If you have ever pressed an ear against someone’s chest, or perhaps listened to your own heartbeat after a long run, you know the sound well. It is the quintessential soundtrack of existence: lub-dub, lub-dub, lub-dub.
The answer is immediacy. The "Lub-Dub" is the vital sign of mechanical integrity. A doctor can listen to a heart for ten seconds and determine:
The "Lub" is the sound of the loading doors closing so the heart can fire blood to your body.