Re: Somewhat OT: Sound and wind

Nick Suess wrote:
"Edd" <eddedmondson@xxxxxxxxxxx> wrote in message news:eask7r$42q$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Because this came up talking about rowing, and because people here are
often shouting or trying to hear shouts in windy conditions, quick
poll:

Who here knows what happens to sounds in windy conditions:
a) is it easier to be heard upwind or downwind or does it make no
difference?
b) if there is a difference, why is there one?

People with an unnatural understanding of fluid dynamics and such are
excluded (Carl, Anu, possibly others).


Surely this ain't fluid dynamics - it's wave mechanics.

Our ears detect sound due to the reception of pressure fluctuations propagated in wave form via our medium of air. And if that medium happens to be moving away from us while those little waves are trying to get to us (ie the source of sound is downwind from us), well then the waves have further to travel than if the wind is assisting by bringing them our way.

That's at least one good reason why there's a difference.

You know it makes sense.


I hesitate to challenge a jet-lagged traveller, Nick, but I venture to suggest that you're not quite right.

Sound velocity in air is way above normal wind speeds so, if the wind speed were the same at all heights above ground, although attenuation obeys the inverse square law (intensity varies with inverse square of distance from source, chaps) you'd get but modest variations in perceived intensity of a sound signal (speech or other) until wind speeds are pretty brisk.

Fluid dynamics gets involved because it explains the phenomenon of wind-shear or wind velocity gradient with height above ground - a fluid frictional effect. This, by varying the speed of propagation of sound waves WRT to ground, creates the necessary condition for diffraction by progressively altering the plane of the wave-front as it moves away from the source. Waves propagate perpendicularly to their wave-front & radially from their source, so the strength of received signal depends on whether that radial wave front is diffracted downwards (bringing the recipient more of the sound energy which otherwise went upwards & over their head) or upwards from the ground (reducing the proportion of sound energy available at ear level.

Oh bugger! I forgot I was told not to intervene in this discussion. Back to Archimedes then. Bugger again! The ARA thinks Archimedes got it all wrong & that boats only float because they have high saxboards. Hold on - why then do submarines float? It's because they have very high (but invisible) saxboards around their conning towers. Silly me!

See what happens when you let science intervene in matters more properly left to the scientifically illiterate & fundamentally dishonest? Shells will start sinking when they'd never have done so before. (Just as the Emperor was beautifully clothed, as everyone could see, until that stupid little kid said he wasn't). Except that shells have been sinking in rough weather ever since they were first invented, & rowers made out that a bit of unnecessary danger simply spiced things up for the families of the dead.

Hmmm.

Carl

PS Nick - how's with the termites?

--
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