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FINGERNAILS ON BLACKBOARD
Explained at last?
2003 Bill Beaty

Fingernails on a blackboard, or cat claws scraped across an old chalky car hood! Or... a steel garden hoe hitting a smooth dry rock. Or tear a ball of cotton between your fingers. Write with a nearly-dry magic marker. Trying to saw through a brick with a steak knife. Rub a block styrofoam across cardboard. EEEEEEEURRRRRRRRT!!!

How can a simple sound make you feel so awful? Why do humans seem programmed to avoid it? We want to make it stop RIGHT FREAKIN' NOW.

The answer came to me like a blinding flash. I was eating something at a picnic and I dropped it on the ground. I wiped it off and continued eating. (Oh, you DO SO do it too!)

As I was chewing, suddenly I heard SKKKKEEEEEERRRCH!!!!! ...as I bit down hard on a tiny stone. I think every single hair on my body stood on end, and my jaws froze instantly.

THAT'S IT! Fingernails-on-blackboard: it sounds exactly like the destruction of tooth enamel. We're instinctively programmed to respond instantly. Of course! It's so sensible and obvious. Every little kid knows it. I remember many incidents from my own childhood. Why didn't we adults ever realize? The scraping of fingers on a blackboard is the classic, high-frequency violin-like waveform of hard dry surfaces moving with chaotic stick/slip motion. And that could very well be why our instincts are programmed to repond to it so strongly.

It's the sound of body damage; but it's a particular type of body damage for which there is no pain ...yet no healing.

We get no second chance with teeth. If we bite down on rocks, we wreck our enamel, and that could be why fingernails-on-blackboards makes everyone around us take drastic action to halt that noise. Why ddn't anyone realize the origin of our response? It's because we're too damned civilized, and we rarely have rocks in our food anymore. But whenever we bite down on something which is far harder than tooth enamel, our inborn programming instantly informs us about the problem in no uncertain terms. Sensible? Flesh can heal, but tooth surfaces do not.

I had rather hear a brazen canstick turn'd,
Or a dry wheel grate on the axle-tree;
And that would set my teeth nothing on edge...
- Shakespeare, Henry IV
But SOUND is not pain. Skin is full of nerve endings, and pain normally teaches babies what not to do. The thick outer layer of our teeth lacks the pain sensors of other tissues. Won't animals need something besides pain to inform them that they're damaging themselves in a permanent way? Fingernails-on-blackboards could be entwined with evolution: animals who respond strongly to that particular noise will guard their teeth carefully, and they won't ever bite down on rocks if they can possibly avoid it. Animals who ignore that noise will die early from bad teeth. We're the product of successful ancestors who CAN'T FREAKIN' STAND the loud internal sound of our own teeth scraping on rocks. And hearing fingernails on chalkboard ...makes our teeth feel funny!

Is the above idea actually correct? Who knows. Well, I think mine's a much better theory than the recent paper about the Macaque warning calls. And I haven't seen any stuff about teeth-on-edge written elsewhere. Perhaps scraping our fingernails on blackboards isn't making fellow primates stop and look for danger. Perhaps it's worse than that, since if the sound is a form of pseudo-pain, then it's forcibly informing all the reptillian brains of all the nearby organisms to STOP BITING THAT ROCK! NOW!!!

Predictions

If I'm right, then it isn't just primates who will respond. Any delicate-toothed creature with a sense of hearing will hate the sound. But how can we test whether non-primate dogs and cats don't like fingernails-on-blackboards? Hmmmm, this might not apply to rodents (lab rats.) Rodent incisors grow continuously, so its less of a big deal when a rat finds itself nibbling on rusty steel or small rocks. Same with sharks and their conveyor-belt of replacement teeth.

If I'm right, then the type of audio waveforms which trigger our revulsion will have major similarity to the audio waveforms heard via bone conduction when our teeth suffer damage by scraping across a hard object. (This might not necessarily be a particular sort of spectrum. If phase between spectral components can be sensed by brains, then features in the waveform itself might be more important for detecting tooth-damage than features in it's frequency spectrum.

Ooo! Idea! Maybe this auditory sensing of tooth-damage is why mammal ears evolved to be near their jawbones in the first place? If ears tended to move much farther away with time, then the bone-conduction sound wouldn't work very well, and that horrible noise of squealing tooth-enamel wouldn't be so... informative. Another thought: toothed-organisms have been around for quite some time, so the tooth-damage problem is evolutionarily old as well.

Going even farther: what if ears originally evolved to be tooth-damage sensors? This would explain where the sense of hearing came from in the first place. Preventing constant bodily damage is an important advancement in the evolutionary track. The ability to detect the sounds of approaching underwater predators might have just been a fringe benefit, a spinoff from the vibrational tooth-damage detector. This leads to another prediction: the atmospheric sound channel wouldn't necessarily have the same evolutionary reinforcement as the through-flesh internal or 'bone conduction' channel. Someone should check to see if the frequency and phase detection of the internal signal channel has any vast differences. Evolution would be expected to redesign ears to become excellent at the two very independent tasks (at least in toothed animals, not in all of them.) In particular, ultrasound is suppressed by air, so we might expect that the internal through-tissue channel might evolve to include the extreme shortwave ultrasound signals of enamel-destruction, while the gas-vibration channel would have relatively little need for such wideband detection.


