Changing the loudness of one note in a chord
By controlling the relative loudness of each note in a harmonic chord, we are doing what a composer does while writing a symphony, and decides which instruments should play which notes in a chord: more instruments on this note, fewer on that note.
More prominence or less prominence given to the extension of the chord into a higher octave. Rarely does the composer put the same number of players on each of the notes in a chord.
When we do this ‘orchestration’ at the piano, the cause, and the effect are more subtle. Yet the result does mimic orchestration. This is because if we play a particular note twice, once softly, and once loudly, though the primary difference in heard in terms of dynamics, there is a secondary change, which we can notice if we focus on it, in terms of the relative loudness each overtone of that note has as compared with the loudness of each of the other overtones.
Acoustically, this is what gives rise to what our brain interprets as a change of ‘timbre’, or ‘tone quality’, or instrumental quality: that which makes an oboe playing middle C at mezzo forte sound different than a violin playing middle C at the same loudness. This change of timbre is somewhat noticeable when listening to a single note, but when it is multiplied over the various notes of a chord, the difference in the overall timbre of the chord changes more noticeably. Even when we play all the notes of a chord with equal intensity, the result is not what we might anticipate. Each note in the chord is under the “spell” of the harmonic progression. If a note happens to be the ‘third’ of the chord, it will have a relative predominance of effect over the root and fifth. This explains why if we are, for example, in C major, and have a V chord going to a I chord, we can omit the fifth in the I chord. Even though we have left just the root(s) and third(s) there will be no doubt as to whether the chord is a I chord or a vi chord, since both C E G and A C E share the C and E in common.