In music theory, harmonic rhythm, also known as harmonic tempo is the rate at which the chords change (or progress) in a musical composition, in relation to the rate of notes. Thus a passage in common time with a stream of sixteenth notes and chord changes every measure has a slow harmonic rhythm and a fast surface or "musical" rhythm (64 notes per chord change), while a piece with a trickle of half notes and chord changes twice a measure has a fast harmonic rhythm and a slow surface rhythm (1 note per chord change).
According to Joseph Swain (2002 p.4) harmonic rhythm "is simply that perception of rhythm that depends on changes in aspects of harmony." According to Walter Piston (1944), "the rhythmic life contributed to music by means of the underlying changes of harmony. The pattern of the harmonic rhythm of a given piece of music, derived by noting the root changes as they occur, reveals important and distinctive features affecting the style and texture."
Harmonic rhythm is rarely notated or described exactly;[clarification needed] rather, analysts compare the overall pace of harmonic rhythm from one piece to another, or the amount of variation of harmonic rhythm within a piece. For example, a key stylistic difference between Baroque music and Classical-period music is that the latter exhibits much more variety of harmonic rhythm, even though the harmony itself is less complex.
The piece shown above, "Prelude no. 1 in C major" Play (help·info) (BWV 846) from J. S. Bach's Well-Tempered Clavier, illustrates a steady harmonic rhythm of one chord change per measure, although the melodic rhythm is much faster.
- "Harmonic Rhythm", Bach-Cantatas.com. Examples of different harmonic rhythms from Bach given. Accessed June 2013.