Download The Path of “Sound”

The following has been rewritten extensively and extensively
reorganized from the website:
http://www.hearingcentral.com/howtheearworks.asp .
Sound
The ear is extremely sensitive; with the softest detectable sound, the eardrum only moves
approximately one-millionth of an inch. Even this soft vibration is transferred to the inner ear
for processing by the brain.
Sound travels as small waves of pressure through the air at a speed of about 740 miles per
hour. What we hear are sound waves provided by vibrations of air molecules. We don't
actually measure waves, but the calculated pressure that a wave makes against an object (ear
drum). The measurements are shown in a logarithmic scales called Pascals or Micropascals.
The waves of sound act like ripples on the surface of a pond spreading out after a stone has
been thrown in.
How Sound is measured
Pitch (Frequency) - measured in Hz/kHz
Waves have pitch (frequency) and that is the number of crests that pass a point in a second.
Pitch is measured as "cycles per second- (cps)" which is now more commonly written as hertz
(Hz); 261 Hz is equivalent to middle C on the piano. One thousand cycles per second (1,000
cps) is one kilohertz (1 kHz). The number of vibrations or cycles per second makes up
frequency - the more vibrations, the higher the pitch of the sound.
Intensity (Pressure) - measured in Pascals (Pa, µPa)
Sound waves also have intensity and, when the comparison is made with ripples on the pond,
this equates to the volume of the wave. In real life, it is easier to measure the pressure of the
wave rather than its intensity. This pressure is measured in units called pascals. One pascal is
rather large for sound pressure measurements so micropascals (µPa), that is, one-millionth of
a pascal, tend to be used.
Hearing A Clap (Pitch is constant; intensity fades)
We hear sounds because our ears are sensitive to pressure waves. Perhaps the easiest type of
sound wave to understand is a short, sudden event like a clap. When you clap the hands, the
air that was between the hands is pushed aside. This increases the air pressure in the space
near the hands, because more air molecules are temporarily compressed into less space. The
high pressure pushes the air molecules outwards in all directions at the speed of sound. When
the pressure wave reaches the ear, it pushes on the eardrum slightly, causing you to hear the
clap.
A hand clap is a short event that causes a single pressure wave that quickly dies out. The
image above shows the waveform for a typical hand clap. In the waveform, the horizontal axis
represents time, and the vertical axis is for pressure. The peaks of the wave form are the same
distance apart. (The pitch is constant.) The initial high pressure is followed by low pressure,
but the oscillation quickly dies out. (The intensity fades.)
Hearing a bell (Pitch is constant; intensity is also)
The other common type of sound wave is a periodic wave. When you hear the hall bell ringing,
the sound comes from the continued vibration of the bell. While the bell is still ringing, it
vibrates at a particular frequency, depending on the size and shape of the bell. This vibration
causes the nearby air to vibrate with the same frequency. This causes pressure waves of air to
travel outwards from the bell, again at the speed of sound. Pressure waves from continuous
vibration such as a bell look more like this:
In this illustration, as in the clap, the distance between the peaks of the waves does not change
(the pitch is constant), but as long as energy is added to the bell so that it continues to ring, the
intensity remains the same. (All waves have the same height.) When these vibrations strike the
ear drum, they cause the ear drum to vibrate at that pitch.
Range Of Intensity Detectible by the Human Ear
For convenience pressure levels of sound are recorded as decibels (dB) which are logarithmic
measurements. Response to increasing sound intensity is a "power of ten" or logarithmic
relationship. The logarithmic unit of measurement means, for example, that 80dB is twice as
loud as 70dB. This is one of the motivations for using the decibel scale to measure sound
intensity. A general 'rule of thumb' for loudness is that the power must be increased by about
a factor of ten to sound twice as loud.
The range of pressures that the ear can hear is enormous. The quietest, just detectable sound
that the average healthy 18 year old, without previous ear problems and with normal
eardrums, can hear is 20 µPa (micropascal), but a jet engine heard close by has a level of
20,000,000 µPa. Thus our ability to detect sounds from the softest to the loudest covers an
intensity range of approximately 100,000,000 to 1. Since it is the quietest sound we can detect,
20 µPa. forms the basis for measuring the pressure of other commonly heard sounds in our
environment. .
