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I live about 2 miles from a race track.

Some days the noise from testing is extremely noticeable, Other days despite the exact same testing activity, there is little-to-no noise at all.

This is mostly effected by the direction of the wind.

How does the wind direction affect sound propagation? Given the speed of sound vs the speed of wind is generally only a few percent, why and how does it make such a dramatic difference?

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Since the mechanism is so well written on Reddit in here, I won't even try to rephrase it. Here it is:

Yes, wind has an effect on the speed of sound, and this effect has interesting ramifications for the propagation of sound outdoors.

An acoustic wave, as you know, is a mechanical wave traveling through a medium. The sound we deal with most often is carried through the medium of air, at a speed of around 343 m/s.

Wind is the bulk motion of air in a given direction.

When you combine these two ideas together, you get that sound is a wave moving through a moving medium. Unsurprisingly, that means that the velocity of acoustic wave is equal to the speed of the wave plus the speed of wind in that direction. IE, if the wind is moving at 20 mph (8.9 m/s), then sound will travel downwind at 351.9 m/s, upwind at 334.1 m/s, and crosswind at the regular 343 m/s. Note that it takes a significant windspeed to appreciably alter the sound speed in any given direction.

What's really interesting is how windspeed gradients alter the path that sound takes through the air. As a rule, sound waves bend towards regions of lower sound speed (an effect known as refraction that's a direct result of Snell's law). Couple this with the fact that windspeed tends to increase with greater distance from the ground, and you find that sound refracts downward when moving downwind and upward when it's moving upwind.

Sound tends to emanate from sources in roughly all directions. Some goes towards the listener, some goes away from the listener, and some shoots up into the sky. When you consider the effect of refraction, the question becomes "Does more sound go into the sky or to the source?" As it turns out, it will depend on where you stand with respect to the wind.

If I'm listening to someone far away talking, and there is wind, I probably want to stand downwind of them. Why? Because the sound that normally goes up into the atmosphere will instead refract downwards towards me, focusing on me. Conversely, if I was standing upwind, the sound would refract up and away from me, causing me to receive a lower effective level.

@Farcher suggested temperature gradient in the comments which in my opinion has quite small effect.

Thanks to Reddit user therationalpi.

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  • $\begingroup$ Well that's about as straight-forwards as it gets for a complex system description! Thank you! $\endgroup$ – Digital Lightcraft Jun 27 '17 at 13:23
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    $\begingroup$ Temperature gradients can have big effects but typically on very still days (when the temperature gradient can be very consistent over large distances). This can be very noticeable at sea. However I agree that in the case you are talking about wind will be dominant. $\endgroup$ – tfb Jun 27 '17 at 13:35
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    $\begingroup$ "Very noticeable" means "very." I live near a coastline where at low tide there is wide strip of salt marsh between the "land" and the sea itself. At the end of a hot day, it is quite easy to hear people talking normally (not shouting, etc) on boats that are mile away, because of the sound refraction over the relatively hot marsh land. $\endgroup$ – alephzero Jun 27 '17 at 15:45
  • $\begingroup$ anecdote evidedence like @alephzero I can tell flying in a hang gliding at hundred meters from the ground you can clearly listen do birds, cars and cattle below as if they are a just a dozen meters away when you are at ground. $\endgroup$ – jean Jun 27 '17 at 20:27
  • $\begingroup$ Can we use the Doppler effect to calculate the impact of wind on sound wave? instead of the relative motion of the source and destination, one may take the relative motion of wind and sound waves. $\endgroup$ – user77220 Jun 27 '17 at 22:09

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