Subjectively, fingernails/blackboards seem very similar to a pain signal. If you want to give yourself the willies, imagine biting down hard upon a dirty iron bar or on a smooth, dry pebble. Imagine grinding your teeth back and forth on the hard rough object so it squeaks... and CRUNCHES. The surface of the pebble is just like slate, but this time it's not your fingernails making the noise, IT'S YOUR TEETH. YEEESH! For me it's like visualizing razor blades slicing across my lips/tongue, or across my eyeball (Heh. Another "instinctive avoidance algorithm" programmed deeply. Call it by the name Triggered Creepout Effect.)

Now that I think about it, this might also explain the reason for the existence of "baby teeth." If pain doesn't work very well in stopping certain destructive behaviors, and if our instincts and our biology need to somehow *TEACH* us not to bite down hard on rocks... then it will take some time for us to get the hang of it. But with teeth, we only get one chance. The baby might arrive pre-programmed with chalkboard-noise sensitivity. The baby then bites on rocks a few times, then learns not to do this. But by then it's too late, and Baby's teeth have suffered significant damage. No matter. Baby can shed teeth once. No big need to shed teeth in adulthood, since the acoustic pseudo-pain already had taught Baby the lesson earlier... DON'T BITE ROCKS. Hmmm. In the same vein, I wonder if our sensitivity to fingernails/blackboard sound decreases in adulthood? It could wither away without major consequence. Baby animals would have far more need to be sensitive to the signal. If we test whether non-primate animals are sensitive to the teeth-on-edge effect, we should also test youngsters, check out puppies or kittens for a human-like response.

Suppose an animal ignores the tooth-damage sound. Why not just evolve in such a way that teeth are rapidly replaced? But constantly repeated teeth-shedding would cause problems for an organism. A fighting animal can't afford to be missing some teeth unnecessarily. Over tens of thousands of years, humans who have several "multiple dentitions" might get into trouble during the times their old teeth are falling out and the new ones haven't yet grown back. Unless their teeth were almost certainly going to get ruined over time, there would be no good reason for new teeth to grow. Better just learn to avoid biting rocks, and evolve the system to where teeth aren't being replaced.

Yet there might be one significant event where a new set of teeth would give an overall payback: a one-time repair for the months of trial and error during childhood. Hey, nature gives us a cheap set of training-teeth to destroy before the long-term serious ones appear.

All speculation, of course.

If I'm right, then someday the trivia experts will know that there's a clear connection between fingernails on blackboard, the position of human ears and jawbone, and the presence of human "baby teeth."

And when people say that the sound of fingernails on blackboards always "sets their teeth on edge"... or if they call it "teeth-gnashingly annoying"... we should leap up and shout "That's IT!"

PS
Fingernails-on-blackboard is pseudo-pain. If I'm right, then ANY toothed animal is programmed to respond to it. That means we can build pain guns! Or actually "pain" guns. If you're being attacked by a predator, just take out your patented Bill Beaty Tooth-Annoyatron and press the button. The agonizing high-power recordings of garden hoes scraping across dusty shale sidewalks will send those sharks or cougars racing away... while holding their jaws carefully apart!

:)

LINKS


 



SOME EMAIL...


From: "sh"
To: 
Subject: re: FINGERNAILS ON BLACKBOARD
Date: Mon, 11 Aug 2003 08:43:54 -0400

Sorry, nice try but you're off the mark (although I particularly liked the
tie in between the jawbone and the ear to shift it to a tactile pain
analogy - it's an interesting proposition).  There have been several
studies which demonstrate that the aspect of the "fingernails on
blackboard" (or metal rake on concrete, in my case) sound which causes so
much distress is actually due to aperiodic repetition of sounds in the 8-13
kHz range - well outside any tactile or vibratory input (Halpern et al,
1986).  In fact, it seems to be related to high frequency sounds made by
human (or other primate) infants (Lounsbury & Bates, 1982) - with the
aperiodicity (which could be caused by extreme distress, interrupted
breathing etc) increasing the irritation factor.  You can actually
synthesize sounds in the highest frequency range which induce a similar
reaction that are barely audible to most adults, but induce the reaction
anyway (e.g., exposure to the ultrasonic components of machinery or dental
drills can induce fear or anger).

