Originally Posted by: ics1974What physically is an octive?

Fret Spider gave a good answer. I want to give a few more specific details.

An octave is a musical interval between two notes (or more precisely, two pitche specific tones), one note of which is exactly double the frequency in Hertz (Hertz is a physical distance measurement of the wavelength of sound measured in cycles per second as it moves through the air).

Consider a note A that is defined by a sound at 220 Hertz. Another note that is defined by a sound at double that, 440 Hertz is an A one octave higher than the first A.

Exactly like Fret Spider mentioned, this is easy to see on a guitar because it is at the 12th fret, exactly half the length of the string. Great example!

Also of interest (at least to me :) ) is that research has indicated that the shape of the cochlea (the snail shaped part of the inner ear that functions as the "collector" of auditory data which it then sends it to the brain for processing) is the reason humans identify an octave as such. Research into acoustics and human hearing physiology has suggested that for example if our cochleas were a straight tube, we might have formed our musical alphabet differently. Possible with no repetition of letters. Or at least in a manner that would not place such emphasis on the octave.