The use of acoustic detection for the measurement of infrared absorption in gases has been shown to be of value in measuring atmospheric pollutant gases. Further improvements in detection sensitivity may be achieved by the use of an acoustically resonant system and multipass optics wherein the incident laser radiation is reflected many times through the absorption sample. We have demonstrated acousticQ's exceeding 750 and sensitivity increases of at least 10 using multipass optics. A microphone amplifier pair is described whose noise limitation approaches the thermal noise on the microphone diaphragm.