UNISON

, in Music, is when two sounds are exactly alike, or the same note, or tone.

What constitutes a Unison, is the equality of the number of vibrations, made in the same time, by the two sonorous bodies.

It is a noted phenomenon in music, that an intense sound being raised, either with the voice, or a sonorous body, another sonorous body near it, whose tone is either Unison, or octave to that tone, will sound its proper note Unison, or octave, to the given note. The experiment is easily tried with the strings of two instruments; or with a voice and harpsichord; or a bell, or even a drinking glass.

This phenomenon is thus accounted for: one string being struck, and the air put into a vibratory motion by it; every other string, within the reach of that motion, will receive some impression from it: but each string can only move with a determinate velocity of recourses or vibrations; and all Unisons proceed from equal vibrations; and other concords from other proportions of vibration. The Unison string then, keeping equal pace with the sounding string, as having the same measure of vibrations, must have its motion continued, and still improved, till at length its motion become sensible, and it give a distinct sound. Other concording strings have their motions propagated in different degrees, according to the frequency of the coincidence of their vibrations with those of the sounded string: the octave therefore most sensibly; then the 5th; after which, the crossing of the motions prevents any effect.

This is illustrated, as Galileo first suggested, by the pendulum, which being set a-moving, the motion may be continued and augmented, by making frequent, light, coincident impulses; as blowing on it when the vibration is just finished: but if it be touched by any cross or opposite motion, and that frequently too, the motion will be interrupted, and cease altogether. So, of two Unison strings, if the one be forcibly struck, it communicates motion, by the air, to the other; and both performing their vibrations together, the motion of that other will be improved and heightened by the frequent impulses received from the vibrations of the first, because given precisely when the other has finished its vibration, and is ready to return: but if the vibrations of the chords be unequal in duration, there will be a crossing of motions, more or less, according to the proportion of the inequality; by which the motion of the untouched string will be so checked, as never to be sensible. And this we find to be the case in all consonances, except Unison, octave, and the fifth.