The Range of Pressure That Can Be Heard By An Undamaged Ear
The ear is capable of hearing a wide range of sounds. The ratio of the sound pressure of
the lowest limit that undamaged ears can hear to that of sounds that can cause permanent
damage is more than a million. The decibel scale is logarithmic; it can be used to describe
very large ratios
Decibels
Micropascals
Typical Perception
0 dB
20µPa
The quietest sound an 18 year-old
healthy ear can hear
20 dB
200µPa
A very soft whisper
45 dB
300 - 800 µPa
60 dB
5,000 µPa
An average spoken voice
70 dB
20,000 µPa
A shout
80 dB
100,000 µPa
A noisy motorcycle on a narrow
street
90 dB
500,000 µPa
Jackhammers within 50 feet
100-120 dB
5.000.000 µPa
A heavy metal rock concert
120 - 140 dB
20,000,000 µPa
The noise of a jet engine within
250 yds.
A softly spoken voice
Range Of Pitch Detectible by the Human Ear
Many young, healthy humans (through teens and early twenties) can hear frequencies from
about 20 Hz to 20,000 Hz, and can detect frequency differences as small as 0.2%. That is, we
can tell the difference between a sound of 1000 Hz, and one of 1002 Hz.
The Parts of the Ear
The ear is made up of three basic structures: the outer ear, the middle ear, and the inner ear.
The middle and inner ear are in the temporal bone of the skull.
The outer ear:
o
o
o
The ear, the part we see (pinna or auricle)
The external ear canal, through which sound waves pass to the ear drum
The outside of the ear drum (which separates the outer ear from the middle ear)
The middle ear:
o The Eustachian tube (connecting the middle ear to the throat)
o The inside of the ear drum
o The ear bones or ear ossicles
 The hammer (malleus) which attaches to the inside of the eardrum
 The anvil (incus)
 The stirrup (stapes) which attaches to the outside of the oval window
The inner ear
o
o
o
o
The semicircular canals (which play a role in balance, but not in hearing)
The vestibule (which also plays a role in balance)
The cochlea, which begins at the oval window, is the structure that transduces
(changes) the vibrations to nerve impulses that travel to the brain where they
are interpreted as sound.
All three of these structures are part of the inner ear labyrinth.
The labyrinth
The semicircular canals, the vestibule and cochlea are formed by two structures filled with two
types of fluids.
The structures are:
o The outer, osseous (boney, i.e., stiff) labyrinth (what you see in the model) and
the inner, membranous (membrane-like, i.e., flexible) labyrinth.
The fluids are:
o The perilymph, between the osseous labyrinth and the membranous labyrinth
o The endolymph, within the membranous labyrinth
The cochlea
The cochlea is embedded in the skull in what is called the mastoid area, a spongy part of the
temporal bone just behind where the jaw hinges. The mastoid bone acts as an amplifier for
some sounds, especially those in the lower decibel ranges.
The cochlea is made up of:
o The oval window to which the stirrup from the middle ear is attached
o The round window which acts as a sound dampener.
o
Three fluid-filled canals that run the length of the cochlea, separated by thin
membranes.
1. The vestibular canal (scala vestibuli) This is filled with perilymph
2. The tympanic canal (scala tympani) This is also filled with perilymph
3. The cochlear canal (scala media). This is filled with endolymph and
contains a very important structure known as the organ of Corti.
Since the details of these last three canals and the organ of Corti are critical to understanding
hearing, here they are in greater detail:
o The vestibular canal is bounded on one side by the osseous labyrinth and on the
other by the vestibular membrane.
o The cochlear canal is bounded on one side by the vestibular membrane and on
the other by the basilar membrane, part of which supports the organ of Corti.
Next, since the details of the organ of Corti are critical to understanding hearing, here they
are in greater detail:
o The only new feature here is the hair cells.
Hair cells
These are what we’ve been heading toward because they are the structures that actually
change the vibrations into nerve impulses. Note that they have tiny hairs that stick into the
tectorial membrane. These hairs are what give the hair cells their name. The tips of these
hairs are held in place by the stiff tectorial membrane. These are very special hairs that
respond to being bent in a particular direction by sending a nerve impulse along the sensory
nerve fibers (in the auditory nerve) to the brain. These hairs can only be bent in that
particular direction if the basilar membrane beneath them is vibrating and the basilar
membrane beneath them only vibrates at a particular pitch (frequency). Thus since the
basilar membrane at the base of the cochlea(where the opening in a shell would be) vibrates at
very high frequencies (20,000 Hz), and the basilar membrane at the tip of the cochlea vibrates
at very low frequencies (20 Hz) and the basilar membrane in between vibrates at intermediate
frequencies, only the hair cells at those frequencies would send an impulse to the brain and
the brain will interpret that impulse as a certain frequency.
