Why are the first and second heart sounds different?

The first heart sound (S1) occurs when the atrioventricular (AV) valves close at the beginning of ventricular contraction (systole), creating a low-pitched "lub" sound. The second heart sound (S2) happens when the semilunar valves close at the end of ventricular contraction, producing a higher-pitched "dub" sound. The difference in timing, valve type, and the physiological processes during systole and diastole contribute to their distinct sounds. Additionally, S2 is typically shorter and may have a split due to the asynchronous closure of the aortic and pulmonary valves.