Kind: captions
Language: en
I want you to listen to these two sounds
and decide which is higher so this is
Sound a and this is sound
B okay so to me sound a is clearly
higher but that's strange because sound
a was just a 100 HZ sine wave sound B
had that same 100 HZ frequency but also
150 HZ and 200 HZ so we added higher
frequencies but the sound was lower how
does that work I think there's this idea
that what our ears do is simply detect
the frequency of vibrations in our
environment that are between 20 HZ and
20,000 Herz but there is so much more to
hearing than that and in this video
we're going to go through a series of
audio illusions that illustrate how our
sense of hearing actually works most of
these effects will work on a phone or
laptop speakers but if you have
headphones handy well I'd recommend
putting them on for The Full Experience
[Music]
[Laughter]
it's like a whole body instrument isn't
it absolutely yeah yeah this is the
Sydney Town Hall pipe organ when it was
built in 1890 it was the largest organ
in the
world something I didn't realize about
organs is that they were meant to sound
like many different instruments playing
together organs are sort of a onep
person
[Applause]
Orchestra very fluty right yeah you can
tell compare that to a
trumpet OBO sound
[Music]
[Applause]
[Music]
so you can hear the all these orchestral
sounds on the on the
organ we could get inside the instrument
too should we go look let's have a look
yeah okay for each instrument there are
a series of pipes in the organ which
play all the different notes for that
instrument I mean there are 8,000 pipes
in this organ 8,000 8,000 yeah why do
you need that many to create all the
different sounds of the orchestra what
you see on the outside is just a tiny
fraction of the organ itself wo look at
all these they're all hidden in here
they are yeah and some are wooden some
are metal some have resonators at the
bottom of them uh to create the more Rey
sounds the brassy sounds but then these
wooden ones are more of the deep fluty
sounds as well so this is like what a
keyboard it's a keyboard yeah that's
right a keyboard layout yeah pipes
[Music]
very nice yourself when two pipes of the
same length vibrate they both play the
same
note that's because they're both
producing the same fundamental frequency
that is the lowest and usually highest
amplitude vibration they produce but if
the pipes are made of different
materials they will sound different so
you can tell they are different
instruments and that's because each one
produces a distinct set of high
frequencies called overtones they're not
as loud as the fundamental and we don't
hear them as distinct tones but they
affect the quality of the sound called
tamber it's how you can tell apart a
trumpet from say a
flute they have overtones of different
frequencies and relative
amplitudes for a lot of instruments the
most common overtones are integer
multiples of the fundamental frequency
these are known as harmonics
this was your fundamental note yep the
notes you're going to be hearing with it
would
[Music]
be so all of those notes are within that
fundamental note now harmonics can be
useful when you're trying to play really
low notes the Sydney Town Hall pipe
organ is one of only two in the world
that has a 64t long pipe it's actually
so large that it has to be folded over
itself where is the 64 can we can we see
it you've got the grand question that I
don't know I know it's somewhere here
like that's a really big um chunk of
wood right here oh yeah could that be it
this pipe is used to produce a frequency
of 8 Hertz when you get to that level
it's more something that you feel rather
than something that you hear for sure
the lowest note most big pipe organs can
play is 16 Herz which is just at the
limit of human hearing but even this
requires a 32t pipe which is too big or
expensive for many organs do you know
where the 32f Footers are well they're
they're the ones at the front they're
the they're the ones that you see at the
front that one that's the 32- footer
yeah so that's nearly 10 m nearly 10
MERS yeah it's pretty scary to think
about from the top of it actually in
18th century Europe gor Joseph Vogler
was a popular organist he wanted to tour
the continent but that would require
building a compact portable organ that
he could take with him on the road now
he obviously couldn't haul around the
huge 32t pipe required to produce 16
Hertz so how could he still create the
low frequencies that make the organ so
powerful well Vogler realized that if he
played the harmonics of 16 HZ using
shorter pipes your brain would hear this
missing fundamental
can we try the trick and see if it sure
the fifth is that sort of most common
fundamental which you're going to hear
um to get the low sound but basically
the Quint gets used with a 16 foot and
it creates the lower resultant tone so
that's just the six
metal it's so funny because you add it
and I do not hear it going up you're
you're playing a fifth above yes but I'm
hearing it go down like you just pull
that out and I'm like oh yeah the note
dropped that's the trick yeah with the
two sounds I played at the beginning the
first was a pure 100
HZ but the second sound was made up of
the harmonics of 50
HZ so you actually heard this
fundamental frequency even though it
wasn't there that's how higher
frequencies together can sound lower
than low frequencies if they are
harmonics of a low
fundamental now this might not be as
weird as it seems if you look at the
wave form of the harmonics you find that
adding the higher frequencies changes
the period of the sound it makes the
period longer so that it's actually the
same as the missing
fundamental if you kind of recreate some
of those harmonic pitures you're
actually going to bring out more bass in
the in the sound so so the idea of like
you could play the harmonics and he the
fundamental even even if you're not
playing the fundamental that's right
yeah so different frequency sounds can
combine to make notes that aren't there
but they can also do something even
stranger hello it's me Mario in Super
Mario 64 there's a staircase that seems
to go on forever players can't level up
until they collect enough coins now
