The intervals are traditionally divided into consonant and dissonant, depending on their sweet or harsh effect. For example, the C major chord (that you can obtain superimposing the first five harmonics) is formed by consonant intervals, while the one formed by C-F#-B-F-Bb contains many dissonant intervals and therefore it is extremely harsh. You can better understand the difference by looking at the following two graphs, where the thin lines are the single sound waves and the thicker line is the chord waves. It is evident that the wave shape is fairly regular in the first case, contrary to what occurs in the second.
Note that this distinction is not a value judgment: a consonant chord is not necessarily better of a consonant one (in fact, the appropriate use of dissonance makes a piece more expressive); here what you want to emphasize here is the physical reason why some chords (consonances) give a sense of quiet, others (dissonance) of tension.
You can observe how the same melody takes on a different appearance depending on the types of chords used. In the
first example
, a flute solo is later accompanied by a viola forming octave intervals, with the only result of reinforcing the melody. Finally, the two voices proceed bt fifths, already a sketch of polyphony although very poor: the effect is a bit “empty”, almost as if something was missing. In the
second case the four voices always form consonant chords (except the penultimate note). To conclude you can hear the effect of chords always deliberately
dissonant, that make the melody, for listener not already accustomed to this kind of composition, a bit strange, almost alien.
