The closure of the heart valves creates distinct sounds which can be heard through a stethoscope placed in the correct location (Fox, 2013, p. 420). These sounds are the "lub-dub" that you can hear. The first sound is the closing of the AV valves and the second sound is produced by the closing of the semilunar valves. The first sound can also exhibit "physiological splitting." What is physiological splitting? Why does it occur?
Reference
Fox, S.I. (2013) Human Physiology (13th ed) New York, NY: McGraw-Hill.