24 Tones, 48 Reeds, Octave Tuned For Full Bodied Organ Sound. Key of C. Octave harmonicas are similar to Tremolo models in reed layout and musical range. Instead of having reeds tuned to the same note, however, each double hole has one reed tuned an octave apart from the other. The resulting sound is stronger and full bodied, but without the tremolo effect. This is the harmonica equivalent of a twelve-string guitar and is ideal for regional and international styles like Cajun, Old-Time and Irish music.