An iterative recognition method was then applied for detecting those components that are possibly missing. Since S3 and S4 are various both in amplitude and frequency, an adaptive detection method might occasionally misjudge noise signals as
heart sound components or vice versa. The iterative recognition method examined whether the detected points were periodically occurring. For those positions where S3 or S4 should have been detected, the method determined if there is a missing point. Consequently, S3 and S4 of heart sound signals could be recognized separately with enhanced accuracy.