COMMITMENTS : The Sound and the Fury : Nails on a chalkboard. A knife cutting Styrofoam. You’re probably cringing just thinking about it. Why are some noises so universally annoying?

Male, 34: “It’s primal. I almost go into a fetal position.”

Female, 41: “Eww. Just thinking about it makes me shiver.”

Male, 6: “I hate it.”

What’s vexing people so? A roller coaster? The new Stephen King?

Worse.

It’s a sound--one of those sounds that drive us crazy: Chqueerk, a pitchfork scraping concrete. Wreshk, a knife cutting Styrofoam. And don’t forget the scrrEEeck of fingernails on a blackboard, a noise so universally grating that it’s become a simile for anything that makes us cringe.

“For me, it’s the sound of chewing on a paper napkin, that crchcrchcrch,” says Susan Stern, a Bay Area video producer. “Not that anyone sits around chewing on napkins, but if it happens . . . yech. “

Why do certain sounds give us the heebie-jeebies?

Ear and mind experts agree the phenomenon is common, yet baffling. It’s a kind of auditory Chinese puzzle that could tell us something about a particular person, a particular sound, or even a link between sound and person that reaches back to the beginnings of human hearing.

“Think of it as a two-sided question: Why are some sounds pleasurable?” says Dr. James P. Kelly, director of otolaryngology research at the University of New Mexico School of Medicine. Musical harmony, for example, has “great historical interest,” Kelly says. “Pythagoras was interested in it, in why that 3-to-2 ratio--that pleasant musical fifth--is such a soothing, pleasing tone. He ascribed mystical attributes to the ratio.”

Twenty-six centuries later, exactly why harmony feels so good is unclear. What is clear: The auditory system has a disproportionately high input into the part of the brain that affects emotions. Says Kelly: “That’s why some people stand at the national anthem and are completely overcome.”

The national anthem-- aahhh.

A phonograph needle scraping across an LP-- eewww.

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They come out of nowhere, these micro-assaults on the senses. All of a sudden, shwrk, there goes that sound. And there’s no stopping your reaction.

One theory says the hearer could have had a bad experience with the offending sound, or with something like it.

“Hearing is a psycho-biological phenomenon,” says Dr. Max Ronis, chair of otolaryngology at Temple University. “It’s modified by a whole raft of things. Anticipation can actually make it worse.”

Which is why a dripping faucet at 3 a.m. can sound like a construction site to the wakeful ear. The sound itself, of course, is innocent--not intending to be pleasant or vile. The ear registers it and carries it to the brain, and that’s where it gets mixed into pools and spools of meaning. But the hearer may not have a clue about what a particular meaning means.

“It’s possible you’ve stumbled on a chance triggering of a biologically significant sensory process in the nervous system of the [hearer],” says Robert Provine, a professor of psychology at University of Maryland. “The problem is, trying to figure out what that significant thing is.”

Indeed, maybe the 40-year-old man whose “neurological doors unhinge” when he hears the quorchquorch of high heels on linoleum had a high-heeled baby-sitter somewhere in his past.

And maybe not. Only his neuro-circuitry knows for sure. But then, if noisome noises are a person-by-person matter, programmed by individual events, how do we explain the almost universal aversion to such scraping sounds as the infamous nails on the blackboard? Did all of the estimated 75% to 90% of people who cringe when they hear it have a bad experience with a grade-school teacher?

ScrEEck.

Eew.

Not likely.

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In the complex and sensitive world of the human ear, some sounds just ring bad.

Bad sounds tend to be squeaky--rich in high and middle frequencies. Most important, they are uneven--full of thousands of starts and stops per second. To the ear, Kelly says, the sound is “an incoherent jumble of hisses and scrapes, not linked in time, all running simultaneously.” The auditory opposite of the smooth, continuous low-frequency sound of mystical musical harmony. And it can drive people to distraction.

“A knife scraping against a plate--that awful reek-reek. It sounds like lambs going to the slaughter,” says Jean P. Fisher, a Washington D.C.-area magazine intern. “My sister is even worse than I am. Once during dinner she hunched up and squealed, ‘Do you have to cut your chicken like that?’ She took my plate to the kitchen and sliced the chicken on a board.

“What would she do at a dinner party with the President of the United States? ‘Excuse me, but I have to go cut up your chicken for you?’ I told her, ‘Maybe you need help.’ ”

How much we hate a bad sound may be determined by individual genetic makeup. Just as some people have different pain thresholds, some have different thresholds of sound sensitivity.

“It’s like ‘The Princess and the Pea,’ ” says Temple ear expert Ronis. A few lucky souls, it seems, have been spared the agony altogether.

“We’ve all had that experience of sitting in the classroom when some kid gets up there and gleefully scrapes his nails down the board while everyone else cringes in their seats,” says Randolph Blake, chair of the psychology department at Vanderbilt University. The “everyone else” part of the picture suggests the reflex is more than an accident of nature--that it could be shared by most humans.

“The issue is, to what degree is it a conditioned behavior and to what degree is it hard-wired in the brain?” Kelly says. “It’s possible that way back in the evolutionary environment, these sounds had some significance.”

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If you sat at a computer to analyze the sound waves of nails scraping a blackboard, you’d find a “very complex acoustical event,” says psychologist Blake, who once conducted such a study.

Using a computer-stored recording of a garden fork on slate, he asked headphone-wearing volunteers to rate degrees of unpleasantness as the sound frequencies were manipulated. The “complex acoustical event,” the analysis showed, bore a striking resemblance to the sound patterns of a primate warning cry.

“It’s sheer speculation, of course, more a ‘Gee isn’t that an interesting similarity’ than anything else,” Blake says. “But the fact that the sound is so universally noxious suggests biological relevance. Whether by God or by nature, we have a reflexive reaction to those kinds of sounds. At some point in our heritage, it was probably adaptive. Now, it’s just irritating.”

In the end, then, our reactions to certain sounds may be evolutionary leftovers from the days when heeding a warning cry made the difference between having a good day or no day at all.

Most people learn to live with it.

The alternative: Unlearning the sound by forcing yourself to listen, sort of Pavlov-in-reverse. If you have the stomach. And there’s always earplugs. That is, if you don’t mind blocking out today’s warning cries and risk getting hit by a truck.