The Path of “Sound”
We now have all the information we need to follow the path of sound from the vibrating bell to
our brain where we hear the sound of the vibrating bell.
Damaged hair cells and deafness
There are a series of hair cells contained in the cochlea (inner ear) that are key to most people's
hearing. They are called the "inner hairs" (more on this later). It is damage to, or lack of the
inner hair cells that cause most deafness. High decibels i.e. loud music or sounds above 140 db.
will cause some of these hairs to die, as will some serious infections. Once an inner hair dies, it
cannot be replaced. Because we initially are born with only about 3500 of these hairs, loss of a
few can make a big difference in our hearing capacity. (The cochlea and the role of these hairs
are discussed in detail in this document)
The Path of “Sound”
We now have all the information we need to follow the path of sound from the vibrating
bell to our brain where we heare the sound of the vibrating bell.
Damaged hair cells and deafness
There are a series of hair cells contained in the cochlea (inner ear) that are key to most
people's hearing. They are called the "inner hairs" (more on this later). It is damage to, or
lack of the inner hair cells that cause most deafness. High decibels i.e. loud music or
sounds above 140 db. will cause some of these hairs to die, as will some serious infections.
Once an inner hair dies, it cannot be replaced. Because we initially are born with only
about 3500 of these hairs, loss of a few can make a big difference in our hearing capacity.
(The cochlea and the role of these hairs are discussed in detail in this document)
Within the middle ear, vibrations travel through three small bones (first the hammer,
then the anvil, and finally the stirrup) to the cochlea. The hammer is attached to the
lining of the eardrum, the anvil is attached to the hammer, and the stirrup links the anvil
to the oval window, the opening to the inner ear. These three tiny bones amplify the
sound from the ear drum by the time the vibration is transferred to the stirrup and the
oval window.
The oval window as amplifier
The stirrup is connected to what is called the oval window on the outside of the cochlea.
Because the oval window of the cochlea is smaller than the eardrum, when the stirrup
vibrates from vibrations transmitted by the other bones, it causes a further amplification
of the sound vibration - up to 20 times at some frequencies.
Normally, when sound hits the surface of a liquid, 99.5 per cent or more is reflected.
However, the operation of the middle-ear mechanism results in about 50 per cent of the
sound to be transferred to the inner ear - a very efficient mechanism!
The eustachian tube
The middle ear is also connected by a narrow channel to the throat called the eustachian
tube. Ordinarily, the eustachian tube is closed because the air in the middle ear chamber
must be completely still for the optimal vibration of the middle ear bones.
When you swallow or yawn, it opens briefly to allow an exchange of air, equalizing the
air pressure within the middle ear and the air pressure outside. When you yawn or
sneeze, or hold your nose and push out on your ear drums with your breath when
descending in an airplane, and your ears "pop", the eustachian tube is forced open and
the air pressure between the inner ear and the outside is balanced.
The eustachian tube is also used to drain accumulations in the middle ear such as
mucous or bacterial detrius.
How the Various Canals Work
The cochlea begins at the oval window where the middle ear stirrup is attached and
curves into a shape that resembles a snail shell, where the chambers get narrower
towards the end. The coiled tube inside the snail-like apparatus contains three parallel
canals discussed above, that run the length of the cochlear envelope.
The following diagram is a cross-section of a cochlear showing the three major canals
or ducts.
Graphic courtesy of NYU medical Center
The following is a cross-section of the same diagram above.
The cochlea as microphone
When sound waves from the world outside strike the eardrum, it vibrates. These
vibrations from the eardrum pass through the three bones of the middle ear and into the
inner ear through the oval window. Action of the oval window causes fluids in the
cochlea to create waves where they disturb the basilar membrane. Inner hairs attached
to the basilar membrane convert the waves into electrical impulses that are transmitted
to the brain by the auditory nerve. The hair cells are critical to hearing; it is the inner
hairs that move in the Organ of Corti fluids, and translate the fluid movements to
chemical messengers that can in turn be converted to electrical impulses that the brain
understands.
