Sound heard by the human ear and identified as a discernable tone has a particular quality, its frequency, measured in Hertz (Hz) or Cycles Per Second. Although this is a useful definition in a purely academic sense it doesn’t bear witness to what actually happens when we (for instance) pluck or draw a bow across a string.
A pure sine wave generated electronically would have a pure wavelength but it would always sound dull and flat. The distinctive voice and vigour of a violin string or a bassoon is a result of a combination of frequencies being sounded together along with the particular tonal characteristics of the instrument in question, the attack, sustain and decay of the sounded note. The combination of frequencies mentioned are known as overtones and they are what give the sound life. When the overtones are a multiple of the initial frequency (for instance, the complete string length (known as the fundamental)) they are known as “Harmonics”.
Looking at the image above we can see that we can break down the fundamental into a series of ever-smaller harmonics, many of which can be exploited to generate specific aural effects. If we take a frequency, 440 Hz (the A above middle C (A4)) and exactly double it bringing the frequency to 880 Hz we would hear A5, exactly an octave above A4. The octave is known as a perfect interval (along with the fourth and the fifth divisions of the Major scale) and is pleasing to the ear from both a musical and a mathematical point of view. This is known as “Consonance”, from the Latin word for “Sounding Together”, Consonare.
When the second note of an interval generates a fundamental that is a multiple of the fundamental in the first note, the frequencies move in synchronisation. When the frequencies are out of synchronisation (such as with a Major 2nd or a Major 7th) we subconsciously her a “beating” between the two notes which is formed by the harmonics working against each other. You can hear this beating by very slightly detuning one guitar string while playing an octave or fifth. This is known as “Dissonance”, the direct opposite of Consonance. The recognition of dissonance is also partly cultural in that the Western listener has grown accustomed to a certain series of tones and their relationships. These relationships can be very different in other musical cultures such as in the Javanese tuning systems of sléndro and pélog which purposely slightly detune one of a pair of instruments, producing the “Ombak” effect, a beating tone between the two fundamental tones.
In music as in everything else that’s useful or interesting we require contrast, a darkness, harshness or unease to provide interest in the subject and to provide a foundation for the successful resolution of the contrast which brings a pleasing release. A piece of music, a painting or a dish of food that is uniformly concordant, completely written in octave and fifth, painted with all yellow or just tasting of butter, soon becomes dull and we long for contrast and change. In music, dissonance helps provide this contrast.
Consonant intervals sound at ease and complete whereas dissonant intervals lead the ear on as it stretches for a satisfying resolution, and this can be used to drive music forward. Taken further, a composer might resolve one dissonance to a second chord with a consonant root but which itself has a dissonant interval, creating great tension in the ear of the listener. Wagner was a great one for this.
The story of the constructive use of dissonance blossoms from the history of harmony in Western music which itself has its roots in the history of polyphony. Growing out of Medieval plainchant, three forms of basic polyphony were developed by ecclesiastical composers, Parallel, Free and Melismatic Organum.
With the full development of Melismatic Organum under Léonin and Pérotin at the cathedral of Notre Dame in Paris we are on the verge of harmony proper, moving away from the bounds of the modal system and requiring the use of sharpened or flattened notes to avoid adverse dissonance in the flow of melody. Although the technique of flattening a note when required was developed in the ninth century during the period of parallel organum, it was only used when singing a fourth or a fifth above B, lowering the B by a semitone to avoid the discomfort of the interval.
By the beginning of the Renaissance period tonal polyphony was well established and composers were driven by the emotions that music could generate, the tension and release of dissonance and consonance being a major ingredient.
Baroque composers use dissonance liberally to enhance the colour of their compositions. The very first prelude and fugue in Bach’s Well Tempered Clavier delivers a major 2nd in the second bar when the left hand moves the E down to a D. Taken out of context this would be harshly dissonant as the second triad played, D Major, doesn’t accept a C, but the following arpeggio resolves this nicely by moving the offending C on the left hand down to B3. This sequence of preparation (consonance), suspension (dissonance) and resolution (consonance) is still very much in use today.
Throughout the Baroque, Classical and Romantic periods a more subtle use of dissonance was in evidence with the use of pedal notes (or pedal points). By constantly referring to one note as the melody itself moves around we hear a fast-moving series of tension/release statements.
Moving forward slightly we encounter the dissonance of Richard Wagner, arguably the first composer to explore dissonance as a compositional form in its own right. From the very second bar in the opera “Tristan und Isolde” (at twelve seconds in this video) we hear a heavily dissonant chord which immediately invokes a tension in the listener. This moment in music has become famous as the signpost for what would become twentieth century atonality, known as the “Tristan Chord”. Wagner went on to give the listener almost four hours of dissonant opera before finally drawing to a consonant resolution in the final few bars.
Stepping forward again we encounter Igor Stravinsky and his groundbreaking piece “The Rite of Spring”. Taking his lead from Wagner, Arnold Schoenberg and Anton Webern, Stravinsky’s ballet was so divisive in its use of dissonance that it famously caused a riot in the theatre upon its première. Labelled “a laborious and puerile barbarity” by one newspaper, this was the beginning of the atonal movement of the twentieth century and the final release from the shackles of tonality.