I was going to write a blog post about what are called “irrational meters” (time signatures with denominators that are not powers of 2) – then I re-read a fantastic post by Helen Bledsoe, flutist for MusikFabrik (among others), and realized I should just link to her! She very lucidly explains the mathematical workings of Ferneyhough-style rhythm, complete with “irrational meters”, which really pose very little additional challenge, if you know how to interpret them.
The most basic point to remember about these meters is that the denominator, just like in more familiar time signatures, indicates the number of notes it will take to fill a whole note. 4/8 indicates 4 of something it takes 8 of to fill a whole note (namely eighth notes). A denominator of 5 would indicate that quarter note quintuplets are the basic unit of the bar, and the numerator, as in familiar time signatures, indicates the number of units in the bar. Thus, 3/5 would be a bar of 3 quarter note quintuplets! Of course, you can, instead, treat these changes of denominator as metric modulations and tempo changes – but I’ll leave some of the specifics to Ms. Bledsoe’s lucid explanations!
Head on over to her blog, Flutin’ High, for the full post!