1. Why Do People Feel Differently About the Same Scraping Sound?
Some people enjoy the "crunchy" sound of writing with a glass pen. In contrast, many people feel a strong sense of discomfort, like a shiver down the spine, when they hear the "screeching" sound of fingernails scratching a chalkboard. Both sounds are produced by hard objects rubbing against a surface. Despite this similarity, why do we feel the exact opposite: one is "pleasant," and the other is "unbearable"? This article explores where this difference comes from by looking at research on the unpleasant noise of chalkboards and brain studies on pleasant sounds.
2. Why Is the Sound of Scratching a Chalkboard Unpleasant? How Sound Frequency Matters
The "screeching" sound made when scratching a chalkboard with fingernails or tools is caused by friction vibrations. These vibrations happen when surfaces stick and slip against each other at high speed [1]. In 1986, American researchers conducted an experiment where they digitally processed this sound to remove specific frequency bands. The results showed that the main cause of discomfort was not the highest-pitched sounds, as previously thought, but the medium-pitched sounds [2]. In 2011, researchers demonstrated that the frequency band between 2,000 and 4,000 hertz is particularly linked to strong discomfort [3]. The human ear canal has a structure that naturally amplifies sounds in this specific range. Because of this, chalkboard sounds feel more piercing to the ear [3].
3. How the Brain's Amygdala and Hearing Areas Work Together When We Hear Unpleasant Sounds
Sound pitch alone cannot explain all the discomfort. In 2012, a research team from universities in the United Kingdom used a brain activity scanner (fMRI) to study 13 people. They listened to various sounds, such as a knife scraping a jar or a baby crying [4]. The results showed that when hearing unpleasant sounds, the "amygdala," a brain part involved in fear and alertness, strengthened the activity of the "auditory cortex," which processes sound. This interaction triggered the reaction of discomfort [4]. The two parts exchange signals to create the feeling of unpleasantness [4]. Therefore, chalkboard sound is not just a "strange pitch." The brain processes it as a "sound to be alert to," which creates the strong discomfort.
4. How Pleasant Scraping Sounds Activate the Brain's Reward System
On the other hand, some scraping sounds, like those from a glass pen, can feel pleasant. Specifically, sensations of tingling in the head and neck, often triggered by whispers or rhythmic scraping sounds, are called "ASMR." In 2018, a research team from an American university studied the brains of 10 people who easily feel ASMR. They examined brain activity at the moment of feeling the pleasant "tingles." The results showed that brain parts related to "rewards" were strongly active in both the left and right hemispheres [5]. This means that even with the same "scraping sound," the feeling becomes opposite depending on whether the brain reacts to it as a "sound to be alert to" or as a "pleasant stimulus."
5. How Knowing What Makes a Sound Changes How It Feels
Interestingly, in the 2011 study mentioned earlier, people felt more discomfort when they knew the sound was "fingernails scratching a chalkboard" compared to when they did not know what it was [3]. The strength of the discomfort is influenced not just by the pitch of the sound, but also by the knowledge of "what the sound is." Additionally, there are large individual differences in how ASMR is felt. While some people feel it strongly, many others feel nothing when hearing the same sounds [5]. In the past, it was widely said that chalkboard discomfort comes because the sound resembles the alarm calls of monkeys. However, later research has shown that this theory is not necessarily supported [2]. Thus, "pleasant or unpleasant" is not a simple phenomenon. It is determined by a combination of physical elements like pitch, biological elements like the brain's alert and reward systems, and psychological elements like knowledge and individual differences.
6. How You Can Investigate Which Sounds Feel Pleasant or Unpleasant
Try dividing the sounds around you into "pleasant" and "unpleasant" and think about why you feel that way. You can compare sounds like rain, wind, or creaking doors, and write down which ones feel good or bad in a notebook. Ask your family about sounds they dislike. You might get different answers because people feel discomfort from different sounds. If you go out, you can try different writing instruments at a stationery store or music shop to compare their writing feel and sound. You can also visit a science museum to see exhibits about sound and hearing. A good tip is to write down just one word that caught your interest. For example, "What exactly does the amygdala do?" is enough to start your own research.