Shrillness is a word used to describe the quality of sounds that have a high-pitched, strident, raucous, screeching or harsh character, such as those produced by a trumpet or piccolo, but it can also be used to describe a widely recognised and puzzling phenomenon whereby certain sounds are perceived as psychologically painful or aversive to a degree that cannot be accounted for simply in terms of frequency content or loudness. Such sounds include the sound of fingernails scraping a chalkboard, the sound of chalk on a blackboard, the sound of glass being scratched, and possibly the sound of a baby crying. There have been attempts to explain the phenomenon, often in terms of frequency content, or evolutionary advantage, but so far no complete explanation or mechanism has been found.

A 2011 study^[1] by musicologists Michael Oehler and Christoph Reuter has led its authors to hypothesize that the unpleasantness of the sound is caused by acoustic resonance, as the shape of the human ear canal amplifies certain frequencies, especially those in the range of 2000 to 4000 Hz (the median pitches), at such a level that the sound triggers pain in our ears.^[2]