When music is written in 6/8 time, should the bpm equal eighth notes per minute or dotted quarter notes per minute?
There's no reason why, in theory, it couldn't be either one. As a practical matter, it depends on whether the tempo is perceived to be fast or slow.
It seems that the human sense of hearing wishes to perceive tempo as falling within a range of approximately 80 to 150 beats per minute. Perhaps it's not a coincidence that this is the approximate range of human heart rate and also the spectrum of beats per minute for contemporary dance music.
Whether a sixteenth note or a whole note (or anything in between) nominally receives one beat depends on how the music is conceived and notated. If tempo strays beyond the upper or lower limits of that range, though, we instinctively attempt to reorganize what we hear into different units—subdivisions or multiples, respectively—that result in a bpm value that does fall within the "heartbeat spectrum."
Instead of hearing sixteenth notes at a rate of 200 per minute, for example, it's invariably more comfortable instead to double their durations and perceive the music as eighth notes at 100 bpm; conversely a Largo
tempo written so slow as 40 beats to the quarter note will be counted, whether consciously or unconsciously, as 80 eighth notes (or 160 sixteenth notes) per minute.
Perhaps the most familiar manifestation of this idea is in the difference between common time and cut time. Some time ago, I came across an article that explores the associated principles from an academic perspective as applied to the music of Franz Schubert. It's no longer online, unfortunately, but it can still be read here courtesy of the Wayback Machine:A Schubert Fingerprint Related to the Th... Nigel Nettheim