Part 2 addendum 

Reverberation

I feel that it is important to talk about reverberation as it is key to all of sound.  Reverb is when a wave bounces off of a solid surface  and hits our ear multiple times.  simply, it looks like this:

see all of the reflected arrows on the top of the diagram?  that is the reverberated sound.  and the observer will hear those reflected sound waves at a different time than the initial sound wave labeled "direct sound.