The hair cells - keys to hearing
There are approximately 24,000 fibers or hairs in the Organ of Corti, organized into
"inner" and "outer" hairs. These hairs are arranged in four long rows the length of the
cochlea. In a healthy young human ear there are about 3,500 inner hair cells of varying
lengths and about 12,000 -21,500 outer hair cells. Each hair cell also has a cluster of
small rigid hairs, which project from the thicker upper surface of the cell into the fluids
that fills the cochlear duct. The tips of the hairs of the outer hair cells are lightly
embedded in the under-surface of the basilar membrane whereas the tips of the inner
hair cells do not reach the basilar membrane and stand free in the endolymph.
Inner hair cell operations
The inner hair cells consist of a single row running the length of the cochlea and lie
closer to the core of the cochlea. Sensor neurons are in close contact with the top
surface of each receptor hair cell and they transmit electrical changes to the brain
through the basilar membrane when sound waves disturb the fluids in the vestibular and
tympanic canals.
Outer hair cell operations
There are three or four rows of outer hair cells, which are further away (sideways to the
length of the cochlea, through which the brain sends messages back to the cochlear to
mediate activity that the inner hairs have transmitted. The outer hair cells complete the
feedback loop from the brain back to the inner ear. The returning messages refine the
sensitivity and frequency selectivity of the mechanical vibrations of the fluids in the
cochlea by causing the outer hairs to inject inhibitor chemicals to counteract the action
of the inner hairsti fluids.
Basilar membrane
Overlying the hair cells is a gelatinous membrane called the tectorial or basilar
membrane. One edge of this is attached to the bony core at the center of the cochlea (the
modiolus); the other is loosely attached to the organ of Corti outside the outermost outer
hair cell.
Messages to the Brain
Both the inner and outer hair fibers are all connected to the auditory nerve through the
basilar membrane and, depending on the nature of the movements in the cochlear fluid,
different hair fibers are put into motion. When the hair fibers move they send electrical
signals to the auditory nerve which is connected to the auditory centre of the brain. In
the brain the electrical impulses are translated into sounds which we recognize and
understand. As a consequence, these hair fibers are essential to our hearing ability.
Should these hair fibers become damaged, our hearing ability will deteriorate.
Adjacent to the base of the hair cells are the nerves that carry impulses to the brain (they
are not quite "embedded" in the basilar membrane). At least 90 per cent of these nerves
come from the inner hair cells, despite their smaller number. Each inner hair cell has
about 10 nerve endings attached to it and there are, therefore, about 30,000 nerve fibers
in the acoustic nerve that transmit the electrical equivalents of the sound waves to the
brain.
Putting it all together
What the outer ear does
Sound waves from the external world travel through the auditory canal to the tympanic
membrane (ear drum). The auditory canal can resonate and amplify sounds within a
frequency range of about 2000 Hz to 5500 Hz by up to a factor of 10.
Successive compressions and retractions of air waves reaching the eardrum result in
changes in pressure between the outer ear and the middle ear. The Eustachian tube
helps to keep the middle ear at atmospheric pressure.
The difference in pressure between the sound wave striking the outer surface of the
eardrum and normal atmospheric pressure on the inside of the eardrum causes the
eardrum to vibrate.
What the Middle Ear Does
From the vibrating ear drum, the malleus (hammer), the incus (anvil), and the stapes
(stirrup), vibrate and transfer the vibrations to the oval window of the inner ear.
What the Inner Ear Does
The movement of the stapes, the third small bone, causes the oval window membrane to
vibrate which, in turn, causes the fluids of the vestibular to move, then the tympanic
canal fluids to move. As the fluids of the canals move, still at the same frequency of the
original air waves, the round window membrane at the end of the tympanic canal
functions as a sound multiplier, moving outward or inward at a 20X multiple as the oval
window membrane is pushed in or sucked out by the stapes.
As a pressure wave passes through the fluids of the two canals, there is movement of the
thin basilar membrane and, along with it, the organ of Corti containing the hair cells.
The basilar membrane is flexible enough to move with the pressure of the sound source
(see descriptions of wave movement on the basilar membrane, below). The basilar
membrane is very narrow at the beginning of the cochlea and becomes three to four
times wider at the apex or helicotrema. A wave form with a high frequency will only
affect the initial part of the basilar membrane, which is stiffer than the final segment
that responds to low frequencies.
