sound field

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Author Message   Armin Wiegner (praktaw) Member Username: praktaw Post Number: 4 Registered: 09-2007 Posted on Wednesday, June 11, 2008 - 12:01 pm:    Hallo Mr. Nelson, I try to simulate the sound field (2D) using the wave equation: c*c*dxx(u)+c*c*dyy(u)=dtt(u) It is working with dt(u) on the right side. But FlexPDE gives me an error "Undefined name: DTT". Why? Thank you for your help! I'm using V5.0.18 Student sound field akustik 01.pde (0.6 k)   Robert G. Nelson (rgnelson) Moderator Username: rgnelson Post Number: 1129 Registered: 06-2003 Posted on Wednesday, June 11, 2008 - 01:32 pm:    FlexPDE does not directly implement second order derivatives in time. You need to define an auxiliary variable for the velocity and use two first order equations. In your case the system would be VARIABLES u,v EQUATIONS U: v = dt(u) V: c^2*div(grad(u)) = dt(v) Notice that the variable assignment is chosen to keep the dt() term on the diagonal of the coupling matrix. In this formalism, the boundary condition specifications are as follows: Natural(V) defines normal(c^2*grad(u)) Value(V) defines dt(u) Value(U) defines U Natural(U) is undefined. See notes to the example "Samples | Time_Dependent | Stress | Vibrate.pde"   Armin Wiegner (praktaw) Member Username: praktaw Post Number: 5 Registered: 09-2007 Posted on Friday, June 13, 2008 - 01:39 pm:    Thank you very much. It works great!