With synesthesia, senses overlap: Letters become colors, colors become sounds, sounds become tastes.
When a young Mary Shelley sat down to describe the first conscious moments of Frankenstein’s monster, her imagination flew to synesthesia. “I saw, felt, heard and smelt, at the same time,” she wrote, “and it was, indeed, a long time before I learned to distinguish between the operation of my various senses.” Jolted awake for the first time, in Shelley’s vision, the mind emerges from a primordial unity in which sight and sound are one.
Artists have always been sensitive to hidden links between the senses. Nabokov, for example, claimed grapheme-color synesthesia, a condition he shared with Rimbaud, whose poem Voyelles paired the primary colors to the sound of vowels. Both thought that the letter A was black. To some degree, we all rely on cross-sensual metaphors to describe the things that move us. The colors in one painting seem loud, while those in another seem muted. Some people crave black metal; others prefer the blues.
We are used to thinking that these associations are mostly subjective, but a number of scientific studies point to an underlying human palette of color and sound. It has been established, for example, that music in a major key is associated with lighter and brighter colors. Among other findings, a 2015 study in PLOS ONE discovered that subjects consistently ascribe violet hues to music they consider tender.
Theories abound for why some of these links go so deep. There may be mechanical commonalities in the way our brains encode visual and auditory information so that pairs of color and sound end up in neighboring bins. An evolutionary explanation holds that co-occurring stimuli—like the blue-green of the ocean and the crashing of waves—might over time contribute to a shared impression.
In audio engineering, basic types of noise take their names from colors. White noise, for example, contains an equal amount of sound at every frequency, just as white light contains all colors. Similar relationships exist for blue noise, brown noise, gray noise and so on.
We’ve come to think of white noise as background noise—a radio between stations or the whir of the little machine at the therapist’s office. It’s more accurate, though, to think of it as pure sonic chaos. All sounds at once, with nothing favored. True white noise is theoretical and can only be approximated.
Pink noise also aims to represent the full spectrum of sound, but falls off logarithmically as frequency increases, creating a whoosh that is richer in tone and more soothing than that of white noise. Pink noise is designed to imitate the human ear, which perceives sound in octaves. For the scientifically inclined, this means that the chunk of sound between 40 Hz and 80 Hz contains as much energy as the chunk between 10,000 Hz and 20,000 Hz, even though the latter is a far bigger chunk. This function is known as 1/f.
While white noise remains theoretical, pink noise is everywhere. The beating of the human heart, the rise and fall of ocean tides, the pulsation of light from quasars and data patterns in our DNA have all been shown to demonstrate 1/f behavior. Pink noise has been observed in financial markets and in the aggregate forms of popular music and film. Psychologists have shown it to mirror vacillations in our attention spans. If you were to extract and analyze all the curves and wobbles in a picture of a flower, you would likely find the shape of pink noise. The cubists would have loved it. Insomniacs certainly do.
It’s tempting to read into the strange ubiquity of pink noise. It has been described as a perfect mathematical balancing point between order and chaos, a type of mean state to which the universe tends. It is calming to discover that a sound custom-made for the human ear lies deep at the heart of so many inhuman things. Perhaps it was already present in the womb. Perhaps our ears themselves were shaped by it. Small wonder, then, that it can help us fall asleep.