Note: I'm trying to implement a Discontinuous Galerkin method, as kind of a way to learn about these things.As of now, I've taken the acoustic wave equation $c^2 \nabla \cdot \nabla u(x,t) - \frac{\partial^2 u(x,t)}{\partial t^2} = f(x,t)$, rewritten it in weak form with the test function $v(x,t)$, and used Green's first identity to obtain a flux integral from the laplace operator:$$\int_{\Gamma_K} c^2 v(x,t) \nabla u(x,t) \cdot dS - \int_K c^2 \nabla u(x,t) \cdot \nabla v(x,t) dV - \int_K \frac{\partial^2 u(x,t)}{\partial t^2} v(x,t) dV = \int_K f(x,t) v(x,t) dV$$ where $K$ is an element of the mesh and $\Gamma_K$ is its edges / faces.I'm not super sure how ABC's work, but I believe the reason for using Green's first identity to get the flux integral on $\Gamma_K$ is so that we can replace existing boundary conditions with absorbing boundary conditions on the boundary faces of $\partial \Omega$? If I understand correctly, these absorbing boundary conditions would also be a differential equation that are only imposed on the boundary $\partial \Omega$.My question is: What are some good absorbing boundary conditions (differential equations) I can use for the acoustic wave equation in a 2-dimensional or 3-dimensional rectangular domain? 
Absorbing boundary conditions for acoustics in Discontinuous Galerkin
finite element;boundary conditions;discontinuous galerkin;wave propagation