hey, welcome to 12tone, and happy Halloween!
last year we looked at some of the tricks that film composers use in order to make horror
movies sound scary, but it turns out that the scariest sounds may be the ones you don't
hear at all.
what do I mean by that?
well, let's talk about how sound works.
what we hear as sound is actually just a pressure wave traveling through the air.
our ear picks up the vibrations, and our brain converts them into what we interpret as noise.
these sound waves have two main properties: amplitude and frequency.
amplitude is the difference between the high-pressure peaks and the low-pressure troughs.
that is, it's a measure of how hard the air is vibrating, and we perceive it as volume.
louder sounds create more powerful pressure waves, and we measure that effect in decibels.
decibels are a logarithmic scale: if you double the total sound level, you've increased the
volume by about 6 decibels.
prolonged exposure to sounds over 85 decibels, roughly as loud as a lawnmower, can lead to
permanent hearing damage, while sounds at 120 decibels or higher, like a gun shot, can
cause physical pain.
frequency, on the other hand, is all about the speed of the vibrations.
we measure this in hertz, which is just how many times the wave cycles from high pressure
to low and back to high within a single second.
frequency information is processed as pitch, with faster waves perceived as high and shrill,
while slower waves are low and booming.
actually sound waves are a little more complicated than that, but it's close enough for our purposes
here.
if you've looked at this sort of thing before, you may already know that the octave, our
most fundamental interval, is a doubling in frequency.
that is, if we make the wave twice as fast, it sounds really similar.
and the same thing works in reverse: here's a wave at 320 hertz, which in terms of pitch
is somewhere between a D# and an E in the middle of the piano.
if we slow it by half we get 160 hertz, which is the same note an octave lower . doing that
again gets us to 80 hertz, 40 hertz, and then 20 hertz, and if you can't hear that one,
don't worry.
20 hertz is generally considered the bottom of human hearing.
and it gets worse: not only is it hard for you to perceive it, but there's a good chance
the speakers you're using can't produce it.
manufacturers know that you can't really hear this range anyway so they often cheap out
on making it work.
ok, I'll stop playing the note now.
but even if your speakers can't play it, high-quality subwoofers like the ones used in movie theaters
can.
they can even go lower than that, creating what's called infrasound, because it's too
low to be perceived as sound.
at high enough volumes it does become audible, sort of, but it has to be really loud, it
doesn't carry tonal information, and it can be pretty effectively masked by other, higher
frequencies.
instead, it's mostly perceived as a deep rumbling sensation, almost like a miniature earthquake,
which makes sense because a really loud, really low sound wave is pretty much what an earthquake
is.
that rumbling can cause a lot of anxiety, especially because it has no apparent source.
for obvious reasons, humans like to know what's causing the things we experience.
like, if a large truck drives past you and you feel the ground shake a little, that's
probably not going to scare you because you know where it's coming from.
you don't have to worry because you've seen the truck and you know it's not a threat.
but if you're just walking down an empty street and the ground starts rumbling?
that could mean danger.
that could mean death.
infrasound is thus really effective at building tension before the scare.
studies have shown that exposure to high-volume infrasound can disrupt cortisol levels, which
are associated with stress.
it tells us that something's wrong, even though we haven't actually seen it yet nm.
it's like walking down a dark alleyway in the middle of the night: everything may look
fine, but you don't actually feel safe.
infrasound helps create that sort of environment within the movie theater, where you sense
a looming threat but have no idea where it might come from.
infrasound may even be responsible for more disturbing phenomena.
the resonant frequency of the human eyeball is 19 hertz, just within the infrasound range,
and if you play that frequency loud enough, it can cause your eyes to twitch involuntarily.
if you turn it up louder it can even cause distortions in your vision, as the sound warps
your eyeballs, applying pressure to your photoreceptors and sending signals that your brain incorrectly
interprets as light.
this phenomenon has been blamed for numerous ghost sightings in haunted labs and the like
that turned out to be the result of bent fan blades and other infrasonic events.
it's worth noting here that this is, as far as I could tell, an under-researched field
and that a lot of these ideas are just that: ideas.
in fact, the mythbusters even tested whether infrasound could make a place seem haunted
and came to the conclusion that it couldn't.
however, I've watched the episode and there's one possible caveat I want to point out: volume.
they never give a specific decibel number for the sound they're playing, and they even
talk at one point about having to turn their speakers down to avoid rattling the dilapidated
cabin they're using as a testing site.
given that visual distortions allegedly require amplitudes of 110 decibels or more, it's not
clear that they were actually playing their infrasound loud enough to cause problems.
but while the effects are understudied so we don't know for sure what infrasound does
to the human body, the possibilities are scary.
while shaky eyes and ghostly visions may seem bad enough, the book Acoustic Weapons by Seth
Horowitz details some even more disturbing risks.
after all, the eyes aren't the only vulnerable organ.
at around 130 decibels infrasound can disrupt your inner ear, messing with your ability
to understand speech.
150 decibels causes whole-body vibrations and possibly nausea.
and around 170 decibels the air in your lungs could start to warp, making it hard to breathe
and maybe even inducing artificial respiration as the pocket of air expands and collapses
in time with the infrasonic wave. and, at 240 decibels, with the right frequency, it's
possible you could shake a human head so violently it explodes.
but I wouldn't worry too much about that: at 194 decibels, the wave is strong enough
to create a literal vacuum in the low-pressure troughs, so reaching 240 in order to create
a weapon isn't very practical.
which brings us to infrasound's most famous use: the brown note.
this is a rumored frequency that, when played at a high enough volume, disrupts the human
digestive system and causes you to… well, you know.
however, as far as I can tell, there are no actual documented cases of this happening,
and controlled attempts to reproduce the effect have all failed.
thankfully, it just doesn't seem to be a real thing, at least not at volumes that are otherwise
safe.
but back to horror movies, infrasound is becoming a more common tool as speaker technology improves.
these days a high-end home theater system can pretty effectively produce those lower
frequencies, and as we develop further, generating infrasound in your home is going to get easier
and easier, so expect to hear more of it in the future.
or not hear it, I guess.
you know what I mean.
anyway, thanks for watching!
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