Pulse
Everybody has a heartbeat, which we call a pulse. Your heart pumps blood to help deliver oxygen and nutrients to your body's cells, giving you the energy to move and think. Your heartbeat is so integral to being alive that your pulse can be detected long before birth.
Your pulse will be faster or slower, depending on your actions. If you’re sitting still or sleeping, for example, it will be significantly slower than if you had just run for five kilometres. At rest, the average heart beats between 60 and 100 beats per minute, but it can push to well over 160 beats per minute, depending on a person’s age, during strenuous activity. Within the pace range of a human heartbeat—as long as it remains steady—the periodical nature of a pulse becomes easy to perceive. To put it in practical terms, you can clap along in sync with the pulse of a human heartbeat.
That is not true of the heartbeat of the Etruscan pygmy shrew. This is the world’s tiniest mammal and boasts the fastest heartbeat of the animal kingdom, at 1500 beats per minute, or 25 beats per second. The duration between each impulse is so quick that it becomes impossible to perceive them as discrete events, and instead, we start to hear the heartbeats as a tone. Playing the lowest note on a piano (A0) resonates at 27.5 Hz (or 27.5 wobbles of the string per second). So, if the racing heartbeat of the shrew were made loud enough for human ears to perceive, those 25 beats per second would be perceived as a pitch slightly lower than the lowest note on a piano.
Good luck trying to clap along to a pulse so fast it is heard as a pitch.
An analogy from the visual arts can help illustrate the seamless relationship between an increasingly fast pulse and pitch. Georges Seurat’s painting A Sunday on La Grande Jatte (1884) depicts a crowd enjoying the park on the banks of the Seine River. But a closer inspection reveals it’s merely an illusion. Seurat used pointillism, a style in which thousands of dots of discreet colour are painted close to one another. At a distance, the colours blend in the mind's eye to create the image; while viewing the painting extremely close, the image can no longer be discerned, but the dots can be seen as separate colours. Just as our eyes consolidate separate dots of colour to form an image, so do our ears consolidate the discrete impulses of a beat into a pitch as it accelerates.
In Giorgio by Moroder, a sweeping piece of music from Daft Punk’s album Random Access Memories, we hear the opposite effect: a pitch decelerating/transforming into a pulse. It features a monologue by Italian electronic musician Giorgio Moroder, who speaks of his musical journey, including using synthesizers and click tracks to invent new sounds and genres of music throughout the 1960s and 1970s. In the last twenty seconds of the track, after an extended climactic passage of music, a continually evolving tone emerges into the foreground. As it does, the sound becomes more jagged or gritty, and at some undefinable point, that gritty texture becomes emphatically discrete. The piece ends as the tone morphs, through continual declaration, into the unadorned pulse of a click track.
