Octave

(oktāv.) [From L. octavus, the eighth.] (Mus.) The eighth interval in a scale, which, as it affords a sound agreeing very closely with the first, is denoted by the same letter of the alphabet. The most simple perception that we have of two sounds is that of uni sons; the vibrations beginning and ending together. The next to this is the O., where the more acute sound makes precisely two vibrations, while the deeper makes one; consequently, the vibrations of the two meet at every vibration of the more grave. Hence uni sons and octaves pa almost for the same concord; hence also the proportions of the two sounds that form the O. are, in numbers or in lines, as 1: 2, so that two cords or strings of the same material, thickne , and tension, one of which is double the length of the other, produce the O. The number of upper and lower 0, or the manner in which the several O. of the scale are to be chiefly distinguished, is not absolutely determined, on account of the continually increasing compa of instruments. -(Eccl.) In the Roman Catholic Church, a festival day and the seven days immediately following it; the last or eighth day being termed the octave, or octave day. The circumcision is the octave of Christmas-day. A festival is supposed, in that church, to last during its octave, and therefore the Office and Ma on every day during the octave are invariably those of the festival, unle a greater feast interferes, by occurring within the O. [s. 658]