Source: Suneel Dhand, MD, Attending Physician, Internal Medicine, Beth Israel Deaconess Medical Center Proficiency in the use of a stethoscope to listen to heart sounds and the ability to differentiate between normal and abnormal heart sounds are essential skills for any physician. Correct placement of the stethoscope on the chest corresponds to the sound of cardiac valves closing. The heart has two main sounds: S1 and S2. The first heart sound (S1) occurs as the mitral and tricuspid valves (atrioventricular valves) close after blood enters the ventricles. This represents the start of systole. The second heart sound (S2) occurs when the aortic and pulmonary valves (semilunar valves) close after blood has left the ventricles to enter the systemic and pulmonary circulation systems at the end of systole. Traditionally, the sounds are known as a "lub-dub." Auscultation of the heart is performed using both diaphragm and bell parts of the stethoscope chest piece. The diaphragm is most commonly used and is best for high-frequency sounds (such as S1 and S2) and murmurs of mitral regurgitation and aortic stenosis. The diaphragm should be pressed firmly against the chest wall. The bell best transmits low-frequency sounds (such as S3 and S4) and the murmur of mitral stenosis. The bell should be applied with a light pressure.