as a physicist.

My question to you is, why use the Kinetic Gas Model Why is it more accurate than the more advanced and superior if more complicated methods used in Fluid Mechanics?

I'm curious as to what you're reasoning is.

Oh and btw, gases are fluids, strictly speaking. Fluid Mechanics is concerned with both liquids and gases and is the pre-eminent theory used when studying both.

(Wiki links are for interested people who want to learn more btw, they are not intended as an insult to you Sapien)

Sapien

nesf wrote:

Attacking my posts over their use of terms is thus pointless since I am purposefully not using technical or precise terminology but trying to draw a conceptual picture for non-techies.

I was not attacking your post, I was commenting on its content. Using non-technical terminology is all very well, but you still have to use scientific ideas and thinking, or else your conclusions will be wrong. I don't think my posts have been excessively technical or opaque, have they?

nesf wrote:

I chose to look at it from that perspective for that post, you should be able appreciate such an approach as a physicist.

I appreciate that in some situations the choice of model is not quite absolute. I also understand that in many cases the use of the wrong model will ensure failure to properly describe the system.

nesf wrote:

My question to you is, why use the Kinetic Gas Model Why is it more accurate than the more advanced and superior if more complicated methods used in Fluid Mechanics?

I'm curious as to what you're reasoning is.

Oh and btw, gases are fluids, strictly speaking. Fluid Mechanics is concerned with both liquids and gases and is the pre-eminent theory used when studying both.

(Wiki links are for interested people who want to learn more btw, they are not intended as an insult to you Sapien)

Well actually, I'm not using the principles of fluid dynamics to deal with this problem, and neither are you. If you apply fluid dynamics to this problem, it will work effectively by reducing to kinetic gas theory. You are using *intuitive* understanding of fluidity to describe how wind effects the propagation of sound. The actual mathematical theory will allow for the fact that the situation is not a good fit, by allowing certain complications to drop out. Your intuition, like anyone's, is less sophisticated, and fails to do so.

Using fluid dynamics to describe sound waves is like using special relativity to describe a coin falling a few metres under terrestrial gravity.

Yes, gases and liquids are both fluids, but fluids that behave very differently.

el tel

El Pr0n wrote:

A few very unreliable sources said, "Ah, i must be 'cause of the way that wind's blowin'".

El Pr0n, your unreliable source was not entirely incorrect in his assessment. Knowledge of fundamental underlying scientific principles is no substitue for experience when it comes to these things. Education is not necessary!

nesf

Eh, I'm busy today so I don't have the time to devote to this that I'd like. Etc.

So just a quick thing on this:

My problem with using the kinetic theory model is that what interests me is not the sound wave itself but the propigation of a sound wave through air that is moving as "wind".

It's not a microscopic scale question (although to be fair, sound waves aren't exactly tiny, they do spread out and propigate quite far).

The reason I think fluid mechanics would be the better theoretical bedrock is that we know what a sound wave does. We just want to change the medium in which it's propigating. I don't think kinetic theory is capable of doing this cleanly. I think that the problem is on too large a scale.

Also kinetic theory doesn't work too well with air regardless compared to fluid dynamics for things on this kind of scale.

And I never claimed that I used fluid dynamics in my post. I was more refuting your suggestion that the kinetic model was the one to use. I don't think that wind can be accurately described with it and I don't think it works well in complex environments like what we are discussing.

This isn't a simple question once you get into it tbh.

Sapien

Okay. You seem pretty entrenched, and I think I've explained my opinion pretty well, so I'm going to drop this once I've dealt with the critical issue:

nesf wrote:

My problem with using the kinetic theory model is that what interests me is not the sound wave itself but the propigation of a sound wave through air that is moving as "wind".

You are still thinking of air as a contiguous and continuous medium that moves concertedly in the case of wind. This is wrong. You are thinking in terms of the behaviour of a body of liquid. If you realise that air particles travel at aboout 300ms^(-1) under standard temperature, regardless of oscillations or wind, then you will be better equipped to figure this problem out. The particles are not connected, are not in contact - they barely interact at all.

Sapien out.

rcaz

el tel wrote:

El Pr0n, your unreliable source was not entirely incorrect in his assessment. Knowledge of fundamental underlying scientific principles is no substitue for experience when it comes to these things. Education is not necessary!

Yeah, evidently not

I forgot that sound has to travel through something, thinking that it was unaffected by wind. I say unreliable, I meant old

[I have no problem with old people, but this particular person doesn't seem like the most knowledgeable of people]

I have been proven wrong!

nesf

Meh, you are the one who is entrenched here.

