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ESDU Challenge: Sound and Wind Speed

Posted August 01, 2014 12:00 AM by SavvyExacta
Pathfinder Tags: ESDU

In this series from IHS ESDU, challenge questions will be posted for the community. Some questions may require calculations and others are general knowledge questions.

Here's this week's question:

Compared with constant wind speed as a function of height, if wind speed increased with height above the ground, what would happen to the sound level at a reception point a given distance from a sound source?

And the answer is:

The sound level will be higher. A positive wind speed gradient would, in general, amplify the sound. (From ESDU 04011.)


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#1

Re: ESDU Challenge: Sound and Wind Speed

08/01/2014 1:40 AM

It would improve downwind and at a lower altitude and get weaker upwind and at a higher altitude....

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#2

Re: ESDU Challenge: Sound and Wind Speed

08/01/2014 7:39 AM

Not quite sure I understand the question. Based on what I think it's asking, I'll say:

The speed of sound is not an absolute like the speed of light. If the speed of sound, say 5 miles up, were 1000 miles per hour and if you were moving with the wind at that speed, you would not notice any difference in sound level. If (somehow) you were standing on a mountaintop with the wind going by at 1000 miles per hour there would be a standing shock wave around the mountaintop making it nearly impossible to hear sound - or you'd hear a constant roar, depending on where you stood.

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#3

Re: ESDU Challenge: Sound and Wind Speed

08/01/2014 10:54 AM

The Doppler shift would take effect. The frequency would increase with height on the leeward side and decrease on the windward side.

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#5
In reply to #3

Re: ESDU Challenge: Sound and Wind Speed

08/01/2014 11:15 AM

Ooops! The frequency would decrease with height on the leeward side and increase on the windward.

It would arrive earlier with height on the leeward side too.

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#4

Re: ESDU Challenge: Sound and Wind Speed

08/01/2014 11:08 AM

It obviously refracts down, a kind of acoustic mirage. At a far enough distance, the sound would return to earth. At a far distance (downwind) the sound would be audible whereas at a lesser distance in the same direction, it would not be audible.

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#6
In reply to #4

Re: ESDU Challenge: Sound and Wind Speed

08/03/2014 11:15 AM

Rixter, your response makes less sense to me that the question.

"At a far enough distance, the sound would return to earth."

Huh?

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