carried by the wind

“Why can a distant sound be heard easier when it travels with the wind?
Why does it become weaker if it travels against the wind?”

So my house is in a valley, with a small mountain between me and both the motorway & the Kapiti train line. If the weather and wind is still, the ambient sound from the motorway & train line is subliminal. Similarly i am far enough away from the sea, that it really needs to be a big storm with a roaring sea to hear it from my place. But over the last five years living here I also noticed how the wind alters perception.

My instinct would be to assume sound is ‘carried with the wind’ and in the direction of the wind simply because all those air molecules are being blown in a specific direction. But that is only part of the science, because refraction also plays a part.

#refraction?

“Sound waves travel faster in the air when it is with the wind. Generally speaking, wind speed is lower near the ground because of the presence of blockages, accordingly wind speed increases with height. Therefore, when traveling with the wind, sound wave farther from the ground travels faster.
Refraction is resulted from the difference in speed of sound in the upper and lower region, just like how sound waves travel in different mediums. Sound waves will be refracted towards the region with a lower speed of sound, thus sound waves will be refracted downwards when it travels with the wind, and hence it can be transmitted further away.


ok?

In contrast, when sound wave travels against the wind… the speed of sound will be reduced by the wind speed, resulting in a lower speed in the upper region. According to the principle mentioned above, sound waves will be refracted upwards, and hence cannot so easily reach a distant person…”

 

thanks
http://www.hk-phy.org/iq/sound_wind/sound_wind_e.html

 

 

 

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