Last week, we learned that time signatures only function to tell us how much "real estate" exists in a measure of music. For example, 12/8 time means 12 eighth notes or their equivalent will fit in there. The time signature itself contains no clue as to how those notes group into larger pulses, although it is customary in most compound meters (3/8/, 6/8, 9/8, 12/8, etc.) to group them in threes.
Nevertheless, taking such an eyeball-only approach to the meter can be very helpful when it comes to changes in the time signature. If you have a piece of music that constantly bounces around between 5/8, 2/4, 7/16, 3/2 and the like, you may have trouble deciding which rhythmic end is up, and, frankly, that is where we often need to cut ties with our gut and go use our brains. Vague notions of speed and relation don't cut it anymore.
Many of us still feel that a 16th note must be pretty fast, and a half note pretty slow. or that a measure with four quarter notes in it must have four beats. That isn't true in our era, though several centuries ago it was thought of that way. It isn't true that the earth is the center of the universe and doesn't move, as we all found out a while back, and it isn't true that a quarter note is always slower than a 16th note. It's all relative--thanks to Einstein, and tempo markings. For example, a sixteenth note in a largo tempo is probably slower than a half note at presto. A piece in "cut time" will have two pulses in a measure even though there are four quarter notes. Things like this trip people up constantly. But today, let's worry about one thing:
How to get from 4/4 to 6/8. The two meters, one simple, and one compound, one generally consisting of four beats which split into two eighths each, the other of two beats which split into groups of three eighths, represent opposite ends of the rhythmic universe. How do you get from one to the other?
One item will have to change. Either the pulse will no longer be the same size, or the note values will have to change. In most cases, the note values remain the same. If the composer or publisher has any sympathy, they've printed a little equation at the top of the first meter change, to the effect that eighth note=eighth note. The old eighth and the new eighth is the same length. What you need to do then is make sure that your eighth note remains constant throughout each time change.
Musically, that means you need to keep a constant eighth note pulse in your mind, or in your fingers. At choir rehearsal, I often drum eighth notes through these passages because if we are singing long notes that pulse can get lost. Next we need to do a little math, in which meters are rewritten so that the bottom number is the same.
4/4 is the same as 8/8 (it isn't in every particular, but for our present purpose it sure is).
This means that the first measure contains eight eighth notes, and the second one, in 6/8, has six. Thus we would count
1 2 3 4 5 6 7 8 1 2 3 4 5 6
where every number comes at the same temporal distance from the next--you have to keep those numbers even, in other words. In terms of pulse, respecting the 4/4 meter, we are likely to count
1 and 2 and 3 and 4 and 1 and uh 2 and uh
1 and 2 and 3 and 4 and 1 2 3 4 5 6
But while either of these gives us a better sense of how the overall scheme of accents changes, it might mess with our heads when we are trying to make a smooth and even transition from one to the other, which is why I am suggesting the first method, where there are no uneven subdivisions between beats (no uhs and ands). This way we know exactly how one meter becomes the next.
So for 4/4 to 7/16, we would have to make sure we have even divisions or 16th notes.
4/4 = 16/16, thus
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 4 5 6 7
However, at this point, this method might be more tongue twisting than we would like, so if you will promise to give every sixteenth its due, I'll let you say
1 e and uh 2 e and uh 3 e and uh 4 e and uh 1 2 3 4 5 6 7
The key is to keep each one of those items exactly even so we don't speed up or slow down from one to the other, or, heaven forefend, crash completely because we can't figure out how to make the transition.
When I was learning to read, we were told to break each word down into its smallest parts, and "sound it out." This was known as "phonics." It is too bad more people are not taught rhythmic phonics. Instead, we go through life with a vague notion of how rhythms on the page are related to others. And this only gets us so far, particularly when we are confronted by a 20th or 21st century composer, who assumes we've all "gotten the memo" and are able to fluently decode the component parts of the language of rhythm rather than just relying on what seems right. The same is true, of course, for harmony since the 19th century. You can't be sure you know what the composer meant, because as music gets more advanced, it doesn't rely on stereotyped ideas like sentences you can finish because the outcome is always the same. Instead, you have to rely on problem solving skills, which often means breaking the problem area into its smallest components and then putting it together carefully, and consistently.