Why sound of fingernail scratching blackboard is disturbing

Just imaging the scratchy and screechy sound made by fingernails running down a blackboard is annoying. Now scientists have found the reason why it has such effect. The shape of inner ear is to blame -- human ear canals amplify the high pitches of the sound, making them disturbingly louder to ears, explained researchers from the University of Cologne in Germany reported by CBS. The frequency of the annoying sound ranges between 2,000 Hz and 4,000 Hz, where human ears are most sensitive, suggested Michael Oehler, one of the researchers. As the high-pitch parts of the noise in a record is erased, it sounds less unpleasant, noted the research. The research also suggested that people's perception of sound also plays a role in how unpleasant it is. Researchers asked two groups of participants to listen to the record of fingernails scratching blackboard. One group of listeners were told they were listening to horrible sounds, whereas the others were told they were listening to experimental music. The participants who knew they were listening to noise reported higher level of unpleasantness than those who perceived they were listening to music. These findings were presented last week at the annual meeting of the Acoustical Society of America in San Diego.