US Patent No.  5,680,868  October 28, 1997

A method and apparatus is provided for detecting blood pressure sounds produced by the opening snap of an artery in a patient in the presence of significant noise using the phase information contained in two microphone signals. The two microphones are placed on a patient along the axis of an artery with their centers separated by a distance such that blood pressure sounds picked up by the microphones will be out of phase. Pressure is applied to the artery such that the artery opens and closes during each heart cycle, the opening snap of the artery producing blood pressure sounds. The two microphone signals are filtered, preferably using band pass filters having pass bands corresponding to the frequency of the blood pressure sounds. The filtered microphone signals are sampled and multiplied together or convolved in the frequency domain to generate a microphone signal product. If the microphone signal product is negative, the detection of a valid blood pressure signal for that sampling time is indicated. In an alternative embodiment the slope of the two microphone signals at the sample time is also determined, A valid blood pressure signal is indicated when the microphone signal product is negative and either the two microphone signal slopes have inverse signs or are both less than a selected slope threshold. A selected number of consecutive valid blood pressure signal detection indicates the detection of a blood pressure sound. The detection of blood pressure sounds at a range of cuff pressures is used to determine the systolic and diastolic blood pressure levels of the patient.