listen carefully to the music the scale
sounds like it keeps going up and up and
up just like the endless staircase
again this is the shepherd tone illusion
and here's a shepherd gando on its
[Music]
own an ever increasing tone should be
impossible because we can't hear
anything beyond the 20,000 Herz limit
and yet this sound keeps going always
ascending the trick is a Shepherd tone
isn't just one note there are multiple
frequencies being played all separated
by octaves all of these frequencies are
increasing but as they do their volumes
change so the high notes get quieter and
the low notes get louder high notes soon
Fade Out and new low notes are faded in
this gives the illusion of an Ever
Rising pitch like the audio version of a
barbershop
pole Shepherd tones can also evoke
emotional or physical responses in some
listeners a 2016 study found that after
listening to sheer tones participants
reported feeling nervous anxious and
Disturbed perhaps that's why during an
intense bombing scene in the film
Dunkirk Shephard tones feature in the
accompanying
score hopefully this won't make you
uneasy but I want you to try to figure
out which well-known tune this is all of
the notes have been kept the same but
they've been mixed up into different
octaves
did you recognize the song well here is
the unscrambled
Melody but now that you've heard that
can you follow the scrambled version
to me it's fascinating how the second
time I heard the scrambled Melody the
tune seemed obvious which is very
different from how it sounded the first
time our brains can find patterns in
random sounds too this is the Phantom
word illusion created by Dr Diana
Deutsch listen to this audio and try to
figure out which words are being said
you can put what you heard in the
comments
when one speaker plays a word the second
speaker plays a different word at the
same time according to Dr Deutsch
because the signals are mixed in the air
before they reach your ears you're given
a pile of sounds to choose from so you
can create words in your
mind a lot of what we hear depends not
on the frequencies of sound but on how
our brains process them Dr Deutsch
noticed that when she played this
illusion near exam week students were
reported hearing words like no brain I'm
tired or no time and we can actually
Prime the brain to hear what we want it
to hear for example using text take the
case of this crowd
chanting you're primed to hear the
lyrics you
see these are called mandin after a
misheard poem in which there's a line
they have slain the Earl of
and Lady monag green except in the real
poem The Earl dies alone and his Killers
actually laid him on the
green sometimes mtag greens happen when
sounds are divided logically but
incorrectly such as hearing pullet
surprise instead of pullet surprise
language familiarity would help you hear
the correct one from the start so while
UK football fans might hear the common
chant that is
embarrassing an American football fan
might
not what's even more amazing is how
subtle visual cues can affect what we
hear what am I saying in this clip bear
bear bear if you heard the word bear
that's because that's what I was saying
but what am I saying in this clip bear
bear bear now I bet you heard fair but
if you play back both those clips
without looking
you'll find it's the exact same audio
all we changed was the mouth movement
and I can prove it to you by playing
those two clips at the same time and
what you hear will change depending on
which clip you focus on bear bear bear
bear bear
bear so what we see affects what we hear
and the reverse is also true in this
illusion if no sound is played it looks
like the two circles are passing through
each other but add a sound when they
intersect and immediately it seems like
they're bouncing off each
other what we see and hear are
intrinsically linked because in the real
world one sense can reliably inform the
other but what if there are no visual
cues to go on in the 1950s air traffic
controllers were communicating with
multiple Pilots simultaneously in the
same room unfortunately messages from
all of the pilots would play from a
single loudspeaker and the overlapping
audio made it really difficult to pick
out just one voice so researchers
started looking into the so-called
cocktail party effect because this
problem resembled focusing on a single
voice in a noisy room most of us can do
this with little effort but how it's
kind of like taking the recording of the
entire party and pulling out a
particular voices wave form the sound
waves interfere with each other before
reaching your ears so this should be a
difficult task in this recording try to
find the voice talking about a flight In
This
Crowd I find that really hard but if you
hear the voice first then the rest of
the conversation is easier to follow
this is much easier because you can
predict what words will come next based
on context and language
structure the second way we can focus on
one voice is by identifying where the
sound is coming from listen again but
this time focus on the pilot played in
your left
ear in a cocktail party you can focus on
your friend by ignoring sounds that come
from other locations once researchers
realized this they advocated that
different Pilots be broadcast through
different speakers spread out throughout
the control room this allowed air
traffic controllers to more successfully
tune in to their
[Music]
pilot but how do we actually locate the
source of a sound well I'm going to put
on this blindfold and ask my wife to
walk around me and clap in different
locations and I am going to try to point
to the location of the sound so let's
give it a
try normally you can pinpoint a sound to
within a degree or two and there are
actually four different cues that help
me identify the location of the sound
how was it the first is volume a sound
on my right will be louder in my right
ear my head sort of casts a sound Shadow
over my left ear and the second cue is
that this Shadow attenuates higher
frequencies more than low frequencies
it's kind of like when your neighbor is
having a part you can't really hear the
high frequencies like the lyrics but you
can hear the bass because low
frequencies are less attenuated by
