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I am curious as to what current research shows regarding why scraping noises such as fingernails on a chalkboard, a knife/fork scraping against a plate, metal grinding against metal or stone etc are so intolerable to humans. In doing some initial research for this I read the wikipedia article on Sound of fingernails scraping chalkboard (there's a wikipedia article on everything!) but I am looking for more references or updated research.

From the reading I have done, there seems to be two competing theories:

  • The sound is reminiscent of the warning calls of primates and triggers an evolutionary response left over from an earlier age designed to protect us from predators
  • The sound hits a resonante frequency of the inner ear which causes it to be particularly intolerable

To me personally the first theory makes very little sense. Hearing the warning calls of primates does not personally trigger anywhere near the same feelings of extreme frustration and irritability that I feel when I accidentally scrape my knife on my plate. Furthermore the feelings caused by the "nails on a chalkboard" sound do not at all trigger a fight-or-flight response or put me on edge as a loud noise or something that startles me does. If anything it is almost paralyzing, which seems counter to the evolutionary theory.

How accurate is the wikipedia article? Is there any newer / more widely accepted research available on what causes the sound of "fingernails on a chalkboard" to be so intensely awful to human beings?

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The feeling I have when I hear chalk screeching on a chalkboard is very similar to the feeling I have when my baby cries (I think more research should be done by people with children, it would be enlightening). –  what Aug 19 '13 at 9:45
@what that is an interesting insight - I wonder if there is a connection in that respect? As for your point in parentheses, that indeed would be interesting (and no, I don't have any children). –  user3554 Aug 19 '13 at 10:04

3 Answers 3

My initial thoughts are that the sound produced a dissonant sound, that is (from the link):

"An unstable tone combination is a dissonance; its tension demands an onward motion to a stable chord. Thus dissonant chords are 'active'; traditionally they have been considered harsh and have expressed pain, grief, and conflict."

—Roger Kamien (2008)

According to the article "Psychacoustics of a chilling sound", (Halpern et al. 1986), it is the low frequency components of the sound that cause the most discomfort. In their tests, removing the low frequency part in a digital simulation, made the sound a bit more bearable.

In the conclusion of the article, there is some support for the Wikipedia article that Josh Gitlin linked to in his question:

the complex acoustic stimulus pictured in Figure I very closely resembles some of the spectrograms of warning cries emitted by macaque monkeys (Green, 1975). As another possibility, the signal may be similar to the vocalizations of some predator.

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Hmmm, dissonance, very interesting, thanks! I'll do further reading on this and will likely have followup questions. I still feel like the relationship to a predator doesn't compute to me as it doesn't trigger anything like fear/fight-or-flight, but I could easily be wrong there because I am basing my beliefs on pretty much zero empirical evidence ;-) –  Josh Gitlin Aug 15 '13 at 21:48
@JoshGitlin as an aside, as a teacher, this sound has no effect on me at all. Interestingly, just the mention of scratching a blackboard is often enough to bring students in line. The dissonance is a personal theory of mine, based on the definition (above) - I tend to agree with your assumption about the predator relationship for the same reasons. I look forward to more questions! –  user3554 Aug 16 '13 at 8:15

Furthermore the feelings caused by the "nails on a chalkboard" sound do not at all trigger a fight-or-flight response or put me on edge as a loud noise or something that startles me does. If anything it is almost paralyzing, which seems counter to the evolutionary theory.

Yes, I agree, it seems to, however what about the fainting goats? We could have traits which aren't advantageous, yet we were able to survive in spite of them. Like the goats.

Even though it seems perfectly possible (though not logical) to be paralyzed by fear in response to a warning signal, I don't agree with the theory that the sound is indeed a warning signal.

In my experience with hearing fingernails on the chalkboard, its almost as if my teeth hurt. This leads me to believe it is not predators I'm fearing, but injury. It could be a fear by association produced from when I was very young and putting many different objects in my mouth. Some objects may have produced a similar sound and the sound was associated with pain. If you wish to survive in nature, its very important to have teeth, and injuring our teeth may be an especially important thing to avoid... not only if you expect to eat, but also attract a mate. A good smile is important.

In my search for references to back up my theory, I found this presentation, the author of which seems to agree with my assessment.

I also found at least one other person who claims to feel something in his teeth:

I feel the discomfort in my teeth and have always assumed the screeching is at the right frequency to build up uncomfortable vibrations in the teeth.

That is actually a random comment (pages 11-20 in the comments section at the bottom) to this article.

I don't agree that the sounds would have enough acoutical power to vibrate the teeth. I think the pain is merely imagined. Of course, that's my opinion, for what its worth. Since the prevailing theory is predator-based and not tooth-injury-based, I wouldn't expect to find any studies on the matter.

In addition to the evidence presented so far, I have another reason to doubt the predator theory. If the sound mimics the cry of danger and the response is instinctual, then why don't we all have the response? Out of 3 people contributing to this question, 1 has admitted "this sound has no effect on me at all." This argues in favor of my "fear by association" theory.

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very good points - particularly your final paragraph. –  user3554 Aug 19 '13 at 10:05
Fainting goats were I believe actually bred deliberately as decoy animals: put one in your herd and if a predator ambushed the herd, the fainting goat would fall over and get eaten, distracting the predator from the rest of the herd. So no, fainting when startled isn't a survival trait without humans around to value and breed selectively for it! With humans around, though, it is in fact a survival trait. –  Krysta Aug 22 '13 at 19:40
@Krysta That's an interesting bit of thinking, kinda like my cats who learned not to hunt in favor of begging at the door. But, how does one breed goats to fall over when scared instead of running? And how can I train my cats to do that? Running is usually what gets them killed in this modern world of automobiles. They see a car and immediately dash across the road. –  Randy Aug 22 '13 at 21:01
Apparently the fainting trait (also called myotonia) is genetically quite simple. It shows up like any mutation, and some sharp goatherd realized its use and bred for the trait selectively. I think you have to be a catherd to do this in felines, though, and we all know how hard that is! ;-) –  Krysta Aug 22 '13 at 21:06
@Krysta Opossums also have this trait of playing dead. So, who was herding the opossums? ;) –  Randy Aug 22 '13 at 21:14

I don't understand why there's a bias to assuming that something that doesn't actually touch us can't cause pain. I assure you: the sound that Styrofoam makes is enormously painful to me. (And I grew up in the egg business, where I would have to plug my ears on a daily basis when my father or someone else had to do something with Styrofoam cartons. Ugh.)

We all have different tolerance levels for pain in other ways. The guys who go for Navy SEAL (or equivalent) training, or who play NFL football, clearly have a different tolerance for pain than I do. Why would anyone assume that this is different in terms of one's hearing?

The simplest answer is the best: some of us are caused actual pain when certain sounds are made, most likely because there are frequencies in the sound that we can perceive more or more clearly than others can. I'm an audiophile, and have lots of anecdotal evidence that I can hear higher-pitched sounds than my friends... my guess is that for those of us who have massively painful reactions to these sounds, our ears are genetically wired to be able to hear those frequencies far more clearly than those who aren't susceptible.

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