Pest sounds 
Scientists test their grate expectations
Fingernails on a blackboard. Why does the very phrase send chills down one's back? The question has annoyed scientists for at least 2,300 years. Aristotle mentioned "hard sounds", but didn't try very hard to explain them.
In the mid-1980s, three scientists subjected volunteers to a battery of electronically synthesised nails-on-blackboard screeching. D Lynn Halpern, Randolph Blake and James Hillenbrand, at Northwestern University in Evanston, Illinois, published details in the journal Perception and Psychophysics. They called their study "Psychoacoustics of a Chilling Sound".
First, they ran some tests to establish exactly where nails-on-a-blackboard ranks in the hierarchy of annoying sounds.
They recruited a panel of volunteers - a different group from the one that would later undergo exposure to the sounds. The panel listened to recordings of 16 different sounds. They rated just how annoying each was. This ranged from not very (rotating bicycle tires, running water) to excruciating. Jingling keys mildly annoyed some people. Then, increasingly less pleasant, came: a pencil sharpener; a blender motor; a dragged stool; a metal drawer being opened; scraping wood; scraping metal; and rubbing two pieces of Styrofoam together. But fingernails-on-a-blackboard topped them all.
Halpern, Blake and Hillenbrand, having established this simple fact, then converted the tape recording to a digital signal, so that they could manipulate and experiment with constituent high and low pitches. The formal report notes the researchers' belief that the signal was of good quality. "To the authors and several other reluctant volunteer listeners," they write, "the digitised, filtered signal sounded very similar to, and just as unpleasant as, the original."
The original recorded, pre-digitised sound was not actually of scraping fingernails, but of something known, from previous experiments, to be very like it. In a footnote, Halpern, Blake and Hillenbrand confide that "the instruction set used in this study included a description of [a] three-pronged garden tool being dragged across a slate surface. Virtually all subjects shuddered upon reading this portion of the instructions."
The shuddering volunteers listened to several different, digitally doctored versions of the sound, and rated the unpleasantness of each.
The conclusion, when all was scraped and done, is perhaps worth quoting:
"Our results demonstrate that the unpleasant quality associated with the sound of a solid object scraped across a chalkboard is signalled by acoustic energy in the middle range of frequencies audible to humans. High frequencies are neither necessary nor sufficient to elicit this unpleasant association. Still unanswered, however, is the question of why this and related sounds are so grating to the ear."
Twenty years later, the mystery endures, giving cold discomfort to almost everyone who hears about it.
· Marc Abrahams is editor of the bimonthly magazine Annals of Improbable Research (http://www.improbable.com) and organiser of the Ig Nobel Prize
Marc Abrahams
Tuesday June 27, 2006
The Guardian
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