Why Does Wavelength Affect Sound Diffraction Around Obstacles?

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Discussion Overview

The discussion revolves around the effects of wavelength on sound diffraction, particularly in relation to obstacles and the behavior of sound waves at boundaries between different media. Participants explore concepts of reflection and refraction in the context of sound waves and their interactions with obstacles of varying sizes.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant expresses confusion about why sound waves bend around obstacles when the wavelength is larger than the obstacle, seeking clarification on the underlying principles.
  • Another participant suggests that diffraction occurs at any boundary and is more noticeable with smaller objects, emphasizing that reflection and refraction also occur simultaneously depending on the boundary conditions.
  • There is a challenge regarding the categorization of sound behavior at boundaries, with some participants questioning the accuracy of statements about reflection and refraction based on the speed of sound differences between media.
  • A participant raises a specific question about the relationship between the speed of sound differences and the amount of reflection observed, particularly in the context of sound traveling from air to water.
  • Concerns are expressed about the interpretation of diffraction, particularly regarding the statement that "the wavelengths of sound are long enough to bend around the post," prompting further inquiry into the meaning of this statement.

Areas of Agreement / Disagreement

Participants do not reach consensus on the categorization of sound wave behavior at boundaries, with multiple competing views on the relationship between wavelength, diffraction, reflection, and refraction remaining unresolved.

Contextual Notes

Participants highlight potential misunderstandings regarding the conditions under which reflection and refraction occur, particularly in non-normal incidence scenarios. The discussion also reflects uncertainty about the implications of wavelength size relative to obstacles and the definitions of sound behavior at media boundaries.

Misr
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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/feschools/waves/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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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.).
 
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?
 
Misr said:
Okay,how about my second question?


Should I just accept this?

What do you mean by that?
 
Misr said:
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.
 
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 :

[url]http://hyperphysics.phy-astr.gsu.edu/hbase/sound/imgsou/difr.gif[/url]

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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