Interpretating time signature change
I'm in a situation where there are various groups of singers and musicians who will be performing a song together, but who are not rehearsing together until the day of the performance. We're doing a song that none of us (or at least none that I know of) have heard before. So we're supposed to be closely following the sheet music. The song starts out in 3/4 time. It then switches to 5/4 and back to 3/4 a few times.
At the beginning of the sheet music, the tempo is 104 beats per minute. Other than a ritard at the end, there are no instructions to change tempo. Would the number of beats per minute remain the same and therefore the 5/4 measures be longer than the 3/4 measures because there are more beats in each measure? Or would the 5/4 measures take up the same amount of time as the 3/4 measures and therefore the beats per minute increase because there are now 5 beats played in the same amount of time as 3 beats?
I turns out that at least two of the groups have different interpretations of this. Which is correct?
If the unit of the beat stays the same - that's the bottom number of the time signature - then in the absence of any other instructions, the beat stays the same. So your 5/4 measures will be 25% longer than the 4/4 ones.
It gets more confusing when you change beat units - for example, if you go from 4/4 to 7/8. This can be interpreted as keeping the beat the same... so if you were at 100 bpm in quarter notes, you'll have 100 eighth notes per minute after the change... or it can be interpreted as keeping the relative notation the same (so you'd have 200 eighth notes in both signatures).
Best practice for music copyists and publishers is to include an additional marking to clear things up. That's usually done with a printed note, an equal sign, and another printed note directly over the signature change - so you'd have (quarter note) = (quarter note) or (quarter note) = (eighth note).
Unfortunately, not everyone follows best practice. Sometimes you have to guess.
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Thanks, my interpretation was the same as yours. I'm not in charge of any of this, just playing guitar, but I really wanted to know what the right answer was. Could be I'll get to the rehearsal with all of the groups and the master director will do it my way. Or not. I'm planning to do my best to follow him and not try to discuss music theory when everyone's there. I did have a tough time playing with one of the choirs involved two days ago though. I had really worked on changing back and forth between the two time signatures so that I didn't start playing a different tempo.
Just to follow up on this, the master director was excellent and enthusiastic. He understood the markings on the sheet music. He was even nice enough to ask if I wanted to run through the song once with him on piano while everyone took a break. I had arrived and got set up during the break because I was only involved in one song. By the time we finished the morning rehearsal, everyone had caught on that the measures were longer when the music switched to 5/4 time. The performance was in the afternoon. There were five choirs. Three of them did some songs by themselves and then all of the choirs sang three songs together. The one that I played was the very last one of the show. Because I've been in choruses/choirs since I was in grade school, I enjoyed listening to everything and watching the technique of the three directors and the master director. It turned out to be a very satisfying day.