THE EAR AND HEARING

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					THE EAR AND
  HEARING
Biophysical mechanisms
Which sense is most valuable to you,
        seeing or hearing?


Many people who are totally deaf claim
that they would rather be blind.
What they have lost is our basic means of
communication; in silent word it is difficult
to learn to speak and language itself is at
risk.
Structure and function of the ear
The ear is designed to convert weak mechanical
vibrations of the air into electrical pulses that can
be sent to the brain. The frequency range of the
vibrations is between 16 - 20 000 Hz.
It consists of a mechanical collection and
amplification system (in the outer and middle ear)
and transducers to produce electrical potentials in
nerves (in the inner ear).
The auditory nerves lead to the auditory cortex,
the part of the brain which interprets the signals.
Vertical section through the ear
1. The outer ear
             Sound energy spreads out
             from its sources. For a
             point source of sound, it
             spreads out according to
             the inverse square law.
             For a given sound
             intensity, a larger ear
             captures more of the
             wave and hence more
             sound energy.
             The outer ear structures
             act as part of the ear's
             preamplifier to enhance
             the sensitivity of hearing.
             The auditory canal acts as
             a closed tube resonator,
             enhancing sounds in the
             range 2-5 kiloHertz.
       Outer ear - function
The external ear, which includes the pinna and
auditory canal, carries out two physiological
functions: acoustic and non-acoustic.
The auditory function allows efficient sound
transmission from the environment to the
tympanic membrane. The nonacoustic functions
of the ear canal include protection of the
tympanic membrane and the maintenance of a
clear passage for sound.
     Inverse Square Law; Sound




The sound intensity from a point source of sound will obey the inverse
square law if there are no reflections or reverberation. A plot of this intensity
drop shows that it drops off rapidly
         2. The middle ear
There is a mechanical linkage of three small
bones-the ossicles, between the eardrum and a
smaller membrane, called the oval window.
The bones are called the malleus, incus and
stapes, or, reflecting their shapes, the hammer,
anvil and stirrup.
They act as combined lever and pistons in the
air-filed cavity between the membranes.
2. The middle ear
           The Ossicles
The three tiniest bones in the body form
the coupling between the vibration of the
eardrum and the forces exerted on the
oval window of the inner ear.
Formally named the malleus, incus, and
stapes, they are commonly referred to in
English as the hammer, anvil, and stirrup.
With a long enough lever, you can lift a big rock with a small applied
force on the other end of the lever. The amplification of force can be
changed by shifting the pivot point.
Ossicle Vibration
        Ossicle Vibration
The vibration of the eardrum is transmitted
to the oval window of the inner ear by
means of the ossicles, which achieve an
amplification by lever action. The lever is
adjustable under muscle action and may
actually attenuate loud sounds for
protection of the ear.
A physiology book describes the ossicles as
small enough to fit collectively on a U.S. dime.
The image to the right actually makes the
ossicles a bit too large - they may be half that
large in some persons.
The ossicles can be thought of as a
compound lever which achieves a
multiplication of force. This lever action is
thought to achieve an amplification by a
factor of about three under optimum
conditions, but can be adjusted by muscle
action to actually attenuate the sound
signal for protection against loud sounds .
   The Tympanic Membrane
The tympanic membrane or "eardrum" receives
vibrations traveling up the auditory canal and
transfers them through the tiny ossicles to the
oval window, the port into the inner ear.

The eardrum is some fifteen times larger than
the oval window of the inner ear, giving an
amplification of about fifteen compared to a case
where the sound pressure interacted with the
oval window alone. The tympanic membrane is
very thin, about 0.1 mm, but it is resilient and
strong.
The eardrum is some fifteen times larger
than the oval window of the inner ear,
giving an amplification of about fifteen
compared to a case where the sound
pressure interacted with the oval window
alone.
The tympanic membrane is very thin,
about 0.1 mm, but it is resilient and strong.
                    The Inner Ear




The small bone called the stirrup, one of the ossicles, exerts force on
the thin membrane called the oval window, transmitting sound
pressure information into the inner ear.
            The Inner Ear
The inner ear can be thought of as two organs:
the semicircular canals which serve as the
body's balance organ and the cochlea which
serves as the body's microphone, converting
sound pressure impulses from the outer ear into
electrical impulses which are passed on to the
brain via the auditory nerve.
The basilar membrane of the inner ear plays a
critical role in the perception of pitch according
to the place theory.
       The Semicircular Canals
The semicircular canals are the body's balance organs, detecting
acceleration in the three perpendicular planes. These accelerometers
make use of hair cells similar to those on the organ of Corti, but these
hair cells detect movements of the fluid in the canals caused by angular
acceleration about an axis perpendicular to the plane of the canal. Tiny
floating particles aid the process of stimulating the hair cells as they
move with the fluid. The canals are connected to the auditory nerve.
                 Organ of Corti
             The Body's Microphone




It is situated on the basilar membrane in one of the three
compartments of the Cochlea. It contains four rows of hair cells
which protrude from its surface. Above them is the tectoral
membrane which can move in response to pressure variations in the
fluid- filled tympanic and vestibular canals. There are some 16,000 -
20,000 of the hair cells distributed along the basilar membrane
which follows the spiral of the cochlea.
            Mechanism
Tiny relative movements of the layers of
the membrane are sufficient to trigger the
hair cells. Like other nerve cells, their
response to stimulus is to send a tiny
voltage pulse called an "action potential"
down the associated nerve fiber (axon).
These impulses travel to the auditory
areas of the brain for processing.
      The Auditory Nerve
Taking electrical impulses from the cochlea and
the semicircular canals, the auditory nerve
makes connections with both auditory areas of
the brain.
Auditory Area of Brain




