The acoustic attenuator is so highly nonlinear and of high order that the observer design using the linear observer theory can hardly guarantee the robustness against the modeling error and uncertainties. In this paper, a state-space representation of an acoustic attenuator utilizing the electrical analogy is presented first, and the time domain response analysis is performed. The state-space model developed was validated by comparing the simulation results with the experimental results. Then, a nonlinear observer of the attenuator was designed based on the optimization problem which bridges the gap between nonlinear observer theory and its application. The observer performance was evaluated when modeling error existed to verify that the observer designed is insensitive to modeling errors and uncertain parameters. Satisfactory robustness and convergence properties of the observer were found.