To be more specific ... by 'propagate more effectively' I mean with greater fidelity -- if the sound were a marching band, its music seems more audible when coming from the upwind direction (wind in your face) than from the downwind direction (wind to your back.)

Hi David

This is not my area of expertise. As no one has answered your question, I will venture a guess that perhaps may serve as a starting point for discussion.

For what it is worth.

Wind velocity superpose on the wave speed. Wind at an angle bends the propagation direction. Wind speed varies with height. Wind layers with different speeds cause turbulence at the intersection. .

Now, downwind conditions may interact to focus sound into particular areas. Such a condition would then be a conduit with a particular wind and wave speed combination that would represent the acoustic most effective path from source to receiver and not be greatly affected by turbulence, hence, produces the greatest clarity in the sense of being the closest to the near-the-source sound.

Other paths would imply a transfer across layers, at which losses occur, and transfer across turbulent layers.

Turbulence interacts with sound just as wind does. Turbulence has a greater effect on high frequency sound as its correlation length matters. Sound wave lengths of similar size to- and shorter than- the turbulence correlation length become affected the most as, in essence, this increases the propagation distance for the wave lengths turbulence is able to divert.

Damping in air is not very high. It is affected by moisture and, I guess, precipitation and snow. We will ignore such effects in the below other than comment that passing such layers affect high frequency than it does low frequency.

Damping losses absorb energy per wave length, i.e. its effect on sound is the higher, the longer the propagation distance and, the higher the frequency. The latter because it takes the more wave lengths to cover a fix distance, the higher the frequency. To exemplify, the distance 1 m, at the wave speed 340 m/s is 1/340th wave length at 1 Hz, 1 wave length at 340 Hz and 100 wave lengths at 34 kHz.

Upwind conditions will also have a path of clarity, only it tends to orient away from people.

What we hear is for upwind conditions transmitted via a longer and acoustically less effective path in which turbulence show a greater effect than is the case for the downwind case. The end result is reduced clarity.

Just my 2 cents

Claes

I think Claes Richard Fredö has done a pretty good job of explaining it, and yes, as Joseph Charles French mentions, there will also be a Doppler shift, upwind source sound higher than a downwind source. There will also be a difference in amplitude for a fixed distance in either direction, since the downwind sound will have effectively propagated for a greater distance due to the movement of the medium. So combine all three - less amplitude, lower pitch, and less high frequency content due to damping - and you have the perception of a "less clear" sound.

Thanks, Claes and Paul, for interesting answers. Yes, I agree that a combination of turbulence, waveguiding, frequency shift and effective path length must be contributing. Still interesting to me that the up- and down-wind differences are easily noticeable. Appreciate your thoughts. (I wonder if the submarine/sonar community deals with similar effects with thermoclines and water currents and which are considerably slower than wind in water where the sound velocity is higher than air.)

Hi

Yes, I imagine that you would get similar effects also for underwater conditions though things like salinity, that water shifts into layers with temperature and so on makes underwater sound propagation quite a different beast from wave propagation in air.

There is also the difference that wave propagation in air is adiabatic while it is isothermal in water.

On top of this, wave steepening matters, in particular where gas is dissolved in the liquid. Steepening is the largest where line pressure ('density x gravity x depth) is low, i.e. at shallow waters.

https://qringtech.com/2010/09/15/wave-steepening-increase-peak-pressure-piping-pumps/

As pointed out by Joseph Charles French

/Claes

The usual answer involves the effect of refraction. The velocity profile (wind speed increasing with height) means that sound gets refracted upwards when it travels upwind, but downwards when it goes downwind. This results in sound being removed from ground level upwind, but concentrated at ground level downwind.

Actually, I was in error. There will be no Doppler shift because both the sound source and the listener are moving at the same speed relative to the medium. That's what happens when you try to answer a question while giving half an ear to a committee meeting. ;-)

Hi Paul

That makes more than one of us in error.... #MeToo

I got lost in the Mach effect from flow which is a different thing...

