Tempo is a deceptively tricky musical concept. On the face of it, it seems straight forward enough. Tempo is measured as the number of beats occurring in one minute given a steady rate, and that beat can be equal to any note duration, such as eighth, quarter, half, or whole note. There are tempo markings that broadly indicate that the tempo should be lively, very fast, moderately fast, moderate, slow, or very slow. There are the more precise metronome markings that indicate a precise number of beats per minute, and the note value that will be used as the unit of measurement. All of this makes tempo uncomplicated and clear to performers, because as the musician plays or sings, they are forming rhythms over a concrete pace of pulses that coincides with the instruction in the printed score or, as in the case of dance music, of the standard convention.
Tempo for the listener is more complicated. The listener does not necessarily know what the unit of pulse is, and so must match a pulse rate with the rhythm patterns they are perceiving. So while the performer may be playing a passage of 32nd notes at a slow 8th note tempo (a common situation in classical slow concerto or sonata movements), those 32nd notes are going by rapidly for the listener, who might organize the music into beats of 16th notes, making the tempo faster than for the performer, who is measuring those same 32nd notes in slowly moving 8th notes. In this case, it would be tempting to say that the tempo (measured in 8th notes) is slow, while the rhythm (as perceived by the listener) is fast. But the difference is not between tempo and rhythm, but instead between the unit used to measure (and perceive) rhythm. The same music can be said to be fast or slow depending on what note value is being used as the unit of the pulse.
A good example of this is the opening of the 4th movement of Mozart’s Symphony no. 41 in C major (Jupiter). The tempo marking is motto allegro, and the pulse is generally around 120 beats per minute. Yet the first four measures are whole notes, and so one note progresses to the next slowly, even as one perceives the pulse to be fast, in contrast to the accompanying eighth notes, which are flying four times faster than the fast pulse, almost to fast to track. Yet if we listen to the same music and track two measures as one beat, though many notes pass by, the tempo now seems extremely slow. It is all in what is perceived as the unit of pulse.
A second factor in the perception of tempo is meter. Meter is part of the rhythmic structure of music, and influences how listeners perceive the unit of pulse. In the Mozart example just cited, the tempo is only perceived as fast if the meter is perceived as alla breve. If the meter were perceived as two or four whole notes per measure, then the tempo is perceived as Andante at most. Meter defines how the listener groups note durations into patterns that can be divided and subdivided into equal parts. There are times when musicians will use a faster, subdivided tempo to improve accuracy, while they intend the audience to perceive a slower, unsubdivided tempo. The introduction to Dvorak’s symphony no. 9 (From the New World) comes to mind. Notice how the conductor conducts eighth notes an an Allegro tempo, while the music, when listen to without following the conductor, is perceived as being Adagio, as Dvorak intended.
I began by asking the question, “can rhythms be fast?” We are now in a position to answer that question by saying no, it cannot. The reason is that tempo is a measurement of degrees of fastness measured in beats per minute, whereas rhythms are a relationship between a beat and a duration which is shorter, equal to, or longer than one beat. Rhythms as they are perceived by a listener are not individual notes, but patterns of note durations perceived as patterns by their relationship to a beat, regardless of tempo. In other words, the rhythm pattern of one quarter note, two eighth notes, two eighth notes again, and one more quarter note will be heard as such at any tempo as long as the quarter note is used as the unit of pulse. The notes can be made faster by increasing the tempo. The first sound is equal to a beat, the next four sounds are divisions of the beat into two equal parts, and the last sound is again equal to a beat. We cannot say the rhythm is faster or slower, because the fastness or slowness is entirely dependent on the tempo, the speed of the beats, not the durations, which set the interval of time from the end of one note to the beginning of the next.
While it is true that we arrive at the next note sooner if the last note was a sixteenth note than if it were a quarter note, the reason we arrive sooner is a shorter note duration, not a faster tempo. The tempo, which is the measurement of fastness, has not increased, the durations of notes, the measure of rhythm, has decreased. There is more activity within the beat divided into four equal parts than within the beat divided into two equal parts, but that is not an indicator of faster, of tempo, but of duration, of rhythm. Fast does not exist apart from a reference to pulse. Fast is a relative concept that is not dependent on duration, but on pulse. A flourish of 32nd notes is a group of very short durations, not very fast notes. Notes are not fast or slow apart from the pulse to which they are sounded, only the pulse itself can be considered fast or slow.