A spectrophone assembly comprises a single detector chamber, a plurality of lasers, a gas inlet for supplying a gas sample to the single detector chamber, and at least one microphone. The detector chamber has an internal geometry arranged to be simultaneously acoustically resonant at a plurality of different resonant frequencies. Each laser operates at a different wavelength and is positioned to emit radiation into the single detector chamber, and is operable to emit radiation that is amplitude modulated at a frequency rate corresponding to a particular resonant frequency different from the resonant frequency of each other laser, simultaneously with each other laser. The microphone(s) are positioned in the single detector chamber so that each microphone is located at or near a maximum of a corresponding acoustic resonance defined by the internal geometry of the detector chamber.