The sentence can be completed as follows:
"As thunder travels away from a lightning channel, the amplitude (loudness) of the sound DECREASES by A FACTOR 4 as the distance from the channel doubles."
In fact, the intensity of a sound wave follows an inverse square law: it means it is inversely proportional to the square of the distance, [tex]1/r^2[/tex], where r is the distance from the source. We see from the formula that, if we double the distance, the intensity gets a factor [tex]1/4[/tex], which means that the intensity decreases by a factor 4.
