What is metric modulation?
Metric modulation is the modulation of the original metre, either upwards to a faster tempo or downwards to a slower tempo, using a subdivision of the original pulse to decide the speed of the modulation.
The mechanics of the process are as follows: 1. Subdivide the original pulse into smaller or larger pulses. 2. Reorganise these pulses into regular groupings, so that the impression of a new tempo or metre is created. This "new" tempo maintains a strict relationship to the original pulse at all times. This type of metric modulation is a very common one, and is probably one that you have done yourself at times. A simple example of metric modulation is the technique of "double time" and "half time". When we go to double or half time we maintain a strict relationship to the original pulse and the changes go by at the original speed. This is metric modulation. In double time the modulation speeds up, and in half time the modulation slows down. The technique of metric modulation can be used extremely effectively to create tension and release, and to provide variety and contrast within a piece.
From the book Creative Rhythmic Concepts for Jazz Improvisation by Ronan Guilfoyle (© 2000 Newpark Music)