You probably just shuddered after reading the title of this article, and for that I apologize. What is it about the sound of fingernails scraping on a chalkboard that makes your blood curdle?
This question has been the basis of several studies done in the field of psychoacoustics, the branch of psychology which deals with the perception of sound and its physiologic effects. There are two main hypotheses for the sound’s visceral reaction: the primate heritage hypothesis and the physical hypothesis.
Research supporting the primate heritage hypothesis has shown that the frequencies of a screaming or crying mammal are similar to the frequency of the sound of fingernails scratching a chalkboard. This suggests that the response to this sound may be tied to survival. A 1986 study by Randolph Blake tested this theory using a tape recording of a three-pronged garden tool being scraped across a chalkboard. The recordings were manipulated to remove the median, extreme low and extreme high pitches. The new recordings were then played to volunteers who rated how much they disliked each sound. Contrary to what had been previously theorized, Blake determined that the median pitches were the cause of the irritation and “flinching response,” rather than the high frequency sounds. Since both humans and primates responded to the median frequencies and not the extreme frequencies, Blake believed that they perceived the scratching sound as a warning cry and thus the “flinching response” was an unconscious and automatic reflex. Blake then studied the warning cries of primates and found that the cry of the chimpanzee, a closely related primate ancestor to humans, sounds very similar to the sound of fingernails on a chalkboard. The pitch of the chimpanzee’s cry is also equal to that of the chalkboard sound. Though Blake hypothesized that the “flinching response” can be attributed to predation during early human evolution, recent research seems to contradict his theory.
The second theory, the physical hypothesis, was proposed by musicologists Michael Oehler and Christoph Reuter. They claimed that the unpleasant response to the sound is due to acoustic resonance caused by the shape of the ear canal. Resonance is the physical phenomenon in which a vibrating system or external force causes another system to oscillate with a greater amplitude at a specific frequency. The median frequencies mentioned above, those ranging from 2000 to 4000 Hz, are amplified in the ear canal to a level where the sound actually causes pain in the human ear, thereby causing the “flinching response.”
Other studies have helped elucidate what actually happens in our brains when we hear the sound of fingernails scratching against a chalkboard. Research suggests that this sound initiates a communication between the brain region involved with hearing and another region involved with emotion. In a 2012 study published in the Journal of Neuroscience, thirteen participants listened to several sounds including nails on a chalkboard while functional magnetic resonance imaging (fMRI) examined how their brains responded to the sounds. Results indicated that there was an interaction between the auditory cortex, the region where sound is processed, and the amygdala, the region where negative emotions are processed. This study also found that for a more irritating or painful the sound, a stronger connection was formed between the two brain regions.
More research still needs to be done—it is not entirely clear what causes the strong reaction to the sound of fingernails against a chalkboard. But no more research is needed to show that this sound is universally detested.