Final answer:
The ventricles have just emptied when the second heart sound, known as the 'dub' or S2 is heard due to the closing of the semilunar valves during ventricular diastole.
Step-by-step explanation:
The chambers that have just emptied when the second heart sound is heard are the ventricles. The second heart sound, S2, is characterized as a "dub" and is caused by the closing of the semilunar valves during the ventricular diastole.
This occurs after the ventricular ejection phase, where blood has been pumped out of the right ventricle into the pulmonary trunk and from the left ventricle into the aorta. The ventricles have just emptied when the second heart sound, known as the 'dub' or S2 is heard due to the closing of the semilunar valves during ventricular diastole.