Abstract: A method and an apparatus are provided for optically indicating that the gas pressure of a gas reservoir of a gas mask worn by a user has reached a predetermined value. This value is indicated by an acoustic signal within a predetermined frequency range and at a given lowest sound level. The method is characterized by monitoring for sound within the predetermined frequency range and above the lowest sound level with the aid of a sound detecting device, and activating a light source in response to such sound detected by the device. The apparatus includes a device for delivering an acoustic signal within a predetermined frequency range and at a sound level which exceeds a predetermined lowest value, a sound detecting unit which actively monitors for the sound within the predetermined frequency range and at the sound level exceeding the lowest value, and a light source which is connected to the sound detecting unit for producing a light warning signal.