Transcript PowerPoint-Ear - Living Stones

THE HUMAN EAR
What can we hear?
We can hear sounds from a frequency
of 20 Hertz [that is twenty vibrations
per second] to an upper limit of
20,000 Hertz [that is twenty thousand
vibrations per second]. This upper limit
decreases with age.
We say that the human hearing range is from 20 Hz to 20 kHz [kiloHertz].
20Hz is deep bass and 20kHz is very high treble.
We are most sensitive to sounds in the 2 – 3kHz region and much less sensitive to low
bass and high treble sounds.
We can detect a very wide range of sound levels over a ratio of about 1000000000000:1
This is often called a ratio of 120dB.
What is sound?
Any surface that
vibrates in air
produces sound
waves
A loudspeaker is a
good example.
Travelling Sound Wave
A sound wave in air consists of pressure variations which travel away from the source.
This causes the air molecules to “ bunch together “ in the higher pressure regions.
These are called “longitudinal waves”.
The parts of the ear
Schematic diagram of the ear
“3D” view of the ear structure
The outer ear
The sound waves
go into the ear
and travel down
the ear canal.
They strike the
eardrum and
some of the
wave is
reflected.
This reflected wave combines with the incoming wave to produce a
resonance at a frequency around 3.7kHz.
This means that our hearing in this frequency range is very sensitive.
The eardrum
The eardrum vibrates when the sound
waves bounce off it
The vibration is at the same
frequency as the incoming
sound wave
This vibration is transferred to the
bones of the middle ear [called the
Ossicles]
The middle ear
The three bones in the middle ear are
necessary in order to allow the eardrum
vibrations to actually get into the COCHLEA
where the sound waves are detected.
Without these bones the sound
waves would simply bounce off the
cochlea and there would be no
transfer of vibrations or energy.
The bones allow a much higher
pressure to be exerted on the
cochlear fluid, thus enabling it to
be moved.
The Cochlea: Why It’s Shaped the Way It Is
Starting with a simple model of a spiral, the research team calculated how the
cochlea’s curve affects the movement of sound energy inside the inner ear. Like the
Whispering Gallery in St. Paul’s Cathedral in London, whereby a whispered voice
travels along the curved wall to be heard on the opposite side of the room, the
researchers found that sound energy preferentially travels along the outer wall of the
cochlea. As this occurs, the fluid sloshes higher on one side, the basilar membrane
wave tilts more to one side, and the hair cells, which are sensitive to motion at right
angles to the direction of the wave, get an added push, causing them to bend. The
effect is strongest at the centre of the spiral, where the lowest frequencies are
detected, said the researchers, who calculated an increase in sensitivity by as much as
20 decibels. The ability to hear low-frequency sounds, which travel the farthest of all
sounds, is especially important to mammals for communication and survival.
VERY CLEVER DESIGN
What biologists miss out when they describe the ear!!
Interesting fact: The speed of sound in water [like cochlear fluid] is about 1500 m/s
The unwound length of the cochlea is about 3.5 cm
It takes a sound wave about 23 microseconds to travel down the cochlea and
the same time to go back down to the round window.
This corresponds to a frequency of about 22kHz which is roughly our upper limit
So the shock wave
This means that all the fluid in the cochlea can move together at normal frequencies.
It goes to the end of the spiral and through a gap and then back down again. This is
why the round window is needed. When the oval window is pushed in, the fluid
motion pushes out the round window. Without the round window, the cochlea could
not function.
BUT There is another kind of wave that travels down the cochlea
Basilar membrane waves
These travel down the cochlea at a much slower speed than sound waves.
Due to the steadily increasing width of the basilar membrane, different sections
respond best to different frequencies, allowing us to hear pitch and tone.
The width of the Basilar membrane increases along its length. This enables the
various points along its length to respond to different sound frequencies.
The cochlea
The hair cells and nerves that detect vibrations are located in the Organ of Corti.
The Organ of Corti
Outer hair cells do not send nerve
signals to the brain, but they
mechanically amplify low-level
sound that enters the cochlea. The
amplification may be powered by
the movement of their hair
bundles, or by an electrically
driven motion of their cell bodies.