Incidentally, just to let you know I'm not just spitballing here, I'm an
auditory neuroscientist who has worked on this type of thing for a long
time, and have a company which specializes in using sound to evoke specific
emotional states with applicaytions in film, music and software(NeuroPop). 
But I really love your website and have been coming to it for many years -
keep up the good work, fun links and interesting questions

SH, PhD


Date: Tue, 12 Aug 2003 01:15:22 -0700 (PDT)
From: William Beaty <>
To: "sh"
Subject: re: FINGERNAILS ON BLACKBOARD

On Mon, 11 Aug 2003, sh@.com wrote:
> Sorry, nice try but you're off the mark (although I particularly liked the
> tie in between the jawbone and the ear to shift it to a tactile pain
> analogy - it's an interesting proposition).

I certainly admit that it's total speculation!   :)

>  There have been several
> studies which demonstrate that the aspect of the "fingernails on
> blackboard" (or metal rake on concrete, in my case) sound which causes so
> much distress is actually due to aperiodic repetition of sounds in the 8-13
> kHz range - well outside any tactile or vibratory input (Halpern et al,
> 1986).

Uh... why do you mention "well outside tactile/vibratory"?  I don't
understand what this means, or how this disproves the bitten-stone idea.
Biting a stone generates the high-freq sound in question.

If "bitten-stone" is wrong, I'd like to know the details of why this is
so. (Also, if you try to disprove the idea that a bitten stone makes
people cringe, won't you also disprove the idea that the fingernails/
chalkboard sound makes people cringe?  To me they sound almost identical.)

By "tactile" are you talking about skin sense as opposed to hearing?  The
noise of chalk on chalkboard is certainly SOUND, not tactile or skin
vibration, and it's certainly broadband and has components well above the
one-KHz range.   It would be interesting to put a contact microphone on my
molars and then examine the sound spectrum of stone-biting.



> In fact, it seems to be related to high frequency sounds made by
> human (or other primate) infants (Lounsbury & Bates, 1982)

I've NEVER heard any sounds made by my baby daughter or by animals on TV
nature shows which can trigger my "fingernails/chalkboard" response.  
That's why I concluded that the primate-screech idea was a load of BS as
soon as I heard it proposed.  It's simple: those sounds don't make me
cringe. It's a nice theory, but it doesn't work in the real world.  I've
also never heard anyone else ever complain that monkey screeches make them
cringe in that way.

But a metal tool dragged across a rock DOES make me cringe.  So does a
tooth dragged across a dry pebble.  My own "fingernails/chalkboard"
response is all about mechanical oscillators which suffer an outbreak of
chaos, and I've never felt that "cringe" response from any vocal-style
noises.

If someone has a recording of animal sounds which reliably trigger the
usual fingernails/blackboard "cringe" feeling, I'd like to know about it.
(If they exist at all, I'd expect that they already would be as well known
an irritation as dragging fingernails on blackboards!)

On the other hand, there are a wide variety of squealing noises which
trigger the effect for me personally, and all of them involve Dynamical
Chaos in mechanical oscillators.  Stick-slip Chaos of damped/driven
oscillators is usually called "bearing chatter."  Dragging a piece of
chalk backwards across a chalkboard is a classic example of a Chaos
singnal.

> - with the
> aperiodicity (which could be caused by extreme distress, interrupted
> breathing etc) increasing the irritation factor.  You can actually
> synthesize sounds in the highest frequency range which induce a similar
> reaction that are barely audible to most adults, but induce the reaction
> anyway (e.g., exposure to the ultrasonic components of machinery or dental
> drills can induce fear or anger).
>
> Incidentally, just to let you know I'm not just spitballing here, I'm an
> auditory neuroscientist who has worked on this type of thing for a long
> time, and have a company which specializes in using sound to evoke specific
> emotional states with applicaytions in film, music and software(NeuroPop).

Cool!


Are you aware that the aperiodic "sounds of chaos" always have a particular 
spectrum which contains fractal features?

I've long wondered exactly what it is about the fingernails/ blackboard
sounds which triggers my own response.  Would a simple high frequency tone
with random chopped modulation do it?  If not, then maybe my brain is
keying in on something in particular, such as a signal which contains a
*fractal distribution* of frequency peaks, one-over-F pattern.

Might you know if fractal frequency distributions tend to trigger the
fingernails/chalkboard response?

(((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))
William J. Beaty                            SCIENCE HOBBYIST website
                            http://amasci.com
EE/programmer/sci-exhibits   amateur science, hobby projects, sci fair
Seattle, WA  206-762-3818    unusual phenomena, tesla coils, weird sci



Date: Sun, 7 Mar 2004 20:01:41 -0800
From: Tracy Butler 
To: 
Subject: Comments from billb amform

--- comments ---
I came across your site while searching for blackboard paint (to disguise
an ugly file cabinet plus make it useful), but leaving off the t.  Your
explanation for the nails on chalkboard effect is perfect... and testable:
functional MRI during the sound labeled as "nails on chalkboard" versus an
identical sound labeled as something innocuous like "teakettle" should
activate the teeth area of secondary somatosensory cortex.  I'm a
neurologist doing fMRI research, and I'll credit you if I ever end up
doing this.  There's actually quite a bit of funding for dental pain
research.  Thanks for a fantastic website.


 





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