Recursive least squares (RLS) adaptive acoustic signal filtering for a physiological monitoring system reduces residual heart sound in a primary signal remaining after application of a respiration sound bandpass filter to a first instance of a mixed signal containing respiration sound and heart sound. Residual heart sound in the primary signal is reduced by minimizing a component in the primary signal that correlates with a reference signal containing heart sound but almost no residual respiration sound after application of a heart sound bandpass filter to a second instance of the mixed signal. The correlative component in the primary signal is minimized by applying an adaptive filter to the reference signal and subtracting the filtered reference signal from the primary signal to produce a residue signal, wherein the coefficients for the adaptive filter are selected to minimize the least square error of the residue signal.
