speed of sound vs the wind

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Tahlas
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speed of sound vs the wind

Postby Tahlas » Wed Jan 07, 2009 3:53 pm UTC

How can the direction of the wind have so great an influence on the spreading of sound?

Sound waves travels through air at a velocity of around 300 m/s, a wind speed 10 m/s is ca. 3% of that. As far as I can figure, this should mean that everything is 3% louder if I'm standing downwind. This gives a difference of 6% between standing upwind and downwind.
EDIT: This is not true.

This would be noticeable, but I have noticed very great differences at significantly lower windspeeds, what causes this?

I especially noticed this when I was living with my parents, where a nearby railroad could clearly be herad if the wind was in the north but not at all if it was in the south, but also on lots of other occasions.

Iv'e wondered about this for quite a while now (first thought of it almost five years ago) and I have asked highschool teachers, physics students, and are couple of actual scientist (mostly astro- physicist so this isn't really their field of course), none of them have been able to give me a proper explanation.

My best explanation is that the sound gets drowned out by other sounds that I don't notice when the wind is coming from the other direction. In the railroad example theres a wood and a road to the other side the noise from this might filter out the train. But I think thats unlikely.

Anyway hope you can help, any ideas are welcome :D
Last edited by Tahlas on Wed Jan 07, 2009 4:53 pm UTC, edited 1 time in total.

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idobox
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Re: speed of sound vs the wind

Postby idobox » Wed Jan 07, 2009 4:40 pm UTC

You have to consider the distance traveled by the sound. If there is 10m/s of wind, and sound travels at 340m/s, it will travel at 330m/s against the wind, and 350m/s with the wind.
If you are at a distance of 340m of the source, it will take 1.03, 1 or 0.97s, depending on the wind.
It also means the sound did not traveled that distance, the same way you feel like swimming a lot more when swimming against the current. The distance seen by the sound has the same 3% difference. But the intensity varies with the square of distance. 0.97²=0.94, you've got 6% difference, double that to take back-forward difference, and you've got a 12% difference.

I'm not sure this is enough to explain what you observed.
As the sound is a pressure related phenomenon, I guess it is possible the wind disturbs greatly the actual speed of it.
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uncivlengr
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Re: speed of sound vs the wind

Postby uncivlengr » Wed Jan 07, 2009 4:40 pm UTC

Let's say the wind is coming toward you at 10% of the speed of sound. If you crunch through the numbers, it would seem as though the sound source is 9% closer to you than it actually is. Instead of the train being 100 m away, it's 91 m away.

Sound dissipates relative to the inversed square of the distance, so the sound would seem 20% louder than if there's no wind.

For wind speed of 3%, it seems to be 97 m away, and about 6% louder - you can see that the change in intensity of the sound is double the wind's speed relative to that of sound (assuming the wind is travelling directly toward you).


edit: oooh, beaten by seconds...
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Tahlas
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Re: speed of sound vs the wind

Postby Tahlas » Wed Jan 07, 2009 4:52 pm UTC

idobox wrote:You have to consider the distance traveled by the sound. If there is 10m/s of wind, and sound travels at 340m/s, it will travel at 330m/s against the wind, and 350m/s with the wind.
If you are at a distance of 340m of the source, it will take 1.03, 1 or 0.97s, depending on the wind.
It also means the sound did not traveled that distance, the same way you feel like swimming a lot more when swimming against the current. The distance seen by the sound has the same 3% difference. But the intensity varies with the square of distance. 0.97²=0.94, you've got 6% difference, double that to take back-forward difference, and you've got a 12% difference.

I'm not sure this is enough to explain what you observed.
As the sound is a pressure related phenomenon, I guess it is possible the wind disturbs greatly the actual speed of it.


Yes of course you're right my bad, I don't think thats the whole explanation though.

Rgeminas
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Re: speed of sound vs the wind

Postby Rgeminas » Wed Jan 07, 2009 8:16 pm UTC

I believe that the pressure may have a bigger effect on that. The difference in pressure for an incompressible fluid is proportional to the square of the speed it has, and since the speed of the waves in a fluid changes with the pressure, it may be greatly affected, affecting maybe intensty.

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idobox
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Re: speed of sound vs the wind

Postby idobox » Wed Jan 07, 2009 9:48 pm UTC

yes, but air doesn't really fit the "incompressible" criterium.
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Rgeminas
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Re: speed of sound vs the wind

Postby Rgeminas » Wed Jan 07, 2009 11:11 pm UTC

Nevertheless, I believe it won't change things much, only the decrease in pressure would change depending on the variation on its specific mass (also, would it expand or contract at a wind speed of 10m/s?)

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Re: speed of sound vs the wind

Postby Rentsy » Thu Jan 08, 2009 1:17 am UTC

In an ideal medium, sound "waves" propagate in an orderly fashion, hitting our ears with definite compressions and rarefactions. Air, at rest, carries these to our ear. The lambda, aka wavelength, aka the distance between compressions, aka the rarefaction determines the pitch of the sound.

