Metric modulation is the process of changing tempo and meter without the use of accelerandos and ritardandos.  Elliot Carter is seen as the pioneer of this technique.  In metric modulation the composer would take a note value of one tempo and make it the now tempo of the following section.  For example, an eighth note of an eighth-note quintuplet becomes the quarter note value of the new tempo.  This process can be used to not only speed up tempos, but also to slow it down.

The philosophy behind this technique is that composers are able to suddenly change tempo in more of an organic way.  Metric modulation is not a technique that randomly picks rhythmic values to use as the new tempo.  The rhythmic value that becomes the new tempo is one that is often heard in the piece prior to the modulation.

Elliot Carter is considered the first composer to develop this principle and use it extensively.  His String Quartet no. 1 (1951) is the first piece to employ his fluctuating tempos more extensively.  Carter explores the possibilities of multiple tempos simultaneously.  This, in turn, gives each line more independence.  His use of metric modulation gives the work a feeling of constant mutation.

Alban Berg used a form of metric modulation in some of his earlier works.  In his Lyric Suite (1926) Berg used a large-scale temporal organization where the metronome markings of the different movements share a common numerical basis, such as 23 or 10.  These interlinked tempos allow easier movement from one metronome marking to another.

The preface for Harrison Birtwistle’s Silbury Air (1971) includes a pulse labyrinth that controls the meter and tempo.  The reason for such a device is to help ensure smooth metric modulation and focusing attention on pulse as a foreground feature.

Definition provided by Brian Bice