What is an acoustically soft/hard scatterer?

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In summary: This is a great summary. In summary, the ideal scatterer for acoustic scattering problems is a perfect reflector that does not absorb energy. The soft and hard scatterers are two extreme cases of this. The soft scatterer has to have total field vanish at the boundary, while the hard scatterer has normal derivative of the total field vanish at the boundary.
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Hedzlone
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Hi,

I'm currently interested in the numerical solution of acoustic scattering problems. Here, an acoustic wave propagates in a certain medium and hits (at some time) an obstacle. I am then interested in the computation of the scattererd wave. In the papers I read they distinguish between acoustically soft and hard obstacles/scatterers. Mathematically this means that I have to consider different boundary condition but I don't really know what this means physically. What is an example of a soft/hard scatterer?

It would be great if somebody could help me here. Thank you!
 
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  • #2
Would it have to do with the relative rigidity of the surface? IE: an ideal reflector would be perfectly rigid, with a reflection coefficient of 1 at all frequencies. "softer" surfaces would not be acoustically rigid, and would have a reflection coefficient less than 1 across a frequency range. In other words, the rigid surface would have an infinite acoustic impedance, while the soft surface would have some finite impedance. At the other extreme end (essentially the same as no obstacle), the impedance would be the characteristic impedance of the media (rho * c).
 
  • #3
Hi,

thank you very much for your answer. It definately goes in this direction. The soft and the hard scatterer for these problems are both extrem cases (there are also absorbing scatterers which are some kind of mixture).
It now seems to me that both, the soft and the hard scatterer, are some kind of perfect reflectors, i.e. they don't absorb energy but reflect in a different way (depending on the material properties?). I'm not a physicist so maybe this is nonsense, but it cannot be that one is a perfect reflector and the other doesn't reflect at all.
Maybe it helps if I say something about the boundary conditions. For the soft scatterer the total field u_s+u_i has to vanish at the boundary (u_s is the scattered wave, u_i is the incoming wave). For the hard scatterer the normal derivative of the total field vanishs at the boundary.
I found an http://www2.econ.univpm.it/servizi/hpp/recchioni/w2/virt1.htm" which solves this problem in 2D. You can choose the right edge to be a soft resp. a hard scatterer.

Which material combinations could lead to these different types of scattered wave?

Thank you!
 
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1. What is the difference between an acoustically soft and hard scatterer?

An acoustically soft scatterer is an object that causes minimal scattering of sound waves due to its low acoustic impedance, while an acoustically hard scatterer is an object that causes significant scattering due to its high acoustic impedance.

2. How does the size of an object affect its acoustic scattering properties?

The size of an object is an important factor in determining its acoustic scattering properties. Generally, larger objects tend to scatter sound waves more than smaller objects. This is because larger objects have a greater surface area for sound waves to interact with, resulting in more scattering.

3. What materials are considered to be acoustically soft/hard scatterers?

Materials that are considered acoustically soft scatterers include air, water, and most soft tissues in the human body. Materials with high acoustic impedance, such as bone, metal, and some types of rock, are considered acoustically hard scatterers.

4. How does the shape of an object affect its acoustic scattering properties?

The shape of an object can greatly influence its acoustic scattering properties. Objects with irregular shapes tend to scatter sound waves more than objects with smooth, regular shapes. This is because irregular shapes create more opportunities for sound waves to reflect and scatter.

5. Can an object be both an acoustically soft and hard scatterer?

No, an object cannot be both acoustically soft and hard at the same time. It is either one or the other, depending on its acoustic impedance. However, some objects may have regions that act as soft scatterers and other regions that act as hard scatterers, resulting in a mixture of scattering properties.

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