Moving air jumbles these signals up, due to chaotic, turbulent motion.

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Re: speed of sound vs the wind

Postby gmalivuk » Thu Jan 08, 2009 2:06 am UTC

Rentsy wrote:Moving air jumbles these signals up, due to chaotic, turbulent motion.

Well sure, hence it's hard to hear things over loud wind.

But that doesn't really explain why there's so much of a difference between being upwind versus downwind of a sound source, because there's chaotic, turbulent wind motion in both cases.
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Rentsy
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Re: speed of sound vs the wind

Postby Rentsy » Thu Jan 08, 2009 4:47 am UTC

Oh, well then the speed of the wind changes the lambda. Doppler effect. The speed of sound in the medium changes with respect to the listener.

The sound will propagate at ~344 m/s, with respect to the air itself. The wind may be moving at 5 m/s.

If

(You) <-<-< Wind <-<-< (Sound)

The sounds will be lower in pitch, as each compression will be farther apart than usual.

If

(You) >->-> Wind >->-> (Sound)

The sounds will be higher in pitch, as each compression will be closer together than usual.

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Re: speed of sound vs the wind

Postby jaap » Thu Jan 08, 2009 8:36 am UTC

Rentsy wrote:Oh, well then the speed of the wind changes the lambda. Doppler effect. The speed of sound in the medium changes with respect to the listener.

If

(You) <-<-< Wind <-<-< (Sound)

The sounds will be lower in pitch, as each compression will be farther apart than usual.


I don't agree. The compressions will indeed be further apart in the air (i.e. longer wave length), but since the speed is higher the frequency is the same. Both wavelength and wave speed change, but not the frequency.
You will only get doppler effects if the wind speed changes somewhere between you and the source, so gusts of wind can make the sound warble a little.

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Re: speed of sound vs the wind

Postby ++$_ » Thu Jan 08, 2009 9:44 am UTC

Noises are hard to hear at distance because sound from distant sources is refracted upwards and away from your ears. If the air is still, this refraction depends on the air masses currently in place in your area. In fact, if there is an inversion, sound will be refracted downwards and you will be able to hear things at a very great distance. For example, on a Saturday night with an inversion, I can hear parties being held over 3 miles away, at the university.

The velocity of wind increases with height.

As sounds traveling upwards move downwind, they are moving into a medium in which they travel progressively faster. This causes them to refract downwards, which enables them to reach your ears.

As sounds traveling upwards move upwind, they are moving into a medium in which they travel progressively more slowly. This causes them to refract upwards and away from your ears.

rnh
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Re: speed of sound vs the wind

Postby rnh » Thu Jan 08, 2009 4:15 pm UTC

I think there are two things that have not yet be taken into consideration.

Most importantly, the intensity of sound is not proportional to the "subjective" intensity, or decibels. Instead (if I remember correctly) perceived loudness goes as the log of intensity. So there has to be a _huge_ change in intensity for one to be able to judge if a sound is louder or not.

Also, intensity does go as 1/r^2, but that's without obstructions. It's double when you're on the ground and the sound only has half a sphere to propagate through. I'm assuming that the ground doesn't absorb a lot of sound, but I'm not sure if that's true.

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Tass
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Re: speed of sound vs the wind

Postby Tass » Thu Jan 08, 2009 7:11 pm UTC

rnh wrote:I think there are two things that have not yet be taken into consideration.

Most importantly, the intensity of sound is not proportional to the "subjective" intensity, or decibels. Instead (if I remember correctly) perceived loudness goes as the log of intensity. So there has to be a _huge_ change in intensity for one to be able to judge if a sound is louder or not.

Also, intensity does go as 1/r^2, but that's without obstructions. It's double when you're on the ground and the sound only has half a sphere to propagate through. I'm assuming that the ground doesn't absorb a lot of sound, but I'm not sure if that's true.


Yes, and you need roughly 3dB difference to hear it. That is about a factor of two.

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Re: speed of sound vs the wind

Postby roflcopter » Fri Jan 09, 2009 4:46 am UTC

Did anyone think that the wind being a moving medium that the sound waves are traveling through could actually 'bend' the waves with it? I know that i have seen ripple and waves in water that had a swift moving current move into more of an oblong ellipse than concentric circles. Could the moving air do this to a point to the waves and make more actual 'sound' travel downwind and less make it to an observer upwind?

I was thinking about the edges of the expanding circles, not necessarily the waves going directly from the source to the observer but the ones that may slightly miss him. Like if the ones traveling downwind get bent from radially going out to more than one going at a close to parallel direction to him it could provide more for him to actually 'hear'.

I am just a highschool student so none of this is really based on anything so please don't tear it apart too bad. :oops:


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