Explain in short: $Lub$ and $Dub$ sounds.

(N/A) During each cardiac cycle,two prominent sounds are produced which can be easily heard through a stethoscope.
The first heart sound $(lub)$ is associated with the closure of the tricuspid and bicuspid valves.
The second heart sound $(dub)$ is associated with the closure of the semilunar valves.
These sounds are of clinical diagnostic significance.