A sound, however, is more than the sum of its vibrations. Blake says that if you just mix the frequency components randomly, you get noise—like the hissing of a radio between signals. All of the frequencies that are present in the fingernails-on-a-chalkboard sound might be present in static, Blake says, but they aren’t arranged in the same way. What gives a creak, a squawk, or a squeal its signature is how the peaks and troughs of the component sound waves appear in relation to one another over time.
Think of it like cooking: the proportions of the ingredients and when they’re added can mean the difference between marinara sauce and a Bloody Mary. The same is metaphorically true for sounds, with the frequencies as the ingredients.
The question Blake was trying to get at was: What frequencies were poisoning the pot? Signal processing helps answer this question by providing the recipe for the sound, revealing the mixture of component frequencies and at what levels they are added.
To try to understand which frequencies were the offending ones, the researchers tried removing ingredients. They wondered, At what point when we remove certain frequencies does the sound lose its aversive quality?
Blake hypothesized that the high frequencies were the root of the problem. Think about how you might describe the fingernail-scraping sound. The adjectives that come to mind are shrill, piercing, sharp. They’re pejorative, and they suggest that the offensive parts are the high pitches. “Intuition told us that if we removed the really high frequencies, that would de-fang the noise,” Blake says.
The researchers were wrong. Blake filtered out the high pitches and—weirdly—the sound was still really annoying. The sound was different: it was more muffled but not more pleasant, according to the study subjects.
It turned out that the sound got less annoying to the listeners only when the middle-range frequencies were taken out. The annoying frequencies were the mundane 500 to 2,000 Hz (Hz, or hertz, is the scientific unit for frequency, or in this case, pressure variations per second). These frequencies are right in the middle of our hearing range: humans can hear frequencies from about 20 Hz to 20,000 Hz—and that upper limit drops as our ears age.{13}
All ears basically work the same way: think of them as funnels. Sound waves (pressure oscillations) travel through the air, shoot down the ear canal, and get absorbed by the eardrum, a membrane gate at the end of the ear canal. The vibrations travel through tiny bones that connect the eardrum to the cochlea—a hollow tube that is sort of like a snail shell. “If you were to unravel the snail, it’s essentially this long tube that’s divided by a membrane,” says Josh McDermott, a neuroscientist at New York University. “Different parts of the tube are sensitive to different frequencies. The part closest to the eardrum is sensitive to high frequencies and the part that’s farthest away is sensitive to low frequencies.”
So far, everything is mechanicaclass="underline" pressure oscillations are jiggling a membrane in your ear. Enter hair cells. The membrane is coated with them. These receptors get their name because they have little organelles that stick out from the cell wall like cowlicks. They’re called stereocilia and they’re small—about 5 microns long, one-twentieth the width of a human hair. Small but important: the stereocilia are responsible for detecting the jiggling. The hair cells do the remarkable task of translating those physical signals into electrical pulses that are sent to the brain. After the hair cells translate the pressure oscillations into electrical signals, the signals are transmitted to subcortical regions in the mid-brain, and then they make their way to the thalamus and the cortex, “and then a lot of stuff happens that we don’t understand,” McDermott says.
Not all sounds are heard equally: the human ear seems to have preferences for certain frequencies. The frequencies that contribute most to the annoyingness of the scraping sound are on the low end of the frequency range that our ear is most sensitive to. Humans can detect frequencies between 2,000 Hz and 5,000 Hz at lower volumes than other sounds. Around 3,000 Hz also happens to be the natural resonant frequency of the ear canal, studies show. This means that when a 3,000-Hz signal goes into the ear, it’s naturally amplified, due to the shape of the ear canal.{14}
“It means that you can hear a sound at 3,000 Hz that has much less than a quarter of the energy of a sound at, say, 1,000 Hz,” says David Huron. “Here’s what’s fun. We record a bunch of different sounds made by humans, and we find out which sound has the most amount of energy right around 3,000 Hz that humans make. The answer is a scream. This is true of males, females, and children. When men scream, they break into a falsetto. They end up producing the max amount of energy in the same region as women and children when they scream. What this means is that the sound that we can detect at the greatest distance is the human scream. We’re most sensitive to a human scream.” Sounds in this frequency range will “distract you from any other task you’re involved in,” says Huron. This may explain why the fingernail noise got less annoying when the middle frequencies were removed—when the energies we’re most sensitive to are removed from the noise, it’s easier to tune out.
The 2,000 to 5,000 Hz range is also where the ear is most vulnerable, McDermott says. “If someone listens to lots of loud sounds, and you look at where they have hearing loss, it’s usually going to be in that range. At least, that’s the first place where people have problems.” He adds that it’s possible that our aversion to those frequencies could be a protective response. Ear preservation is one hypothesis to explain why the fingernails-on-a-chalkboard screech makes us cringe. A sensible way to protect our hearing is to evolve an aversion to sounds that are damaging.
Randolph Blake proposed another hypothesis. Some sounds remind us of something we don’t like, which seems to partly explain the annoyance factor.
If you are sensitive to noise annoyances, take heart in the fact that few people are immune, including the most controlled among us. Recall, for example, an anecdote related by Mark Leibovich in January 2009 in the New York Times:
Barack Obama was meeting this month with House Speaker Nancy Pelosi and other lawmakers when Rahm Emanuel, his chief of staff, began nervously cracking a knuckle. Obama turned to complain to Emanuel about his noisy habit. At which point, Emanuel held the offending knuckle up to Obama’s left ear and—like an annoying little brother—snapped off a few special cracks.{15}
Knuckle cracking is annoying, even to presidents—but why? Here are a few theories. First, bodily noises annoy us. “The disgust reaction is universal,” says Trevor Cox, an acoustician at Salford University’s Acoustic Research Centre who has been hunting for the worst sound in the world through his Web-research project BadVibes. Cox posted a variety of sounds on the Internet and asked people to rank them in terms of how bad they were. Nearly half a million votes were tallied, and a gross body sound was the biggest loser: vomiting came in first for the worst sound in the world. (If you remember, earlier we concluded that nothing good ever comes from being nauseated. Vomiting is what we meant when we said, “Nothing good.”)
There are intuitive reasons that humans would dislike the sound of vomiting. “There’s a whole class of sounds that are annoying because they have certain bodily associations,” says David Huron. “There are excellent reasons why listening to the sound of someone retching or anything associated with illness should make us wary of these sounds or find them not very pleasant.” Maybe we don’t like disgusting sounds as a defense mechanism. It’s possible we’re programmed to avoid things that can make us sick because that helps us survive. Knuckle cracking isn’t a contagious disease, but it may piggyback onto our aversion to other bodily noises.