Third heart sound occurs early in diastole. In older patients it indicates the presence of congestive heart failure. It is normal in young people and athletes. The sudden deceleration of blood flow into the left ventricle from the left atrium produces the third heart sound. Our animation of the third heart sound depicts a thin-walled, dilated left ventricle with generalized decreased vigor of contraction. Use the 'Visual' tab above to view this video. When a third heart sound (S3) is present, the first heart sound is diminished and the second heart sound is increased in intensity. The third heart sound is a low frequency sound. Use the stethoscope's bell, pressing lightly on the patient's skin, to best hear this sound.