# How to obtain the Poisson equation as used by FEATool? Classic List Threaded 4 messages Open this post in threaded view
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## How to obtain the Poisson equation as used by FEATool?

 Hello, I am new to modeling and simulation in general and am trying to find the modes of an acoustic resonator including dissipative effects and thermal conductivity. I am using the Poisson equation as given in FEATool by dts_poi*p' - d_poi*(px_x + py_y + pz_z) = f_poi with dts_poi as the time scaling coefficient, d_poi as the diffusion term, and f_poi the source term, set to 0. I have tried at length to figure out how this equation is obtained, or even the Helmholtz equation as provided in the "Resonance Frequencies of a Room" tutorial, given by u' - (ux_x + uy_y + uz_z) = 1. Neither one seems like a standard variational construction of the equation. I get the modes predicted but don't understand why. So how can one obtain these equations from the standard (nabla)^2 * f = x?
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## Re: How to obtain the Poisson equation as used by FEATool?

 Administrator How do you mean "to figure out how this equation is obtained"? These are standard PDEs https://en.wikipedia.org/wiki/Poisson%27s_equationhttps://en.wikipedia.org/wiki/Helmholtz_equation