In the rhythmic symphony of the human heartbeat, two sounds dominate: "Lub-dub, lub-dub." The first of these, the "Lub" (S1) , marks the beginning of ventricular systole. While it is often summarized as a single closing sound, S1 is a complex, high-frequency event that provides clinicians with vital clues about the mechanical and electrical health of the heart. What Creates S1? Contrary to popular belief, S1 is not the sound of the heart "beating" or contracting. Instead, it is generated by the turbulent flow of blood and the subsequent vibration of cardiac structures immediately following the closure of the atrioventricular (AV) valves.