distance and obstacles the third is time
delay it takes a sound half a
millisecond to cross your head so sound
will usually arrive at one ear before
the other listen to a beep on your
left and then on your
right now as the delay between those two
beeps is shortened
it's less of an
echo and more just one sound that's
really on your
left the fourth cue we use to identify
the source of a sound is at what point
in the wave cycle the sound arrives at
each ear or the phase of the wave is it
arriving at a peak or a trough the phase
of the wave at one ear will typically be
different than the phase at the other
year now you run into a bit of trouble
when the sound is either directly in
front
or behind you or on any point in a
vertical plane that passes through the
middle of your head and that's because
the distance from the sound to both of
your ears is the same and therefore
those four qes aren't very
useful owls solve this issue with
asymmetrical ears their left ear is
actually lower on their head than their
right ear so sounds from below are
louder in their lower left
ear humans typically have symmetric ears
but their shapes are important this is
where the outer part of your ear comes
in I mean what we normally just refer to
as the ear technically this is called
the PA depending on the location and the
frequency of sound it will bounce off
these ridges and bumps on your ear and
end up inside your ear actually going
into the eard drum and those Reflections
will actually change some frequencies
differently than others depending on the
location scientists placed tiny
microphones inside volunteers ears to
measure this they you could see for
example that a 6,000 Herz sound located
above you might be Amplified by 10 DB
but that same sound below you would be
attenuated by 10 DB these figures depend
on the unique bumps and Ridges of
cartilage in your ear so each person's
ears have a unique response curve to
different frequencies at different
locations and over the course of our
lives our brains learn the way different
frequencies reflect off our ears and we
use that information to identify the
source of the sound now every person has
a unique P shape so what if our ears
changed in a 1998 study researchers
placed small molds into the ears of a
group of participants changing the shape
of their penas here's one subject's data
the rigid background grid represents
where sounds were actually played the
dots are the subject's guesses and the
darker warped grid is the averages of
those guesses before the study they were
fairly good at locating sounds but after
changing their pen shape they were
downright terrible over a series of days
and weeks with their new penas the
participants all adjusted and became
better at locating sound so it is
something your brain can adapt to
thankfully after the molds were removed
participants had no trouble reverting
back to their original
ears penis shape is so key to an
immersive sound experience in virtual
reality the companies like apple and
Sony actually scan your ears to create
personalized spatial
audio and for a long time people have
been trying to harness and amplify our
ability to locate sounds in 1880
Professor Alfred Mayer presented a
device called a topop phone to locate
ships in the fog it was made from two
adjustable hearing cones by changing the
distance and angle between them Sailors
could narrow down on the direction of a
ship's fog
horn Unfortunately they weren't very
useful because sound waves interact with
fog but then during World War I locating
bombing planes on approach was of
central importance so armies developed
special equipment called sound mirrors
to amplify sound in Britain Sound Mirror
stations coordinated together to locate
an enemy up to 15 minutes in advance but
as planes became faster sound mirrors
couldn't detect them early enough and
they were eventually abandoned after the
invention of radar but even though the
technology became obsolete the system
was not the radar team used the
coordinating stations idea that was
first developed from The Sound Mirror
program linked radar stations were a
critical defense in the Battle of
[Music]
Britain this is is the Vox Angelica on
the Sydney Town Hall organ when two
pipes are slightly Out Of Tune there's
this pulsing effect to the
sound you can hear that in a more
pronounced way if I play Pure tones here
is a pure 261 Herz sine
wave and a pure 263 HZ sine
wave when both of these tones play those
compression and refraction waves
interfere with each other sometimes the
Peaks line up to produce a louder
sound and when a peak lines up with a
trough they cancel out because these
frequencies are separated by two Hertz
you hear two louder pulses every second
this is known as
beating now the beats are really
clear and this makes sense when the two
waves are interfering in the air but
what happens if a 261 Herz tone is
played in one ear and a 263 HZ stone is
played in the
other what did you hear well the tones
never had a chance to interact but you
can still hear some subtle beating your
brain is firing at a rate corresponding
to the phase difference causing the beat
perception when your brain mixes these
frequencies together it's called
binaural
beats and maybe you've already heard of
binaural beats as a quick search of
YouTube shows that some people claim
they can improve focus or memory but a
2023 review was inconclusive and
emphasized the need for more
standardized testing
methods audio illusions aren't a sign
that our sense of hearing is faulty I
mean the world is a messy noisy place
and our brains have developed complex
methods to deal with ambiguity you fill
in the gaps with your past experiences
or expectations without your brain
making these subconscious adjustments a
cocktail party would always just sound
like a total mess audio illusions show
us where our perception goes wrong but
the system as a whole is pretty good at
getting to the
[Music]
truth now Illusions remind us that we
can't always take the world at face
value and while our unconscious minds
might fill in the gaps from time to time
it's our critical thinking skills that
do the heavy lifting of separating fact
from fiction and if you're looking to
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