We know that it is not simply the number of notes (or event-rate) that defines a listeners impression of tempo. There are quite a few musical examples that have a lot of notes but that are generally judged to have a slow tempo (e.g, Javanese gamelan music). The inverse, an impression of a fast tempo caused by only a few notes, is more difficult to find (but I’m sure some of you know of an example).
One correlate of tempo is the ‘metricallity’ of the music, especially the tactus, the rate at which events pass by regularly at a moderate tempo (typically around half a second or 120 bpm). Models of beat induction try to explain this: how listeners arrive at perceiving a beat or pulse in the music. Interestingly, the most salient pulse might not be explicitly present in the musical material itself. It can be ‘induced’ by music while listening (hence the term ‘beat induction’). It’s one of those classical examples that shows that cognition influences our perception of music.
At the recent Rhythm Perception and Production Workshop (RPPW) in Dublin tempo perception was one of the topics. Subjective judgments of duration were shown, once more, to be influenced by event density. Listeners had to continue tapping after hearing a regular beat with the intervals filled with soft random clicks, so-called ‘raindrops’. Participants tapped slower when more ‘raindrops’ were inserted. Apparently they judged the regular beat to be at a slower tempo when more events occurred between the beats. This is of course a relatively artificial setup, but the effect of event or note density on tempo judgments was also shown in more musically realistic contexts. What we can conclude from this is that tempo —defined as the subjective judgment of speed— is at least a product of two things: a sense of pulse (or tactus) and event density. It still is quite a challenge for music cognition researchers to come up with a model that actually can predict and explain these tempo judgments in real music, to, for instance, be able to predict when listeners will perceive music as nice and slow.