sound of thunder

[scm1001]

listening to thunder recently, I noticed that near by strikes started with a sharpish crack, followed a second later by a much louder roar. Why is the louder part after the first?

[bibliophage]

From * Physical Meteorology* by Houghton

The “crack” heard from a close flash owes its characteristic sound to high frequencies that are lost at greater distances. The “clap” is heard when major segments of the [lightning] channel are more or less normal [perpendicular] to the observer’s line of sound. The rumbles come from the more distant segments of the channel. … Thunder of a fabric-ripping sound may be heard from nearby leader strokes.

[femtosecond]

Speed of sound vs. frequency–effects in thunder?
Why does thunder roll?