When are the 'lubb' and dubb' sounds produced by heart?

Explanation

1) On ventricular cotraction due to pressure, the blood moves into the aorta and pulmonary artery.
2) The aperture between the atria and ventricles is closed by valves.
3) When the valves are closed forcibly, we can listen to the first sharp sound of the heart 'lubb'.
4) When the ventricles start relaxing the pressure in the ventricle is reduced.
5) The valves which are present in the blood vessels are closed to prevent backward flow of blood into the ventricles.
6) Now we can listen to a dull sound of the heart 'dubb'.