Does the screeching of chalk on the blackboard or the scratching of the fork on the plate give you a shiver? You are not alone in this. Some noises cause most people an extremely uncomfortable feeling and goose bumps. But why actually? The reason is how our brain processes these sounds.
o shudder at a harmless squeaking noise is actually illogical – after all, the chalk or knife does us no harm. But this reaction dates back to prehistoric times. Loud and shrill noises are usually associated with danger. In such moments, the fur of animals automatically rises to make them appear larger and more frightening. We humans lack the fur to do this, but we also have this innate reaction to unpleasant noises: our body hair stands up and we get goose bumps.
This refers to the how pleasant or unpleasant a person’s voice sounds.
A reaction from prehistoric times
Scientists have discovered that a region of our brain is particularly active at such moments: the so-called limbic system and the associated amygdala, a small, almond-shaped brain region. The limbic system kicks in whenever we process feelings. The amygdala reacts most strongly to sounds that are very high, namely between 2000 and 5000 Hertz. This is the same pitch as a high-pitched screech or shout that usually means danger. Again, when we scream, it makes sense for us to flinch and shudder. For our brain, this pitch means “alarm!” In any case, even if it is really just a squeak of chalk.
Our experiences play a role
However, not every sound is equally pleasant or unpleasant for everyone. Perhaps you are one of those people who can barely file your fingernails because the noise is unbearable, while your friends or siblings have no problem with it. This is probably related to the experiences you or your friends have had. For example, if you’ve ever hurt yourself while filing your nails, this increases the negative reaction to the associated noise. And if you find the squeaking of chalk on the blackboard particularly bad, you may unconsciously associate the noise with a particularly unpleasant lesson. Very young children therefore show such strong reactions less often because they have not yet saved many experiences. People with migraines, however, are particularly sensitive to noise, for example.
The physics of squeaking
Why is chalk squeaking on the blackboard – or a felt pen on paper, or a fork on the plate? The so-called “stick-slip effect” is responsible for this. If the chalk is drawn across the board at an inconvenient angle, it will not slide evenly, but will always stop for a very short time, bend a little, and then slide on. This alternation of sliding and stopping happens many thousands of times per second. Each time the chalk is bent a little and then relaxes again. So the chalk starts to vibrate, and we can hear this vibration as a squeak.
Similar shrill noises caused by the stick-slip effect are the squeaking of brakes, unoiled doors, or even rubber soles sliding on smooth floors.