Since you didn't follow the wiki links I kindly provided. Here is the relevant pieces of information:

From: Wikipedia article on Acoustic Theory, a sub branch of fluid dynamics.

Acoustic theory is the field relating to mathematical description of sound waves. It is derived from fluid dynamics. See acoustics for the engineering approach.

The propagation of sound waves in air can be modeled by an equation of motion (conservation of momentum) and an equation of continuity (conservation of mass). With some simplifications, in particular constant density, they can be given as follows:

The theory you are talking about is useless in the modelling of sound waves in complex gases like air. It is only of use when dealing with perfect or near perfect gases in specific states. It's "molecule" model which assumes no interaction between gas particles is not a suitable approximation since gases do interact and their behaviours can be accurately modelled using flows.

The other main option would be statistical mechanics, which is messy, but accurate where fluid dynamics isn't. Generally.

I indulged you and asked you to clarify your position for long enough. I thought it better to get you to think about this problem before just linking you to the answer.

Kinetic theory is not very useful when dealing with mixed gases which are not "noble". ie air

I do appreciate that this wouldn't be covered in a Physics degree and that you probably have never heard of acoustic theory before now. So I am not implying that you don't have expertise here. It's just that from your posts you don't seem to have done much work with actually modelling fluids imho.

Hope the above is of interest to you

nesf

Eh, a quote to finish this thread with. It's always summed up a lot for me and I think it's quite apt here:

"Education is a progressive discovery of our own ignorance."

Will Durant (1885 - 1981)

SkepticOne

El Pr0n wrote:

In the shop I work in, only a few minutes away from croker, we couldn't hear anything from the U2 concerts. A few very unreliable sources said, "Ah, i must be 'cause of the way that wind's blowin'". I didn't think sound waves were affected by wind, which poses the question:

Are they?

Yes. Here's my reasoning:

The speed of sound is normally refers to the speed relative to the block of air not the observer or the sound source.

Winds don't generally cause uniform movement of air. Generally the higher up you go the higher the windspeed. This is caused by objects (houses, trees etc.) slowing the air down at low altitudes.

Since the speed of wind varies with height, the velocity of sound (relative to an observer on the ground) will also vary with height. If you are upwind of the concert, sound waves traveling at a higher path will take longer to reach you than those traveling a low path.

This creates a lensing effect. In normal conditions sound upwind of the concert will tend to curve upwards and away from the observer. You may not hear the concert very well upwind. The sound is being 'bent' away by the varying speed of the sound as it travels through the medium.

A lensing effect is caused when waves propagate through a medium at differing speeds.

Hope this helps.

SkepticOne

I should add that even without the varying windspeeds, sound still has to travel further (relative to the mass of air) upwind than downwind. Sound upwind will therefore be more dissipated.

The Termignator

Just to clarify a point two posts above, the variation of the speed of sound with altitude is a function of temperature and has nothing to do with wind speed.

In simple terms, I think the post above answers the initial question perfectly.

SkepticOne

The Termignator wrote:

Just to clarify a point two posts above, the variation of the speed of sound with altitude is a function of temperature and has nothing to do with wind speed.

Sure it does if you measure it relative to the observer on the ground. Sound will travel slower upwind than downwind though, relative to the moving mass of air, it will be the same in all directions.

When we talk about the speed of sound, we normally mean the speed relative to the mass of air through which the sound is travelling. This, of course, as you say, will vary with altitude due to the differing densities of air. This is not what I was talking about since this effect regardless of windspeed and direction, and therefore not relevant to the question posed.

Since you have differing speeds with high speeds high up and low speeds low down, you will get the sound curving upwards in the upwind direction and downwards in the downwind direction over and above any other effect.

I saw a diagram of the effect in a book a while back but can't find any links on the web at present.

SkepticOne

Here we are:

When a wind is blowing there will always be a wind gradient. This is due to the layer of air next to the ground being stationary. A wind gradient results in sound waves propagating upwind being 'bent' upwards and those propagating downwind being 'bent' downwards. url="http://www.sfu.ca/sonic-studio/handbook/Sound_Propagation.html"]source[/url

Also

Wind shear, in the context of this paper, describes a situation in which the winds aloft are considerably faster than the winds near the ground. The presence of wind shear can bend (or refract) sound waves downwards or upwards, depending on whether the wave is traveling upwind or downwind.url=http://www.acoustics.org/press/136th/ross.htm]source[/url

_Dubh_

Maybe I'm over simplifying it but....

Everybody has heard the wind howl so something that produces sound can interfere with incoming sound vibrations seems reasonable to me.