the s sound occurs, by definition, immediately before s, while the atria of the heart are vigorously contracting. it is manifest as a vibration of 20 to 30 hz within the ventricle. while the mechanism is not absolutely certain, it is generally accepted that s is caused by stiffening of the walls of the ventricles (usually the left), which produces abnormally turbulent flow as the atria contract to force blood into the ventricle.