NOVELTY - The resonant vessel (4) contains two acoustic transducers (10, 11) to detect the acoustic signals produced, placed in containers (40, 41) connected to each other to form a Helmholtz resonator system where the containers are in phase opposition. The system includes means of adapting the wavelength of the light source as a function of the gas to be detected.    USE - Detecting the presence of a gas.    ADVANTAGE - This device allows the detection of two or more gases simultaneously using one laser diode.    DETAILED DESCRIPTION - The measuring device contains a radiant laser energy source (1) to supply an excitation energy to the measurement zone occupied by the gas. The excitation energy is selected in a wavelength band of maximum absorption for the gas. A modulator (2) modulates the excitation energy according to a modulation frequency. The measurement zone comprises a resonant vessel (4) whose resonance frequency is apt for the modulation frequency of the optical excitation. The optical excitation is modulated electronically by variation of the laser diode control current. The outlet from the transducer (10) in the container (40) subject to the laser beam is connected to the positive input of a differential amplifier (8) and the outlet from the transducer (11) in the other container (41) is connected to the negative input of the differential amplifier. The resonant vessel is made of two containers in the form of measuring tubes connected near each end by a capillary tube. A first capillary (43) receives an inlet tube (5) with a tap (45) and the second capillary (44) receives an outlet tube (6) with a tap (46). The outlet tube is connected to a pump (60). The device is mounted on a vehicle, with the inlet tube connected with the outside of the vehicle to draw in air to detect the gas to be detected. The modulator is mechanical.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the device described    Laser source (1)    Modulator (2)    Resonant vessel (4)    Inlet tube (5)    Outlet tube (6)    Differential amplifier (8)    Transducer (10)    Transducer (11)    Container (40)    Container (41)    First capillary tube (43)    Second capillary tube (44)    Tap (45)    Tap (46)    Pump (60)