What exactly causes the sounds heard by a stethoscope

There are 2 sounds heard by a stethoscope, the so called 'lub dub'. The lub being the closing of the atrioventricular valves (i.e. the valves between the atria and the ventricles). This is caused by an increase in pressure in the ventricles due to contraction of the ventricles. The dub is caused by the closing of the semilunar valves (i.e. the valves in the aorta and the pulmonary trunk), which occurs when the ventricles relax and the pressure in the ventricles is lower than in the aorta or pulmonary trunk, hence causing a back flow which shuts the valves and produces the quieter 'dub'.