US Patent No. 5,873,836 February 23, 1999
A method and apparatus is provided for distinguishing blood pressure sounds from noise in an automatic blood pressure monitoring system using the information contained in two microphone signals. Two microphones are placed on a patient along an axis of an artery to pick up blood pressure sounds. Pressure is applied to the artery such that the artery opens and closes during each heart cycle, the opening snap of the artery producing a blood pressure sound. The two microphone signals are sampled, filtered, and multiplied together to produce a microphone signal product. The microphone signal product is wave width filtered to remove therefrom portions of the signal having a wave width which is less than a selected minimum wave width value. A blood pressure sound is indicated as being detected when an amplitude of this wave width filtered microphone signal product exceeds a noise signal threshold level. The microphone signal product is also wave width filtered to remove therefrom portions of the signal having a wave width which is greater than a selected maximum wave width value. The amplitude of the resulting noise amplitude signal is used to adjust dynamically the noise signal threshold level in response to changing noise conditions. The detection of blood pressure sounds at a range of applied pressure levels is used to determine the systolic and diastolic blood pressure levels of the patient.