Why is the sound of fingernails on a blackboard so uncomfortable?

Ever wondered why do we have such a strong reaction to a specific sound? Are you one of those people that can’t resist the sound of nails scratching on a blackboard?

The exact mechanism of why this event happens is still unknown for scientists but that doesn’t mean they are not doing anything in their power to find an answer. What I have found so far is that it involves many areas of research including sound engineering, psychoacoustics, human response to sounds, neurology, among others.

Sound is very important for humans, it allows us to communicate with others, to stay alert, and also to give us pleasure. Social interaction, cooperation, and communication with others are very important for the human species so it is understandable the brain “enjoys” sounds produced by these activities. 

So which sound is pleasant and which is unpleasant?

The arrangements of harmonics can define if what we hear is pleasant or unpleasant. We know that humans prefer consonant to dissonant chords and the amount of consonance or dissonance chords present in a sound affects the behavior and mood of the people.

The sound of nails scratching in a blackboard is composed of many dissonant chords. It is similar to a scream, or a child’s cry. Those types of sounds often trigger a response from the amygdala, one of the oldest structures in our brain (1). Therefore, scientists believe this might be related to how we react to threatening sensations from very ancient times. See it as a very antique warning system.

How does the brain understand that it is an alarm? Inside our ears is the cochlea, a spiral-like structure surrounded by the basilar membrane that connects directly with the auditory nerves converting sound into nerve impulses. Sound waves travel through the cochlea and stimulate different parts of the structure. “Depending on the wave’s frequencies, a different part of the basilar membrane will be stimulated”, says Antonio Torija, lecturer in acoustic engineering at the University of Salford Manchester, UK. “High-frequency waves (20,000-1,500 Hz) stimulate the very first part of the cochlea, and low frequencies (200-600 Hz) the very end of it. Humans are very sensitive to high frequencies”

Image 1: Sound and the ear. Image by OpenStax

Here comes the tricky part: since almost every sound wave we receive is composed of many frequency waves, the brain has trouble trying to identify each one of them. Collectively, those frequencies can create dissonant chords that the brain has trouble understanding. Scientists believe this is why the brain makes the body react in such an exaggerated manner.

“Sounds like a baby’s cry and scratching a blackboard have some similarities, they both have a high pitch, and also what it is called roughness, that’s when the sound loudness changes very quickly”, says Professor Trevor Cox, professor of Acoustic Engineering also at the University of Salford.

Using “functional magnetic resonance imaging (fMRI)”, researchers from the Institute of Neuroscience of Newcastle University, the Wellcome Trust Center and the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany, measured the activity of the “neuronal responses to sounds that varied over a large range in the degree of their unpleasantness”. They found that indeed is the amygdala, the structure that encodes such types of sounds via the auditory cortex.

Image 2: “Spectral frequency–temporal modulation representation of sounds with high (top) and low (bottom) unpleasantness ratings”. Image by Kumar S. et al. (2)

Noises that give a disgust reaction, like hearing someone being sick, are probably driven by our need to stay away from things that might cause disease. But responses can be very individual. Take chomping noises as someone eats, which most people find a bit unpleasant. But, for some, it can create a very strong emotional response, both while hearing the noise or just even with the anticipation of listening to it. This is a condition called Misophonia and the body responds in three distinctive ways: with anger, with disgust, or with anxiety (3).

"Some of those noises are horrible because we rapidly associate them with previously known experiences, like a woman’s scream or a baby’s crying”, says professor Cox. “The emotions we feel complement what we hear to produce a distinctive reaction. But other sounds don’t have anything to be associated with, like fingernails scratching in a blackboard. It varies from person to person; some people can even stand in a room listening to it, I certainly can.”

Let’s hear out some of those sounds and try to find out how do you feel about them:

• Here’s the sound of fingernails scratching on a blackboard:  https://soundcloud.com/wellcometrust/blackboard-nails
• A fork on glass: https://soundcloud.com/wellcometrust/fork-glass
• Blackboard chalk: https://soundcloud.com/wellcometrust/blackboard-chalk
• A ruler and a bottle: https://soundcloud.com/wellcometrust/ruler-bottle
• A knife and a bottle: https://soundcloud.com/wellcometrust/knife-bottle

Now, which sound is the worst for you?

References:

(1) Mirz et al., 2000; Functional brain imaging of tinnitus-like perception induced by aversive auditory stimuli. Neuroreport 11:633– 637. https://europepmc.org/article/PMC/3505833#abstract

(2) Kumar S, von Kriegstein K, Friston K, Griffiths TD. Features versus feelings: dissociable representations of the acoustic features and valence of aversive sounds. The Journal of Neuroscience. 2012 Oct;32(41):14184-14192. DOI: 10.1523/jneurosci.1759-12.2012 https://www.jneurosci.org/content/32/41/14184

(3) Misophonia UK http://www.misophonia-uk.org/

Images' copyright info:

Image 1: By OpenStax - Creative Commons Attribution 4.0 International License (CC-BY).  https://commons.wikimedia.org/wiki/File:1405_Sound_Waves_and_the_Ear.jpg

Image 2:  Creative Commons Attribution 4.0 International License (CC-BY).