"The
first heart sound or "lub" results from closure of the tricuspid and mitral valves. It is a rather low-pitched and a relatively
long sound which, as indicated in, represents the beginning of ventricular systole.
The second heart sound, or "dub," marks the beginning of ventricular diastole. It is produced by closure of the aortic and pulmonary (pulmonic) semilunar vanes when the intraventricular pressure begins to fall. This "dub" sound is typically
heard as a sharp snap because the semilunar valves tend to close much more rapidly than the
AV valves. Because diastole occupies more time than systole, a brief pause occurs after the second heart sound when the heart is beating at a normal rate. Therefore, the pattern that
one hears is one of: "lub-dub" pause, "lub-dub" pause, and so on.
Sometimes, especially in
young normal individuals, a third heart sound can be
heard. This sound is produced by the very rapid influx of
blood into the partially filled ventricle. It is typically very faint and as such difficult to hear"
Reference: http://
www.jdaross.cwc.net/normal_heart_sounds.htm