Interesting2Links: Why Do Fingernails on a Chalkboard or Scraping on a Plate Make Us Cringe?

Masa C asks: Why do fingernails on a chalkboard make us cringe?

Listen to this. Irritated? Angry? Just glad it’s over? Here’s why.

Your Ear

After examining the results of a 2011 study, researchers have theorized that the human ear is specifically built so that:

Sounds in [a certain] range are amplified due to the anatomy of the ear canal; they are literally louder to us than other sounds are.

Experts have opined that in particular, the ear:

May have evolved to amplify frequencies that are important for communication . . . [which] could have been advantageous for survival, allowing people to come to the rescue of their screaming infants quicker, and thus improve their offspring’s chance of survival, or coordinate more effectively during a hunt.

Frequency Range for Maximum Revulsion

The band between 2,000 Hz and 4,000 Hz contains the frequencies for which the human ear is most acute, and this range just happens to include the sounds of both “human speech, as well as . . . a baby crying.”

When sickening sounds like fingernails on a chalkboard, utensils scraping a plate or squeaking Styrofoam are made within this range, humans demonstrate the most dramatic reactions because the sounds are hitting “right in the sweet spot of human hearing,” so every repugnant nuance is perceived.

Interestingly, however, when the tonal parts (harmonic components related to pitch and frequency) of these noxious sounds were deleted, “listeners perceived the sound as most pleasant.” Notably, though, “removing the noisy, scraping parts of the sound made little difference.”

Physiological Response

Anyone who cringes just at the thought of hearing these sounds will be unsurprised that researchers have demonstrated actual physiological responses to unpleasant noises:

They played back the . . . sounds for the participants, all while monitoring certain indicators of stress, such as heart rate, blood pressure and the electrical conductivity of skin. They found that the offensive sounds changed the listeners’ skin conductivity significantly, showing that they really do cause a measurable, physical reaction.

I bet you have goosebumps just thinking about it.

Psychological Factors

The researchers found that our perception of the obnoxiousness of a sound was subject to suggestion:

The listeners in the study rated a sound as more pleasant if they thought it was pulled from a musical composition (though this didn’t fool their bodies, as participants in both study groups expressed the same changes in skin conductivity).

Fingernails Aren’t the Worst, Not Even Close

According to another study conducted by scientists at the Wellcome Trust Centre for Neuroimaging at UCL and Newcastle University, which used “functional magnetic resonance imaging (fMRI) to examine how the brains of 13 volunteers responded to a range of sounds,” there are at least four sounds more pernicious than fingernails on a chalkboard:

Knife on a bottle

Fork on a glass

Chalk on a blackboard

Ruler on a bottle

Rounding out this noisome top ten includes our fingernails on the chalkboard, a female screaming, an anglegrinder, squealing brakes, a crying baby and an electric drill.

The study also found that the least objectionable sounds included, in order, applause, a baby laughing, thunder and flowing water.

Through the fMRIs, the scientists found that:

Activity of the amygdala and the auditory cortex varied in direct relation to the ratings of perceived unpleasantness given by the subjects. The emotional part of the brain, the amygdala, in effect takes charge and modulates the activity of the auditory part of the brain so that our perception of a highly unpleasant sound, such as a knife on a bottle, is heightened as compared to a soothing sound, such as bubbling water.

The Newcastle University researchers’ findings supported those of the earlier study, showing that “anything in the frequency range of around 2,000 to 5,000 Hz was found to be unpleasant.”

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Bonus Hearing Facts:

A person with normal hearing begins to experience ear pain at around 120 decibels, although the National Institute on Deafness and Other Communication Disorders notes that: “Long or repeated exposure to sounds at or above 85 decibels can cause hearing loss. The louder the sound, the shorter the amount of time it takes for NIHL [noise-induced hearing loss] to happen.” This is bad news for the average American since the noise from “busy city traffic” comes in at 85 decibels, a hair dryer and gas lawn mower above that, and a cranked up MP3 player at 105 dB.

About 15% of Americans, or 50 million people, suffer from tinnitus (ringing in the ears). The condition is caused by, among many other things, exposure to loud noises from “heavy equipment, chain saws and firearms . . . MP3 players or iPods.”

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