The design of a gauge capable of measuring gas pressures in the range from atmospheric pressure to 10−4mm Hg by acoustic techniques is presented. The principles of acoustic transmission in the classical and geometrical relaxation regimes of gas dynamics are reviewed for the case of a confined volume of gas. The geometrical conditions for the feasibility of a sonic pressure gauge following a linear law are obtained. Practical limitations in the sound source used give rise to a change in the sensitivity of the gauge in the transition from classical to geometrical relaxation regime. Transducer and electronic design and a calibration curve of the gauge are given. The experimental results are in general agreement with the analysis.