The inner hair cells transform the
sound vibrations in the fluids of
the cochlea into electrical signals
that are then relayed via the
auditory nerve to the auditory
brainstem and to the auditory
cortex.
The design and function of the human ear is so complex and well
designed that it could not possibly have evolved.
An integrated functional structure like this could not have developed
“gradually”. The whole system needs to be in place for hearing to
function properly.
But there is yet more
We need to have an auditory analysis system to make sense of the nerve
signals sent out by our ears. Ears without analysis are as much use as a TV
broadcast without a television.
What can the ear/brain system do?
Identify musical notes/frequency
Appreciate harmony
Identify a musical instrument by its harmonics and its attack/decay profile
Hear individual instruments when several are playing simultaneously
Recognise speech
Isolate a conversation in a noisy environment
Locate a source of a sound in front of us [stereo hearing]
This level of auditory processing/analysis cannot be matched by any computer system.
And finally ……………..
to give a balanced presentation
THE SEMICIRCULAR CANALS
These are three hollow fluid filled
tubes next to the cochlea.
They don’t help our hearing but the
loose particles can stimulate nerve
cells .
When we rotate, the fluid in the
tubes resists being turned, and
flows in the tube thus stimulating
nerve cells.
This tells our brain whether we
are stationary or starting to
rotate about any axis.
HOW EVOLUTIONISTS BELIEVE HEARING STARTED
The anatomy of the first amphibian ears was quite a bit simpler than modern ears. It is
believed that early amphibians and even some amphibians living today, such as
salamanders, heard by picking up vibrations through the ground and water. These
vibrations would be picked up by the creature’s entire body but mostly by the dense parts
such as bones. As the vibrations travelled through the body, they would reach the inner
ear. This primitive inner ear contained a fluid filled region that was surrounded by dense
bone. The vibrations would cause the dense bone to resonate, which in turn would
transfer the vibrations to the fluid. Tiny hairs in the fluid region would convert the fluid
vibration into electrical signals that would travel through nerves to the brain. The brain
would then interpret these signals as sounds. This structure was sufficient in the water
because the vibrational energy in water was adequate to cause the dense bones to vibrate.
In air, however, the vibrations have much less energy and are therefore more difficult to
interpret sounds with this structure. Indeed the first amphibians, like modern salamanders,
probably heard the world by picking up vibrations through their skulls from the ground.
Regardless of these limitations, it is clear that vertebrates had the early stages of an inner
ear shortly after they crawled up on to the land (Manley, 1998).
So you must have a fluid filled bone cavity plus specialised hair cells plus dedicated nerve
pathways plus a brain that can interpret these signals as sound and respond in some
positive way so as to give a motive for natural selection to work. This mechanism could not
have evolved one step at a time.
It is likely that the middle ear and the Eustachian tube evolved from the respiratory apparatus
of the fish, while various inner ear structures were developed from parts of the fish jaw.
Eventually, the inner ear began to change and develop, in combination with new
environmental pressures. It is probable that a small region of the inner ear partially
responsible for balance evolved into the membrane of the oval window, which was flexible
enough to transmit changes in air pressure to the fluid in the inner ear. At the same time, the
inner ear was increasing in size and complexity. In amphibians, a small bulge appeared in the
vestibular region of the ear, and as evolution proceeded, the bulge eventually developed into
the spiralled cochlea which today forms the hearing mechanism of the inner ear of all
vertebrates.
The range of frequencies which the ear is able to detect and analyse is likely the result of
evolutionary pressure to decode complex speech sounds. Similarly, the amplitude range
probably evolved in response to the loudest sounds in the natural environment. This would
include the cracks and booms of a thunderstorm at close range, as well as the loud roar of
predatory animals. These sounds tend to rise slowly rather than abruptly. And this may explain
why the ear has no defence against extremely loud sounds which occur suddenly, without
warning.
If you believe that all of this complex and cleverly designed organ simply evolved
gradually then to be honest you have not understood the elegance of the biological
technology or the inability of blind chance mutations to create anything!
Only our creator God has the power and intelligence to design
and build such a wonderful sensory organ and to integrate this
into our total brain information system
The hearing ear, and the seeing eye, the
LORD hath made even both of them.
Proverbs 20 v12