diffraction of sound


by Misr
Tags: diffraction, sound
Misr
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#1
Nov8-11, 08:51 AM
P: 391
When the wavelength is bigger than an obstacle, then the sound waves bend around the obstacle.

so why
I can't imagine this at all

I read this page and watched the animations of it but it doesn't tell me the answer
http://www.acoustics.salford.ac.uk/f...s/diffract.htm

reflection of sound takes place when the difference in the speed of the sound between two media is great,while refraction takes place when the difference in the speed of the sound between two media is small

why???
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sophiecentaur
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#2
Nov8-11, 09:37 AM
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I think you are trying to categorise things too much. Diffraction always happens, at any boundary*. It is just more noticeable for a 'small' object.

Also, the effect of diffraction at the boundary between two 'transmissive' media will produce both a reflected wave and a refracted wave. The amount of reflection and refraction will depend on how big a change there is across the boundary and the angles involved.

* diffraction is just the result of all the waves adding up across the whole of the wavefront. When you shine light through a pinhole the resulting shape of the wave that gets through is very rounded. When you shine it through a doorway, much more light, in total, gets through but a tiny amount also 'leaks round the sides' with a very similar pattern at the very edge (90degrees) as from a pinhole. When we discuss reflection and refraction, we are usually discussing very wide wavefronts (big apertures) so we just ignore the off-axis ('diffracted') wave because it is so insignificant compared with all the energy going in the classically defined 'ray' direction.

But, for telescopes, for instance, we have refraction effects (focusing the image) plus diffraction effects (the ultimate blurriness of pictures of the Moon's surface etc.).
Misr
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#3
Nov8-11, 07:15 PM
P: 391
Okay,how about my second question?


reflection of sound takes place when the difference in the speed of the sound between two media is great,while refraction takes place when the difference in the speed of the sound between two media is small
Should I just accept this?

Drakkith
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Nov8-11, 08:11 PM
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diffraction of sound


Quote Quote by Misr View Post
Okay,how about my second question?


Should I just accept this?
What do you mean by that?
sophiecentaur
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Nov9-11, 05:48 PM
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Quote Quote by Misr View Post
Okay,how about my second question?


Should I just accept this?
The quote in that post is not correct. You always get both reflection and refraction for non normal incidence. You continue to categorize rather than try to understand.
Misr
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#6
Nov14-11, 06:15 AM
P: 391
Actually,I'm very confused about that
The quote in that post is not correct. You always get both reflection and refraction for non normal incidence. You continue to categorize rather than try to understand.
but most of the incident rays reflect and some rays refract due to the big difference in the speed of sound between air and water.That's what I mean
so what makes waves encounter more relection when difference in the speed of sound between air and water is big?

Another question :
[img]
http://hyperphysics.phy-astr.gsu.edu...mgsou/difr.gif
[/img]
My problem about diffraction is here
what does we mean by "the wavelengths of sound are long enough to bend around the post"
Thanks in advance


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