Explained at last?
2003 W. 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
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.
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. It's not learned. Instead, 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, as strongly as a
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 didn't
anyone realize this origin? 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
pain-avoidance 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,But SOUND is not pain. Our 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 broken teeth. We're the product of a billion years 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 papers 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!!
PredictionsIf I'm right about all of this, then it isn't just primates who will respond. Any delicate-toothed creature with a sense of hearing/vibration will hate the sound. It might be evolutionarily old; going back all the way to mollsuks or earlier, to the original appearance of hard bioapatite shells, teeth, and predation. But how can we test whether non-primate dogs and cats (and clams? snails?) can't stand 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. Or, perhaps it's tooth-damage plus mirror-neurons of animal groups with group-selection pressure, so dog packs might have the aversion, while lone pumas might not. With others, the aversion would be triggered by knawing on soft material, with bursts of the aversive sound added, in sync to jaw closing. (Animals who don't care when others bite rocks, *would* care when they themselves seemingly bite rocks.)
If I'm right, then the type of audio waveforms which trigger our
revulsion will have major similarity to the audio waveforms heard via
tissue/bone conduction when our teeth suffer damage by scraping across a
This might not necessarily be a particular sort of spectrum shape.
If phase between spectral components can be sensed by brains, then
time-domain features in the waveform itself might be more important for
tooth-damage than features in it's frequency spectrum. Or, perhaps the
internal bone-conduction signal is the avoidance signature, and if sounds
in air gets filtered so they resemble the tooth-damage signal inside
tissues, that triggers our avoidance response.
Ooo! Idea! Maybe this auditory sensing of tooth-damage is why
mammal/reptile/etc. 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.
Going even farther: what if ears originally evolved to be tooth-damage
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 in the jawbone. This
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 from ears. Evolution would be expected to redesign ears to
excellent at the two very separate 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
far about 20KHz,
while the gas-vibration channel might have relatively little need for such
Subjectively to its victims, fingernails/blackboards seem very similar to
a pain signal. Instant inbuilt avoidance! 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. (Almost as bad as
imagining thorns under fingernails, or sharp branches poking towards your
eyes.) The surface of the chewed pebble is just like chalkboard slate, but
this time it's not your fingernails making the noise, it's your
Yeesh. For me all this is like like visualizing stroking my fingertips
along a knife edge, or watching Buñuel's sliced-eyeball film (Heh.
Another "instinctive avoidance algorithm" programmed deeply. I call it by
the name Triggered Creepout Effect: the everyday evidence of mammal
instict remaining in humans.)
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. We could have the
pseudo-pain avoidance response built in, but we don't instinctively know
that bone is OK to knaw, but pebbles are not. But with teeth, we only get
one chance. First, the baby arrives pre-programmed with chalkboard-noise
sensitivity. Then baby bites on rocks a few times, experiences
revulsion-sound, and learns not to do this any more. But by then it's too
late, and Baby's teeth have suffered significant damage. No matter.
Baby can shed teeth once. Finally no big need to shed teeth in adulthood,
since the acoustic pseudo-pain already had taught Baby the lessons
earlier... don't knaw rocks or mommy's socket set. Hmmm. In the
vein, I wonder if our sensitivity to fingernails/blackboard sound
decreases in adulthood? As with childhood pain extrasensitivity, the
pseudo-pain could wither away without major consequence. Baby animals
would exhibit far more need to be sensitive to the signal. If we ever
test whether non-primate animals are sensitive to the teeth-on-edge
effect, we should also test across age, check out puppies or kittens for a
human-like response childhood extreme response.
Suppose an animal ignores the tooth-damage sound. Why didn't we just
evolve in such a way that teeth are rapidly replaced, like rats and
sharks? But this constantly repeated teeth-shedding would cause problems
for mammals. A fighting animal can't afford to be missing some teeth
unnecessarily. Over tens of thousands of years, the few humans who have
ongoing "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 stone adzes and flint scrapers and obsidian arrowpoints and
socket sets, and instead evolve the system to where teeth aren't being
Yet as above, 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 the babyhood knawing stage. Hey, nature gives us a
cheap set of "training-teeth" to destroy, before the long-term serious
All speculation, of course. But it's like a missing puzzle-piece
evolutionarily. Man o man, look at the explanatory power.
If I'm right, then someday the trivia-gam experts will know that there's a
clear connection between fingernails on blackboard, the position of human
ears and jawbone, and the need for our "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 "eureka That's IT!!!"
SOME EMAIL...From: "sh"
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
Date: Tue, 12 Aug 2003 01:15:22 -0700 (PDT)
On Mon, 11 Aug 2003, sh@.com wrote:
I certainly admit that it's total speculation! :)
> There have been several
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.
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
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
> - with the
> Incidentally, just to let you know I'm not just spitballing here, I'm an
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
(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
Date: Sun, 7 Mar 2004 20:01:41 -0800
--- comments ---
 FREQUENCY: I find that vibration frequencies conducted in solids go up into at least hundreds of MHz, not 13KHz. Also, there's a little- known phenomenon where humans can hear 70KHz just fine under tissue conduction, and the detector is the human otolith/utricle