Harmonic rhythm is the speed at which the harmony changes. It is different from the surface rhythmic activity: For instance, in this excerpt from Steve Reich’s Music for Large Ensemble, the there is a lot of surface rhythmic motion; however, the harmony is holding steady.
On the other hand, in this excerpt, there is less surface rhythmic activity, but the harmony is constantly changing.
In this excerpt from Ludwig van Beethoven’s Symphony No. 3, the same progression is played twice—only the harmonic rhythm changes.
The pacing of harmonic rhythm is a key component of our subjective sense of time passing: When the harmonic rhythm is slow, time feels suspended; when the harmony is propelled forward, time seems to push ahead. In this excerpt from Richard Wagner’s Dawn and Siegfried’s Rhine Journey, the harmonic rhythm doubles in speed.
In this excerpt from the Finale of Sergei Rachmaninoff’s Piano Concerto No. 3, the harmonic rhythm slows when the piano enters.
In this excerpt from Giuseppe Verdi’s Rigoletto, the harmonic rhythm slows at the end of each phrase. The second time, the harmony stops moving altogether and the voice performs an elaborate solo.
A slowing of harmonic rhythm is used to great dramatic effect in Richard Strauss’ opera Der Rosenkavalier. The young Sophie is betrothed to the loathsome Baron Ochs. By tradition, the Baron sends a messenger to present a silver rose to his fiancée. As Sophie’s household bustles frantically about, preparing for the presentation of the rose; the harmony moves swiftly. At the moment that the handsome messenger appears and Sophie and he see each other for the first time, Strauss creates the illusion that “time stops” by arresting the harmonic rhythm.
In Thomas Mann’s novel The Magic Mountain, the patients at the tuberculosis clinic have a choice: If they confine themselves to bed rest, they have a chance to recover; if they become excited and their pulse quickens, they risk death. Each choice creates a different experience of time: With bed rest, time seems to pass very slowly; but, in retrospect, it seems to have gone by “in a blink of an eye,” because the days all flow together. On the other hand, being active makes time pass more quickly; but, looking back, time seems to have “thickened” and slowed down, because each day is more jam-packed and memorable. So it is with harmony: very slow harmonic rhythm is similar to bed rest; fast harmonic rhythm is similar to greater activity. Just as daily life has ebbs and flows of psychological time, so most classical works have fluctuating harmonic rhythms.