Heart sounds are the characteristic sounds produced by the heart during the cardiac cycle. Each cycle gives rise to four heart sounds, but the first two (S1 and S2) are the most prominent. S1 is described as a lubb sound and occurs during ventricular systole, associated with the closure of the AV valves. S2 is a dupp sound, occurring during ventricular diastole, associated with the closure of the SL valves.