Slide 3
Slide 3 text
“In music, a fifteenth or double octave is the
interval between one musical note and another
with one-quarter the wavelength or quadruple the
frequency.
The fourth harmonic, it is two octaves. It is
referred to as a fifteenth because, in the diatonic
scale, there are 15 notes between them if one
counts both ends (as is customary). Two octaves
do not make a sixteenth, but a fifteenth. In other
contexts, the term two octaves is likely to be
used.”
wikipedia.org
BTW, did you know?