If you see a nearby flash of lightning, why do you often hear a buildup before the big boom?

Question

The "boom" would represent the main concussion of the lightning - so why doesn't that big boom sound reach your ears first, followed by the softer rumbles? Why is there often a softer building rumble or crackling before the big "boom" sound? How do the softer sounds make it to your ears first?

Answer 1

If you find a diagram of the progress of a lighting stroke, you will see that during the few seconds before the heavy stroke connects (ground to cloud or cloud to cloud) there are other ionization strokes working their way toward various points. When one of them makes a viable connection the others collapse and all the power slams through the connection making the super bang. What you are hearing in advance is the heating of the air by these smaller probes.

Answer 2

What you hear first is the real crack of the lightning and what follows is all the reflections and bouncing of the sound before it reaches you. That is why the lightning strike may have only lasted less than a second but the thunder may last for five seconds. You first hear the large "kerack" and followed by the booming and rumbling.