A method of using an opto-acoustic apparatus for measuring, in an environment containing extraneous noise, the concentration of a gas in a mixture of gases or of particulates in a gas. The method is constituted by the steps of providing a measuring opto-acoustic cell having gas inlet and gas outlet for receiving and discharging a gas containing a particulate or a mixture of gases containing a gas the concentration of which is to be measured, directing laser rays from a laser ray generating device into the opto-acoustic cell, placing a chopper in the path of the laser rays between the device and the cell and operating the chopper for chopping the laser rays at a frequency corresponding to the resonant frequency of the cell, providing a narrow band microphone having a resonator with a narrow resonance frequency range including the resonant frequency of the cell and sufficiently narrow to exclude unwanted noise signals from the environment in which the cell is located, placing the microphone on the cell for detecting the sound signal generated by changes in the internal pressure of the cell, and determining from the sound signal the concentration of the gas in the mixture of gases or the concentration of the particulates in the gas.