When music is written in 6/8 time, should the bpm equal eighth notes per minute or dotted quarter notes per minute?

Oh boy...

Not sure on this one (thus the 'oh boy' which is aimed at me).

Normally, I would imagine it should refer to dotted quarter, but it really depends on the tempo. I mean if you are to get a dotted quarter=20 then you are probably off by having eighth=60 or something...

Still I would imagine that if there is a bmp, then there is also the 'measure' by which this applies.

I doubt this helps, but here's at least another post asking the same question, since I don't know what the answer is.

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

Your whole post was great, but I found this particularly interesting. Is this just common knowledge many people know that I haven't come across yet, or did you read this somewhere?

Thank you! I'm a veteran of the disco era (and experimented with turntablism back in the day), so I was aware of the connection through that genre; I suspect most any disc jockey would be as well.

I feel pretty certain that I've read more general references to this as well, and I thought I would find them when I dug up the article that I linked to. I searched it for the word "heart" before I posted, and no dice. It had to have been elsewhere then.

Chopinist
Like Orange Soda King, I also found your 80-150bpm comfort zone comment very interesting.
I can relate to it. At first when I set my metronome to a low tempo such as 40, I was playing twice as fast as I should have, because I subconsciously treated the quarter notes as eighth notes.

There was a post a while ago here about heart rate and the comfort or discomfort of playing at a certain tempo. At the time I tried to find some literature to see if the theory was backed up by research - but no such luck. The closest I could find was research that found that an individual's physiological state could be changed by listening to music - including heart rate and blood pressure.

Back to the original question:

6/8 time is usually a compound time - two beats with three pulses in each beat. This would be given a metronome mark using a dotted quarter. You will often see that the eighth notes are beamed in groups of three in this case.

It can be used as six beats or three, but I can't remember any examples where 6/8 is used in this way.

Rich

Thank you. I had in mind some of the slow rock songs in 6/8 from the 50's that you could slow dance to. They are accented on 1 and 4. 1,2,3, 4,5,6

You could think of it as 2/4 with two sets of triplets per measure.