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Does wind affect how far sound can travel?

  1. Dec 28, 2006 #1
    I feel that this may be a silly question, but, Does wind affect how far sound can travel? I know that the air temperature affects how far a sound can travel (does wind affect air temperature?), but what about wind? I tried to find an answer via google, but I haven't found anything...
     
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  3. Dec 28, 2006 #2

    Gokul43201

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    Ask a hunter (there are some here that could refute or corroborate this), and s/he'll tell you that sound carries better through still air. If I had to guess, I'd put this down to greater losses from turbulence in the airflow.

    What I do know is that sound velocity is affected by wind speed, and the net speed is pretty close to the vector sum of the velocity in still air and the wind velocity.
     
  4. Jan 1, 2007 #3

    Clausius2

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    That is innaccurate. The sound speed, at moderate velocities of wind, is almost constant throughout the flow field. That is, velocities of wind such as 10 m/s does not produce a substantial variation of the speed of sound compared with that in still air. What makes the desired sound more difficult to be heard is the interference with the rest of pressure waves in your ears produced by wind fluctuations as you said in a windy day. Another point is that the sound wave can be carried faster if the wind is faster, because the sound wave is advected by the fluid. But such difference is negligible at ordinary wind velocities because 345m/s>>10m/s. The thing is pretty different at Mach Numbers of order 1 though...
     
  5. Jan 1, 2007 #4
    With you just saying that, wouldn't there be reasons in both physical space and to the reciever? Setting aside what would happen to the sound as it travels through the wind, it would be more impared to the reciever as both the wind and sound would be making his ear drum vibrate.

    Edit: Ahh, didn't read you above... Oh well...
     
  6. Jan 1, 2007 #5
    Wind doesn't affect air temperature if you're following a parcel of air, but it could do if you were standing still and a cold bit of air blew your way. In fact it's more the other way around, air temp can affect winds but that's not relevant here.

    Also have you read Naked Lunch by William Burroughs? He's not a scientist but he makes a recurring reference to the sound of a piano being played in a windy street. The wind affects the sound of the piano if you're listening from a distance, not sure if it'll make a difference to how far the sound can travel but apart from the advection, I'd guess not though.
     
  7. Jan 2, 2007 #6

    vij

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    Wind is basically moving air. So, the medium through which the sound is propagated is moving and the velocity of wind is added vectorially to the velocity of sound. That's why the frquency of the sound heard by a listener depends on the wind velocity. How far the sound will reach certainly depends on the turbulence of air, produced by the wind.
     
  8. Jan 3, 2007 #7

    Gokul43201

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    Clausius, I don't understand why there's no advection at v(wind) << c(sound). Also, to the best of my knowledge, weather stations measure wind speeds from a Doppler shift, and they have accuracies of better than 1m/s at wind speeds of about 5 m/s.
     
    Last edited: Jan 3, 2007
  9. Jan 3, 2007 #8

    Clausius2

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    I didn't say there is no advection, I said that its effect is negligible. That is, the total distance travelled by the sound is almost the same than if we base the calculation solely on the speed of sound c. Sure there is advection.

    You know, people of fluid dynamics have a particular way of talking, when we say negligible we mean that we know it exists, but the problem on this board the effect is so small compared with others (i.e. dissipation) that it is not worthy to include it in a calculation. I have found myself in hard times trying to explain this things to people of other fields of science (and I have had hard times trying to understand another concepts of their respective areas), but that's the way we discard things (there' s so many effects out there!).

    Maybe the engineer in charge of designing that sensor for weather forecast needed an extraordinary accuracy for measuring the doppler effect and it is sensible to do that btw, but that's another problem, is not the problem stated by the opener.

    Advection is proportional to the inertia of the fluid. Low speeds mean low advective effects, low Mach Numbers, large Mach angles (talking about Doppler effects) and behavior nearly similar to Incompressible=Linear Acoustics. When the velocity of the flow increases, the bulk inertia of the fluid transports information in a non linear way (v^2), that's what happens in sound waves and Compressible=Non Linear Acoustics (Large Mach Numbers). In the latter case, a sound wave always travels at the local speed of sound respect to the fluid (i.e. with a laboratory velocity that is the sum of the local fluid velocity and the local speed of sound). In that range both of them are of the same order of magnitude, and as you know from your courses of PDE's, the slope of the characteristic lines is exactly the inverse of the sum of both speeds. In our problem, the best approximation is the linear acoustics and consider that the sounds waves travel at the speed of sound regardless of the advection. In the problem of the sensor makes sense to consider the small effect of the doppler shift, because it is the main outcome of the calculation.

    Happy new year man.
     
  10. Jan 3, 2007 #9
    So in the most simplest terms, suppose that:

    S0 is the speed of sound when there is no wind,
    W is a given constant wind speed.

    Then how much is Sw?
     
    Last edited: Jan 3, 2007
  11. Jan 3, 2007 #10
    if we take the sound transmitted through still air as a vector and the wind direction as another vector which is normal to the sound vector then we will have a resultant which diagonal. but this is mainly physics pertaining to solid objects we have to take in mind that it is a wave which does not behave as a solid object, but sound wave may attenuate or distort due to the wind producing it own pressure waves which annhilate and reinforce portions of the sound wave
     
  12. Jan 3, 2007 #11
    Another phenomenon is that because cold air is denser than hot air and wave speed is inversely proportional to density sound travels faster in warm air. This means that if the atmosphere is in an unstable condition with warm air below cold air (such as in storm conditions) the sound will be refracted upwards. On a clear day, with cooler air near the ground the sound will travel further - in extreme conditions such as in the evening in the rainforest the warm air layer can totally reflect sound back down to the ground and wildlife calls can be heard from miles around.
     
  13. Dec 9, 2009 #12
  14. Dec 9, 2009 #13

    IMP

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    Yes. I have a freeway about 10 blocks South of my house. I can hear the traffic very clearly with no wind, or a South wind. If there is even a slight North wind, the traffic noise becomes almost inaudible. If there is a brisk North wind (over 15 MPH), the sound is completely gone.
     
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