The 2 sounds are produced by the heart- the first heart sound is produced by the closure of the atrioventricular (tricuspid and mitral) valves and the second heart sound is produced by the closure of semilunar (aortic and pulmonary) valves. These sounds are heard by the clinician by auscultationwith stethoscope. The first and second heart sounds are heard as LUB and DUB, respectively.