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.