Wave reflection and transmission by an open-bottom airtight chamber for an active wave control system are investigated with consideration of compressibility of the air in the chamber. The hydrodynamic interaction of incident and scattering waves with the internal air of the chamber is formulated by employing linear potential theory and the equation of ideal gas state. Numerical results are presented to show the effects of internal air pressure upon the characteristics of wave reflection and transmission in various structural configurations. Model tests have been carried out in a two-dimensional wave flume. The experimental results are in good agreement with the numerical predictions. This paper also discusses an active wave control system to attain an optimum running condition by controlling the internal air pressure.