Consonance and dissonance

Definition

Consonance is the quality of an interval or chord perceived as stable, fused, at rest; dissonance is perceived as tense, rough, in need of resolution. There are two levels:

• Sensory consonance (psychoacoustics): how much two sounds “beat” or sound rough.
• Musical consonance (stylistic): what a style treats as stable. In jazz a Cmaj9 is a chord of rest; in Palestrina the same seventh is a dissonance to be prepared and resolved.

The traditional classification (common practice)

The tritone (augmented 4th / diminished 5th, 6 semitones) is the engine of the dominant: in G7 the notes B and F resolve by contrary motion to C and E.

Why some intervals sound consonant

• Simple ratios (Pythagoras): 2:1, 3:2, 4:3 are the strongest consonances. The more complex the ratio, the more dissonant the interval.
• Beating and roughness (Helmholtz, 1863): when the harmonics of two sounds fall close together, they produce rapid beats perceived as roughness.
• Critical band (Plomp and Levelt, 1965): maximum roughness occurs when two sinusoids are about a quarter of a critical band apart. The critical band is wider in the low register: this is why a third sounds clean at C4 and muddy at C2.
• Timbre (Sethares): consonance depends on the spectrum. With inharmonic sounds (bells, gamelan) the “consonant” intervals change.
• Culture and habit: Western history has progressively accepted as consonant intervals ever higher up the harmonic series (8th and 5th in the Middle Ages, 3rd and 6th in the Renaissance, 7th and 9th in jazz). Bernstein, in The Unanswered Question (Harvard, 1973), ties this evolution to the harmonic series.

Dissonance as an engine

All tonal harmony lives on tension and resolution: dominant → tonic, 4–3 suspension, appoggiatura. In the 20th century Schoenberg speaks of the emancipation of the dissonance: dissonance no longer has to resolve. In pop and electronic music dissonance is often timbral (distortion, detune, clusters) rather than harmonic.

Consonance and production

• Distortion: a power chord (1–5–8) stays clean with fuzz because the intermodulation products fall on consonant harmonics; a distorted major triad generates beating and “mud”. This is why metal uses fifth dyads.
• Register: keep thirds and sevenths above about C3 in pads; in the sub, use only octaves and fifths.
• Detune: detuning two oscillators by a few cents is a “controlled” dissonance that gives width and movement.
• Clusters and seconds: in ambient pads a major 2nd in the mid-high register (add9, sus2) gives brightness without functional tension.

Examples in real songs

• Mozart, String Quartet K. 465 “Dissonance”: the slow introduction stacks chromatic cross-relations that scandalized listeners at the time.
• Bernard Herrmann, “The Murder” (Psycho, 1960): screeching strings in repeated high dissonances.
• The riff of “Smoke on the Water” (Deep Purple) is made of parallel fourths: distorted, they stay compact.
• “Purple Haze” (Jimi Hendrix): E7#9 stacks the major third (G#) and the augmented 9th (G): a dissonance that became a rock signature.

Exercise

1. With a sine wave, play the pairs C4–E4, C3–E3, C2–E2 and listen to how the roughness grows. Repeat with a sawtooth wave.
2. Distort (Fruity Blood Overdrive or Soundtoys Decapitator) a fifth dyad, a major triad and a minor triad: judge clarity and mud.
3. Write eight bars in which every dissonance (7th, 9th, suspended 4th) resolves by step; then rewrite them leaving the dissonances unresolved and compare the effect.

Related topics

Intervals and their inversions; The harmonic series and timbre; Non-chord tones; First-species counterpoint; Equal temperament and cents.