/C

Hi Malcolm, Could you add some reference or a bit more explanation to the effect that breaks the upwind and downwind asymmetry. If velocity gradient is just causing a pressure gradient and therefore a refractive index gradient, it seems like both directions would be deflected in the same direction. So it seems like the direction of deflection must depend on the wind speed gradient and the sound direction. I'd really like to hear your explanation.

Upwind, the wave travels more slowly. Because the wind-speed increases with height, the wave travels more slowly the higher it is. This results in a shorter wavelength at higher altitudes compared to lower altitudes, which results in refraction upwards.

Downwind, the wave travels more quickly the higher it is. The wavelength increases with height, which results in refraction downwards.

The asymmetry is there because the wave is travelling in the opposite direction to the wind upwind, but in the same direction as the wind downwind. It is not a pressure gradient, it is a gradient of speed and wavelength.

These might be helpful

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

https://physics.stackexchange.com/questions/341704/why-does-wind-direction-significantly-affect-sound-propagation

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

Hi again, Malcolm. Your explanation and references were very good. I have made a plot (the only way I learn) of the streamlines of sound direction from a point source in a gentle breeze. In this plot, wind goes from left to right. Sure enough, a listener on the downwind side receives a lot more sound energy that a listener on the upwind side. This answers the question. Now I understand why I can hear a distant train from my house clearly sometimes (whistle and engine) and not other times. Plot follows. (Note, following your advice, I modeled the wind as increasing proportional to elevation which makes sense given ground friction.) Thanks again!

Dear Malcolm, David and Claes,

I tried to explain by 1D Euler eauation.(See the attached PDF fie.)

But there may be mistakes.

Please correct them.

I cannot add references because I only have a Japanese text book.

I am looking forward to your answer.

Ryuzo Horiguchi (Kao COrporation)

Hi Ryuzo Horiguchi

If you want to approach the world from this angle, please take a look at this excellent book by S.W. Rienstra & A. Hirschberg . Sections 2.4 and 8.7 may be of interest.

https://www.win.tue.nl/~sjoerdr/papers/boek.pdf

Sincerely

Claes

Thank you, Richard.

It is very intereting.

Ryuzo Horiguchi

Hi Ryuzo Horiguchi

Yes, in my humble opinion, this is one of the best books on the subject. It is very well made and worth the repeated read.

Other very good books on the subject of aeroacoustics are

• Uno Ingaard, Acoustics https://www.barnesandnoble.com/w/acoustics-uno-ingard/1100114793
• M.S. Howe https://www.cambridge.org/core/books/acoustics-of-fluid-structure-interactions/B2078660935DD108AF0795C191485D91

Ingaard has a very straightfoward introduction to and explanation of the physics. The Howe book contains lots of cases and references.

Have fun, because - this is fun.

Claes

Inversion of the temperature, that is colder air closer to the ground and hotter longer up, can make a significant effect here. Even more than wind does, I think. The soundspeed is temperature dependant, slower in cold air. The sound paths will bend downward in inversion atmosphere and upwards in normal atmosphere. I think temperature gradient is far more important than wind for sound propagation effects like this. It can give effects in order of 20 dB or more at some hundred meters distance. The best simplest explanation I have seen on this was in a Norwegian Physics Paper recently. "Why elephants and skiers sometimes can be heard over long distances".

Thank you Anders. In the case of a temperature and resulting refractive index gradient, what breaks the symmetry in terms of sound direction? With wind (a vector), sound traveling upwind and downwind are acted upon by the wind differently as pointed out by Malcolm White above. It is interesting that the downward bending of sound would be a large effect, however, acting by 20dB in hundreds of meters as you point out. Good contribution to the subject!

Thank you, Claes.

I am reaing the chapters you recomend in the textbook by Rienstra et al.

It is very interesting and the description is detailed.

I would like to read the other two books you recommended.

Thank you.

Have a nice holiday.

Ryuzo