Imagine a film director asking a composer to create a piece of music to underline the mood of a certain scene. Usually, the director communicates in words what the nature of the music to compose should be (romantic, tense, cheerful or melancholy, etc…) and above all the duration of the scene.
Let’s assume that the music has to highlight a romantic scene lasting 48 seconds. Being a romantic scene, the director requires a moderate pace. In this regard, the composer refers to a scheme that provides reference ranges organized by rate:
Choosing, for example, a rate of 90 bpm means entering the range called ‘Moderato’, which means a moderate tempo.
Well, if we follow the request of our hypothetical director who prefers a moderate tempo to musically describe the scene, then the composer will choose a bpm rate that fits in the range between 80 and 100 bpm. Let’s assume that, in agreement with the director, he has chosen 80 bpm and a classic 4/4 time signature.
We have the following data at our disposal:
Since the composer will have to write the music to be played by several musicians, he will do so in the usual way, on a staff divided into bars which will follow the time signatureset at the beginning of the piece (in this case, 4/4). Here’s the question, then: how many bars will the composer have to write to get as close as possible to the duration of the scene in the film?
Let’s consider that 60 bpm coincides with 60 s and that therefore in this case 1 single beat of the metronome coincides with 1 s, therefore:
60 (s): 60 (bpm) = 1 s/bpm
This simple mathematical formula allows us to understand the value in seconds of each single bpm. For example, if we choose 80 bpm and apply the formula we obtain that:
60 (s): 80 (bpm) = 0.75 (s)
This means that, if we choose 80 bpm, we know exactly that the duration of each single beat is equal to 0.75 s. At this point, knowing the value of each beat, we also know the value of each quarter note, since it coincides precisely with each beat of the metronome.
Then we can easily calculate the duration of each single 4/4 bar which, as we stated, is the time signature that the composer has chosen for the piece.
We just need to multiply the value in seconds of each beat by four beats since the total value of the notes within each bar must be at most 4/4:
0.75 x 4 = 3 s
We now know that every single bar of the song lasts 3 s. At this point, knowing that the total duration of the film scene must be 48 s, we just need to do the following:
48 s: 3 s = 16 (bars)
The composer will compose 16 bars in 4/4 time at a speed of 80 bpm.
The duration of a scene does not always coincide perfectly with a whole number of bars, but in this case and in reality, the composer will remedy this by altering the tempo (slowing down or accelerating) so as to adapt perfectly to the duration of the scene.
Try to set up a composing task knowing that the director and the composer have decided on these conditions:
Duration of the scene: 1 m 30 s
Metronome rate: Allegretto/Allegro (your choice within the speed range)
Time signature: 3/4
If you get decimals, you can round the values up or down, remembering music allows for variations in tempo (acceleration or deceleration) in order to align perfectly with the duration of the scene.