instantly negative response from most people? Like retching, fingernails on a chalkboard may remind us of something bad; we’re simply not conscious of it.
“The automatic, almost visceral reaction to this sound makes us wonder whether it mimics some naturally occurring, innately aversive event,” Blake and his colleagues wrote. The researchers went to the library and thumbed through books filled with pictures of frequency analyses of sounds to see whether any looked like the pictures that Blake generated of the scraping sound. “One that jumped out to us was primate warning cries,” Blake says.
Specifically, the garden tool being scraped across slate looked a lot like the cries that some species of monkeys make to signal a predator.
Blake wondered—although he and his colleagues couldn’t prove it—whether the widespread aversive reaction to fingernails on a chalkboard is evolutionary, a holdover from the days when a primate screech meant serious trouble. “Regardless of this auditory event’s original functional significance, the human brain obviously still registers a strong vestigial response to this chilling sound,” the researchers concluded.
How did the research community react to the suggestion that the sound of fingernails on a chalkboard activates an evolutionarily encoded primal fear? “They largely ignored it,” Blake says. Yet it caught some people’s attention. In 2006, two decades after being published, the paper received its due.{16} Blake and his coauthors were awarded an Ig Nobel Prize, honoring science’s strangest discoveries.
Making a credible observation is easier than proving why something is the way it is. Knowing
Cotton-top tamarins (
“Of course, with a monkey you can’t just ask them whether they like something,” McDermott says. “You have to come up with another method to measure that.” The solution was a maze. While he was at Harvard, McDermott and his colleagues built a V-shaped wooden frame with two branches. A speaker was placed at the end of each branch. The monkey is free to run around. When the tamarin is in the left branch of the maze, the left speaker plays one sound. When the monkey moves over to the right branch, the left speaker turns off and the right speaker turns on with a different sound. “The idea was that the animal controlled what it heard by virtue of its position in this maze,” McDermott says. No food treats were given. The only reward was a change in sound.
The point of McDermott’s study was to ask whether monkeys show a preference for consonant chords over dissonant chords. Consonance is derived from the Latin
One side of the maze piped out two-note consonant chords: an octave, “c” and “c”; a fifth, “c” and “g”, for example; a fourth, “c” and “f.” The other side played dissonant chords—minor seconds, tritones, and minor ninths. The monkeys showed no preferences between consonant and dissonant chords. Harvard undergrads, on the other hand, showed a clear preference for consonant sounds.
Even though the purpose of the experiment was to learn about music, McDermott was also curious whether tamarins would show a preference against the sound of fingernails on a chalkboard. He played the sound of fingernails on one branch and white noise from the other branch.
The cotton-top tamarins showed no significant preference for the white noise versus the scraping. They did, however, when given a volume option, choose to spend more time on the side of the maze with lower-volume white noise compared with higher-volume white noise.
This finding tempers Blake’s primate warning call theory. The tamarins didn’t seem to dislike the fingernail sound—meaning, at least, that these primates didn’t show a preference against it. McDermott offers another explanation for why we don’t like the sound of fingernails on a chalkboard. It’s rough on the ears. Roughness— remember from the buzzing fly or abrupt English elocution—is a technical term in acoustics that has to do with the amount of volume, or amplitude, modulation per second.
If you were to watch fingernails or garden tools running down slate with a high-speed camera, says David Huron, you would understand. “Your fingernail is grabbing the surface and then as you continue to move your hand down, it’s stuck to the board, and then all of a sudden it will slip and jump to the next position.” It’s called, Huron says, a “stick-slip sound production.” This produces a highly unpredictable, varied sound. “You get periods where it almost sounds like a whistle. And then there are periods where it becomes very rough—and those are the ones that tend to make people cringe the most,” says Josh McDermott.
Guitar players know roughness as beating. When you tune a guitar, you typically fret a string on its fifth fret and play the next string down. When the two notes are slightly out of tune, you hear the volume go up and down— kind of like wowuuuwowuuu. Those are the individual peaks and valleys in the waveform. The beating slows as the pitches approach each other and stops when the two pitches are perfectly in synch. “People don’t usually call that rough,” McDermott says. “Those are just beats.” As the pitches move farther apart, the beat frequency increases, and the sound will start to sound rough. When the notes are more than 20 Hz apart in frequency, McDermott says, you can’t hear the wowuuuwowuuu, and that’s roughness. Then when the beating goes up to 75 to 100 Hz, the roughness goes away—it’s too fast to resolve as rough.
Roughness has been scientifically proved to be annoying.{17} Car manufacturers, for instance, have done many studies on how to minimize annoying sounds generated by cars. “One of the biggest factors determining whether a sound will be annoying or not annoying is how smooth the amplitude envelope is,” McDermott says. The envelope is the shape of the amplitude of a sound over time. A rough envelope doesn’t look much like an envelope at all. It looks more like an accordion. If it has that accordion shape—if the volume goes up and down rapidly—the sound tends to annoy people. This might be a factor in why the fingernails- on-chalkboard sound is so annoying: it’s quite rough.
As for why we don’t like roughness, there’s no clear answer. McDermott says, “It’s a bit harder to say why sound roughness is considered unpleasant—as far as we know, it is not harmful to the ears.” David Huron suggests that we don’t like it because rough sounds interfere with our ability to hear, to pull information out of the environment.
The fingernail mystery remains unsolved—but the leads are helpful. Distracting, rough, ear preservation, adapted aversion: these are some of the theories for what makes certain sounds intrinsically unpleasant—explaining their widespread ability to annoy.
Smells may be the sneakiest of all annoyances. They’re invisible. They’re silent. You are aware of them only after it’s too late. And they’re powerful. Pleasant smells can transport you instantly to a memory of your grandmother’s kitchen or your favorite swimming hole. Unfortunately, unpleasant ones have the same effect.
When you consider that a ten-pound fur ball can ward off a nine-hundred-pound bear simply by suggesting that a bad smell is coming, you realize how powerful odors can be. Think of that for a moment. A porcupine went to the trouble of evolving a coat of razor-sharp spines, and that’s not as effective as a skunk’s primary weapon. Skunks command the forest through smelliness. The aerosol spray isn’t deadly, it’s annoying. It’s so annoying that it can
