In this paper. we consider a problem with dynamical boundary conditions for a hyperbolic equation. The dynamical boundary condition is a convenient method to take into account the presence of certain damper when fixing the end of a string or a beam. Problems with dynamical boundary conditions containing first-order derivatives with respect to both space and time variables are not self... https://www.roneverhart.com/Doorking-1835-080-Telephone-Entry-System/