http://www.youtube.com/watch?v=ZvJKaurkx6s In our last lesson we learned about octaves and octave numbers. Since a pitch has the same letter name in any octave it considered to be a part of a pitch class. A pitch class is a term used to describe any pitch, for example C, regardless of what octave or register that pitch is in. In the video above the example shows 8 octaves of C's, but all these pitches are part of the pitch class C.
Since all these are the same pitch class they have the quality of octave equivalence. Octave equivalence means that these pitches will share many of the same overtones, as well as be resonant with each other, and that voice leading resolutions will still work with octave displacement.
Listen to the extreme example of octave equivalence in the performance of Happy Birthday in the video. This example displaces the pitches of the melody in several octaves, yet you can still follow the melody due to the phenomenon of octave equivalence.