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I'm learning basic music theory, and I have a naive question:

Does a song with "100 quarter note beats per minute" have a pulse that is slower than one with "80 half note beats per minute"? Or they are incomparable?

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    It might help to read music.stackexchange.com/q/133783/78419 Feb 9 at 20:32
  • Consider a song written in half notes played at a tempo of quarter note = 100. That means you play 50 notes every minute. Now consider another song written in eighth notes played at quarter note = 100. That means 200 notes every minute, so it sounds four times faster even though the tempo is nominally the same. Beats per minute by itself doesn't tell you much.
    – phoog
    Feb 10 at 0:27
  • @phoog: Hi, it seems to me that your answer contradicts the answer of Aaron below... Feb 10 at 23:24
  • It does. Aaron's answer is incomplete if not simply incorrect and I have downvoted it as a consequence.
    – phoog
    Feb 12 at 15:31
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    "Why do we need the tempo while it doesn't tell us anything precise?": It tells you something very precise, which is exactly how fast a given piece of music is. It's just that it's not the only factor determining the speed of a piece of music. The other factors are generally fixed for any single piece of music but can vary from one to the other, which means that tempo alone can't tell you which piece of music is faster. Consider: one car's engine is rotating 1600 times per minute and the other's 2000 times. Which car is going faster? It depends on the gear, the size of the tires, etc.
    – phoog
    Feb 13 at 1:45

3 Answers 3

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First point to understand: any kind of note can be the beat. If you know about time signatures, you know that the bottom number could be a 4 (meaning a quarter note is a beat) or 2 (meaning a half note is a beat). It can get complicated (e.g. 6/8), but it's a starting point.

Next thing to understand: note durations don't tell you the tempo, from one piece to another. Say you've got these two pieces:

enter image description here

... and...

enter image description here

In each piece, you know the quarter notes are half as long as the half note. But you don't know whether a quarter note from the first piece is the same speed as a quarter note from the second. In fact, I could write the first piece in 4/2, like this:

enter image description here

... and it could actually sound just like the first example, as long as these half note beats are the same BPM as the first example's quarter note beats.

So what if the composer wants one piece to be faster? They could just write "fast," but how fast is fast, really? That's where metronome markings in BPM come in handy.

Third point: You don't have to set the metronome (or, for a conductor, wave the baton) to match the bottom number of the time signature. If one of the pieces above is meant to be very, very slow, like say if the quarter note in the first example is meant to be 40 bpm, it's hard to play along with a metronome that is clicking that infrequently. So I might just set the metronome to double that number, 80, and call it eighth notes, and give two clicks for every quarter note. So when you give a metronome marking for a piece, instead of just saying "it's 40," it's helpful to say what length of note that 40 represents.

Let's look at that "complicated" 6/8 that we set aside. If we have this:

enter image description here

... then a beginning understanding of time signature tells us that there should be six beats in the measure. But this is often actually how we write music that has two beats, but with three smaller notes in each beat (like triplets). If this piece goes very fast, the eighth notes might be much faster than we want a metronome or a conductor to do. The real "beat" would be three eighth notes long, or a dotted quarter note, and the metronome marking might look like:

enter image description here

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No. Beats per minute is literal: that is, 100 BPM is faster than 80 BPM, regardless how those beats are represented in notation. Whether those beats are represented as quarter notes or half notes is purely a notational issue.

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  • ... indeed, though peoples' perceptions of "speed" can certainly be influenced (rightly or not) by the number of quarter notes in a minute, since, we have to admit, that's a popular default for thinking about the "speed" of a piece. :) Feb 9 at 20:16
  • could you please tell me what is the purpose of specify what a beat worths ? for example 100 quarter notes beats per minute ? Many thanks!! Feb 9 at 20:19
  • @InTheSearchForKnowledge I'm not sure I understand your question. Are you asking why BPM is specified for a piece? Why different note-types might be specified as representing the beat? Something else?
    – Aaron
    Feb 9 at 21:07
  • Ah, i mean that you have said that 100 BPM means that 100 beats per minute no matter what is a beat. So I wonder why should we specify that a beat worth (for example) a quarter note. Can you explain it to me ? Thanks! Feb 9 at 21:20
  • @InTheSearchForKnowledge Try starting with what difference beat value makes in time signature and the various linked posts.
    – Aaron
    Feb 9 at 21:37
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Does a song with "100 quarter note beats per minute" have a pulse that is slower than one with "80 half note beats per minute"?

Are you making a distinction between 'pulse' and 'beat'? If not, you've answered your own question. In the first, the 'beat' is a quarter note, in the second it's a half note. 100 beats per minute is faster than 80 beats per minute. Doesn't matter how you choose to notate the beats.

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