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!