OCTAVE (Octaves, Lat.), in Music, the eighth note of the scale the most perfect of concords, whose ratio is 2 : 1, therefore the simplest of all the sounds, except the unison. The harmonics of the octave and unison agree invariably, a coincidence which occurs in no other interval, and these sounds have so close a resem blance, that in combination they are hardly distinguishable the one from the other. The following are the properties, says Rousseau, which so singularly distinguish the octave from all other intervals : The Octave embraces all the primitive sounds, that is to say, all the original tones and semitones. Hence, after having established a system or series of notes within the limits of an octave, if it be wished to extend this series it will be absolutely necessary to follow the same order in a second octave, iu a third, or a fourth, &c., and no sound will
be found in any of these but what is, as it were, a recurrence of some one in the first series. It is in virtue of this property in the Octave that the term Diapason was applied to it by the Greeks. [Drarasole.] The Octave has also another remarkable pioperty, the same writer observes, namely, that it may be doubled, tripled, and multiplied at pleasure, without changing its nature. This multiplicatieu, however, is limited as relates to its effect on the car, and an interval of eight octaves, for instance, would be scarcely, if at all, appreciable as such by the auditory organ. A double Octave is less agreeable than a single one ; a triple Octave loses still more of its pleasing' quality ; till, by increasing the distance, the relationship of the sounds becomes nearly undiatinguishable.