Changing the Equation
Have you ever played a song or a piece with a marking in the middle stating ♩ = ♪ or something similar?
If this puzzling marking left you scratching your head, you’re not alone! After all, since our days in elementary school music class, we’ve been taught that whole notes, half notes, quarter notes, eighth notes, and so on, all have distinct rhythmic values.
What this marking actually indicates is a tempo change—a very specific, proportional tempo change—known most commonly as a metric modulation (it may also be referred to as a tempo modulation). A metric modulation makes a note value from the first tempo equivalent in speed to a different note value in the second tempo.
To see why this is done and how it works, try the following exercise. First, tap eight even notes, accenting every two notes. Next, tap eight even notes at the same speed, but this time accent every four notes. Although you are tapping all these notes at the same speed, you’ll notice that the “flow” is different—the first group feels like pairs of eighth notes while the second group feels like sixteenth notes at a slower tempo and in a different meter. Here’s how to write this as a metric modulation:
By indicating that the speed of each eighth note in the first measure is equal to the speed of each sixteenth note in the second measure, we maintain some consistency even as the tempo and the rhythmic feel changes.
Now try this exercise in which we modulate to a faster tempo. First, tap 12 even notes, accenting every four notes, so that you have a sixteenth-note feel. Next, tap 12 notes at the same speed, but this time accent every three notes, so that you have a triplet feel. As the grouping of notes gets smaller, the “big beats” and the tempo get faster.
Metric modulations are used in all sorts of music, from classical to rock. There are two main reasons for using them. First, when the original and new tempos share something in common, the tempo change feels smoother and more natural. Second, metric modulations allow for a precision that can’t be achieved with a typical tempo change. For example, if a song simply indicates that a new section should be played “faster,” this is open to interpretation—but if, say, the quarter note of the new section of music is played at the same speed as the eighth note of the previous section, the proportion of the overall tempo change is exact.