Simulation of a wave displacing the basilar membrane
The displacement of the basilar membrane induced by a sound source creates an
envelope within the membrane which associates the frequency of the sound with the
shape of the membrane.
At a mechanical level, measurements of the basilar membrane can be made similar to a
wave frequency spectrum.
The higher frequency waves are picked up at the cochlear opening; the lower frequency
waves are picked up towards the apex.
Waves build up to a maximum for each particular pitch and then rapidly fall away to
nothing. The location of the peak of the wave varies at different pitches: for highpitched sounds, the wave peaks near the base of the cochlea, whereas for low-pitched
sounds this peak is near its apex (narrow end).
The movement of the fluid in the vestibular and tympanic canals causes the waves to
cancel each other out, except for the specific wave (frequency) that is being transmitted
through the fluids.
The hairs attached to the basilar membrane of the cochlear canal move up and down
(and sideways) only when they recognize (vibrate to) a specific frequency. When hairs
along the cochlear canal match a frequency (at the point where the waves are
cancelled), they push against a specific part of the basilar membrane which in turn,
stimulates the sensor neurons which carry the impulses to the brain's auditory centers.
In other words, only certain hairs respond to certain sound waves, and as a result, the
brain only hears what the hairs and basilar membrane send.
For Biochemists Only :) - How Calcium and Potassium Enable Hearing in the Inner
Ear
The following are the physical and chemical sequences of events that allow hearing to
occur from the cochlea:
As a wave reaches its peak in the cochlea, the outer hair cells near this peak give a
small, physical push to enhance the movement of the basilar membrane. Much like a
carpet on a floor being pushed into folds, the basilar membrane's folds act as an
amplifier and cause the endolymph in the tympanic canal (high in potassium with a
positive charge) to squirt towards the hairs of the inner hair cells which have a negative
charge (in the form of calcium crystals). The hairs are deflected and very small channels
open up near the tips of the hairs. The potassium atoms in the endolymph flow through
these small channels, being propelled by their charges. This action is called
depolarization - a positive change in the voltage across a cell's membrane.
Translating from chemicals to electricity
When the the potassium comes in contact with the hair cell, it alters the membrane of
the hair cell. The chemical sensitivity of the hair cell triggers an action at the bottom of
the hair. Small parcels of chemicals (calcium influx via voltage-gated channels) cause
neurotransmitters (in the form of glutamate) to be released from the base of the hair cell
over a synapse, causing the nerves across the synapse to become active and send an
electrical impulse towards the brain. (See the graphic of a hair cell, above - at the bottom
of the hair cell are spaces - synapses)
The feedback loop
In addition to sending information to the brain, the inner ear is subject to feedback
regulation from the brain. Neurons in the brainstem send messages back to the cochlea
where, based on the "content" of the messages the brain is receiving from the inner hair
cells i.e. what sounds it is hearing. This feedback is terminated at the outer hair cells; if
necessary ducts at the base of these hairs release the neurotransmitter acetylcholine to
inhibit the action of inner hair cells.
It is thought that one of the functions of this feedback/inhibitor mechanism is to protect
the cochlea (and the sensitivities of the user) to loud and sudden noises.
It is also postulated that the brain's signals also induce a physical response to assist with
the cochlear protection mechanism: the postulate is that the effect of the acetylcholine
also causes the outer hair cells to halt their movement, or even move in a countercyclical motion and create interference patterns to dampen the fluid wave action for
loud or damaging sounds. However this has not been proven to date.
Damaged hair cells and deafness
There are a series of hair cells contained in the cochlea (inner ear) that are key to most people's
hearing. They are called the "inner hairs" (more on this later). It is damage to, or lack of the
inner hair cells that cause most deafness. High decibels i.e. loud music or sounds above 140 db.
will cause some of these hairs to die, as will some serious infections. Once an inner hair dies, it
cannot be replaced. Because we initially are born with only about 3500 of these hairs, loss of a
few can make a big difference in our hearing capacity. (The cochlea and the role of these hairs
are discussed in detail in this document)



The
The hairs that move when certain wave lengths hit them (inner
hairs)
Different hairs (outer hairs) that act as inner hair inhibitors
The basilar membrane that transmits fluid movement to the hairs