boat models and lengths

[Piers]

Some googling found these sites with info on sound travelling upwind v downwind.

http://svconline.com/mag/avinstall_weather/
http://www.last-word.com/content_handling/show_tree/tree_id/3198.html
http://mysite.du.edu/~jcalvert/waves/soundwav.htm

There's a lot of words in each (especially the last one!), but if you search (ctrl + F) for 'wind' or 'downwind' you get the gist of what they're saying.

Does this help?

Piers

 

[jfm]

Some googling found these sites with info on sound travelling upwind v downwind.

http://svconline.com/mag/avinstall_weather/
http://www.last-word.com/content_handling/show_tree/tree_id/3198.html
http://mysite.du.edu/~jcalvert/waves/soundwav.htm

There's a lot of words in each (especially the last one!), but if you search (ctrl + F) for 'wind' or 'downwind' you get the gist of what they're saying.

Does this help?

Piers

Thnaks. Nothing useful in the first lnk. Second link contained this interesting bit:

You can do an experiment to demonstrate what happens. For the experiment you will need a swift flowing stream with a flat sandy bottom, so that there are no ripples, a still pond, a bag of small stones, a waterproof flashlight and three bottles of beer.

Go to the pond first, settle on the bank and open a bottle of beer. Now throw a stone into the water and watch the ripples spread out in a circle. At any given distance the riples are the same amplitude no matter what the direction. You are actualy seeing what happens to sound waves in still air. Drink the beer.

Now go to the swift flowing stream. Place the empty beer bottle on one bank and ritire to the other bank. Drink another bottle of beer and place the empty on the bank directly opposite the first bottle. Now throw a stone so that it lands in the water in line with the two bottles. Observe what happens. You will see that the patern of ripples moves downstream with the flow of water. At a given distance from the point at which the stone hit the water, the ripples will be of greater amplitude down stream than they are upstream. You are seeing what happens to soundwaves in a moving stream of air, wind. Now throw another stone in the same way. Look at how the ripples spread across the stream. The ripples will reach closer to the banks down stream than they do directly in line with the bottles. This is what is happening in air when you say that your voice is being "torn away by the wind".


which is interesting in that he says the wave amplitute increases downwind/downsteam, which means louder, but he gives no scientific explanation for that amplitude increase

The third link re-states the boundary layer theory but it's a shame that it completely avoids dealing with the fact that air at normal windspeeds has a very thin boundary layer, milimeters and centimeters. Thus, I'm still sceptical on the boundary layer theory and it would be interesting to see more science on the "increased amplitude" theory

 

[Piers]

Thnaks. Nothing useful in the first lnk. Second link contained this interesting bit:

You can do an experiment to demonstrate what happens. For the experiment you will need a swift flowing stream with a flat sandy bottom, so that there are no ripples, a still pond, a bag of small stones, a waterproof flashlight and three bottles of beer.

Go to the pond first, settle on the bank and open a bottle of beer. Now throw a stone into the water and watch the ripples spread out in a circle. At any given distance the riples are the same amplitude no matter what the direction. You are actualy seeing what happens to sound waves in still air. Drink the beer.

Now go to the swift flowing stream. Place the empty beer bottle on one bank and ritire to the other bank. Drink another bottle of beer and place the empty on the bank directly opposite the first bottle. Now throw a stone so that it lands in the water in line with the two bottles. Observe what happens. You will see that the patern of ripples moves downstream with the flow of water. At a given distance from the point at which the stone hit the water, the ripples will be of greater amplitude down stream than they are upstream. You are seeing what happens to soundwaves in a moving stream of air, wind. Now throw another stone in the same way. Look at how the ripples spread across the stream. The ripples will reach closer to the banks down stream than they do directly in line with the bottles. This is what is happening in air when you say that your voice is being "torn away by the wind".


which is interesting in that he says the wave amplitute increases downwind/downsteam, which means louder, but he gives no scientific explanation for that amplitude increase

The third link re-states the boundary layer theory but it's a shame that it completely avoids dealing with the fact that air at normal windspeeds has a very thin boundary layer, milimeters and centimeters. Thus, I'm still sceptical on the boundary layer theory and it would be interesting to see more science on the "increased amplitude" theory

I read that too, but it was the third link that I found taxing....

 

[BartW]

BartW may well be the best person to explain all this but I bet he is cruising on his superyacht somewhere in the sun. He has become such a playboy

Poor me,
During 7 weeks I have been working so hard, serving and entertaining my demanding guests,

Regarding sound propagation,
The windspeed on the ground is lo and increases with height.
This results in a refraction effect, which deflects the sound travelling against the wind upwards, and this limits the reach of the sound source in that direction.
Same effect causes sound waves with the wind, to be deflected downwards.

Then Refraction by temperature layers.
Sound has a different speed in air with different temperature.
When you have a big audience, just above the audience there is a higher air temp, from body heat.
This air temp difference, will create a boundary that will reflect some of the sound.
I can imagine that something similar could occur when a layer of air just above seawater has higher temp at evening, when ambient air temperature has gone down

And finally there is moisture, the higher the humidity of the air, the more it will ‘damp’ high frequency sound energy

No real figures on this, but we know that these factors are noticeable and important when adjusting sound systems for concerts or festivals in open air.
And they are changing over the period of a day.

Sound level drop over distance is easy to calculate,
For a spherical sound source (in all directions), level drop over distance is -6db for every doubling of distance,
So a horn producting f.e. 120dB at 1m, then it will produce 120-6=114dB at 2m, 108dB at 4m, 102db at 8m, 96dB at 16m, 90dB at 32m, 84dB at 64m …. Etc
This is excluding diffraction / attenuation caused by atmospheric effects.

If you reduce the ‘sphere’ in which the sound can travel,
f.e. I would like to place the horn on the flat surface on top of my lower helm station, I gain 6dB.

 

[Piers]

Hi Bart. Thanks for this explanation. And checking you profile I can see why you know what you're talking about! It's interesting, 'cos in my searching the Internet, many of the comments on just this subject were found on sound professionals websites.

All the best.