Superimposed Metric Modulation
The musical examples that I have contributed here illustrate a sort of temporary change in time signature, or the illusion of the tempo shifting momentarily when in fact it is not. I have chosen to call this rhythmic occurance "Superimposed Metric Modulation" for lack of better terminology, for that is exactly what it is. First of all, I say "superimposed" because one pulse, or time feel, is layered onto another already existing pulse, and because the layered pulse creates an effect of the time changing, I say "metric modulation". But, because the original time base remains intact (the tempo doesn't shift), and the second or layered pulse does not take the place of the already existing pulse, I call it "Superimposed Metric Modulation".
This rhythmic device, when used musically, as with anything, can be very effective during certain places in a musical performance, and can create a very cleverly deceptive shift in feeling, such as in a fade out or out chorus of a tune or during a specific moment in accompanying someone's solo.
Let's take a look at the following examples.
In this example, I have used the quarter note triplet as the vehicle to transport the feeling of one tempo to another. Because of the feel in bar 3, it sounds as if the so-called "new" quarter note is a half-note triplet, not a quarter-note triplet. The accented notes represent quarter notes, as you can see. Also, I have reversed the imaginary tempo shift in retrograde to illustrate the sound of one tempo going to another and back to the original again. This helps in being better able to hear exactly what happens; plus, once you go into a change like that, you have to be able to get back out of it again.
Bars 1, 2 and 3 repeat to show a smoother, gradual change and also to get a chance to groove a little for a moment, so that everything makes more sense.
Here is another example of a modulation based on a dotted eighth-note. The dotted eigth-note in this instance creates a 4 against 3 superimposition, which is the basis of the illusory modulation:
In conclusion, I hope I have shed some rhythmical light on something that can be fun when used properly. And, if you find that you have the place where you can use something like this, above all else, be musical!