Why Chalk Screeches on a Board

This episode explains that the unpleasant screech of chalk is caused by a physical process called stick-slip motion, where the chalk rapidly sticks and slips against the board instead of moving smoothly. This creates high-frequency vibrations that travel through the air as sound.

The reason the sound feels so uncomfortable is because it often falls within a frequency range that human ears are especially sensitive to—similar to distress signals like screams. Additionally, the sound is irregular and unpredictable, making it harder for the brain to process and easier to interpret as a warning signal. This triggers an automatic physical reaction, such as tension or discomfort.

Factors like the angle, pressure, and speed of the chalk can increase or reduce the screech, which is why experienced users can often avoid it. Overall, the sound is a result of simple physics amplified by human biology, showing how even small vibrations can produce strong emotional and physical responses.