An acoustic pressure sensor is described that uses a small volume of ionized gas (microplasma) as a sensing element to estimate the sound-pressure fluctuations. This type of sensor uses electric discharge between a point and a plane to create the ionized gas useful for electroacoustic conversion. Depending on the polarity of the point (in this case negative), the applied voltage, and the distance between the electrodes (millimetric or micrometric), the electric discharge has specific operating modes and hysteretic behaviors that are described herein. The interaction of a sound wave with the volume of ionized gas causes a variation in the electric current of the microdischarge. The electroacoustic model and the expression of the sensitivity to the sound pressure, existing for the millimetric discharges (Trichel impulse regime), are briefly recalled. An empirical expression of the acoustic pressure sensitivity of millimetric and micrometric discharges is proposed. An experimental setup is developed using a waveguide. It enables the comparison of the value of the sound pressure deduced from plasma sensors to the one of a reference microphone (using a calibration method by comparison). The sensitivity to the sound pressure of these microplasma sensors is of the order of 0.4 nA/Pa.