Guitar School magazine (January 1992)
By Troy Stetina
During the 16th century an elaborate system called musica ficta (falsified music) was created in which dissonances (notes that clash) and other "crimes against the ear" were avoided by altering the offending notes one half-step. The early church even referred to the extremely dissonant tritone interval as the 'diabolus in musica' (literally, the devil in music). The tritone's ugly turbulence was seen as the incarnation of evil, and represented the seething chaos lying beneath our mortal reality.
But times have changed. After more than four centuries of musical progress, these "crimes against the ear" sound fairly interesting to the modern ear. Today, dissonance is no longer relegated to the confines of Hades--it's an accepted musical tool, used to create harmonic tension. And since dissonance creates tension, it's not at all surprising that it's found at the core of today's most tense and aggressive music, thrash metal.
Dissonance is easy to hear. Figure 1, below, demonstrates a perfect consonance (notes that blend together well); and then, at the other end of the spectrum, a strong dissonance in this case, the tritone. Listen for its tell-tale unsettledness. Distortion will accentuate this aspect even more. Figure 2, below, shows all the intervals within an octave ('simple' intervals). I've indicated which ones are dissonances and which ones are consonances.
Figures 1 and 2
Dissonances can reveal themselves in two different ways: harmonically or melodically. If dissonant notes ring together, it's called harmonic dissonance. That's what you heard in Figure 1. But if you play the notes in sequence, one after another, it creates a melodic dissonance. In this case, there isn't any turbulent clashing of notes. Instead, you hear an odd or disjointed-sounding leap between the notes. You'll likely find this device lurking within the more interesting riffs you meet.
Figure 3, from "All Things Repulsive," and Figure 4, from "Bug Guts," are both excerpted from the Thrash Guitar Method to demonstrate each type of dissonance in action. An asterisk with an "M" or an "H" directly below it denotes a case of melodic or harmonic dissonance, respectively. Figure 3 abounds with that "diabolus in musica," the diminished fifth. (E to Bb in this case.) Figure 4 uses both types of dissonance.
These examples should help put you on track to creating vicious riffs of your own. But of course we've just barely scratched the surface. Try mixing these dissonances with some ear-twisting chromatics, atonal ideas and a few unusual accents. You'll find these concepts covered in more detail in the Thrash Guitar Method. Good luck with your playing!
For more, see the Thrash Guitar Method.
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