Abstract: A gas analyzer (10) for detecting low concentrations of a specific gas which has a sample chamber comprising passages (14, 30, and 32) through which radiant energy is directed and which is pressure modulated by a loud speaker (36) both of which provide a radiant energy signal which is detected by a beam splitter (82) having a measurement channel (86) directed to a measurement detector (92) and a reference channel (84) with a reference cell (94), containing gas of the type being analyzed, directed to a reference detector (90). Both detectors (92, 90) produce a signal, and these signals and a cross channel normalization constant are processed by an algorithm. The loudspeaker (36) is isolated from the sample chamber by a flexible diaphragm (42) to eliminate phase changes in the pressure modulated output of the loud speaker (36).