This schematic view of some of the auditory areas of the brain shows that
information from both ears goes to both sides of the brain - in fact, binaural
information is present in all of the major relay stations illustrated here. That
is, when the auditory nerve from one ear takes information to the brain, that
information is directly sent to both the processing areas on both sides of the
brain.
TRANSMISION AND
MEASUREMENT OF
    SOUND
           Sound Intensity
Sound intensity is defined as the sound power
per unit area.
The usual context is the measurement of sound
intensity in the air at a listener's location. The
basic units are watts/m2 or watts/cm2 . Many
sound intensity measurements are made relative
to a standard threshold of hearing intensity I0 :
Sound intensity measurement. Threshold
               of Hearing

 The most common approach to sound
 intensity measurement is to use the
 decibel scale:




Decibels measure the ratio of a given intensity I to the threshold of hearing
intensity , so that this threshold takes the value 0 decibels (0 dB). To assess
sound loudness, as distinct from an objective intensity measurement, the
sensitivity of the ear must be factored in.
           Sound Pressure
Since audible sound consists of pressure waves, one of
the ways to quantify the sound is to state the amount of
pressure variation relative to atmospheric pressure
caused by the sound. Because of the great sensitivity of
human hearing, the threshold of hearing corresponds to
a pressure variation less than a billionth of atmospheric
pressure.
The standard threshold of hearing can be stated in terms
of pressure and the sound intensity in decibels can be
expressed in terms of the sound pressure:
             Sound Pressure




The pressure P here is to be understood as the amplitude of the
pressure wave. The power carried by a traveling wave is
proportional to the square of the amplitude. The factor of 20 comes
from the fact that the logarithm of the square of a quantity is equal to
2 x the logarithm of the quantity. Since common microphones such
as dynamic microphones produce a voltage which is proportional to
the sound pressure, then changes in sound intensity incident on the
microphone can be calculated from
         Threshold of Pain
The nominal dynamic range of human hearing is
from the standard threshold of hearing to the
threshold of pain. A nominal figure for the
threshold of pain is 130 decibels, but that which
may be considered painful for one may be
welcomed as entertainment by others.
Generally, younger persons are more tolerant of
loud sounds than older persons because their
protective mechanisms are more effective. This
tolerance does not make them immune to the
damage that loud sounds can produce.
                         Loudness
Loudness is not simply sound intensity!
Sound loudness is a subjective term describing the strength of the ear's
perception of a sound. It is intimately related to sound intensity but can by
no means be considered identical to intensity.

The sound intensity must be factored by the ear's sensitivity to the
particular frequencies contained in the sound.

This is the kind of information contained in equal loudness curves for the
human ear. It must also be considered that the ear's response to increasing
sound intensity is a "power of ten" or logarithmic relationship.

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. To more
realistically assess sound loudness, the ear's sensitivity curves are factored
in to produce a phon scale for loudness. The factor of ten rule of thumb can
then be used to produce the sone scale of loudness. In practical sound
level measurement, filter contours such as the A, B, and C contours are
used to make the measuring instrument more nearly approximate the ear.
Equal Loudness Curves
                     Phons
Two different 60 decibel sounds will not in general have
the same loudness
Saying that two sounds have equal intensity is not the
same thing as saying that they have equal loudness.
Since the human hearing sensitivity varies with frequency,
it is useful to plot equal loudness curves which show that
variation for the average human ear.
If 1000 Hz is chosen as a standard frequency, then each
equal loudness curve can be referenced to the decibel
level at 1000 Hz.
This is the basis for the measurement of loudness in
phons. If a given sound is perceived to be as loud as a 60
dB sound at 1000 Hz, then it is said to have a loudness of
60 phons.
60 phons means "as loud as a 60 dB, 1000 Hz tone"
                          Timbre
Sounds may be generally characterized by pitch, loudness, and
quality.

Sound "quality" or "timbre" describes those characteristics of sound
which allow the ear to distinguish sounds which have the same pitch
and loudness.

Timbre is then a general term for the distinguishable characteristics of
a tone. Timbre is mainly determined by the harmonic content of a
sound and the dynamic characteristics of the sound such as vibrato
and the attack-decay envelope of the sound.

Some investigators report that it takes a duration of about 60 ms to
recognize the timbre of a tone, and that any tone shorter than about 4
ms is perceived as an atonal click. It is suggested that it takes about a
4 dB change in mid or high harmonics to be perceived as a change in
timbre, whereas about 10 dB of change in one of the lower harmonics
is required.
              Audible Sound
Usually "sound" is used to mean sound which can be
perceived by the human ear, i.e., "sound" refers to
audible sound unless otherwise classified.
A reasonably standard definition of audible sound is that
it is a pressure wave with frequency between 20 Hz and
20,000 Hz and with an intensity above the standard
threshold of hearing.
Since the ear is surrounded by air, or perhaps under
water, the sound waves are constrained to be
longitudinal waves. Normal ranges of sound pressure
and sound intensity may also be specified.
           Audible Sound
  Frequency: 20 Hz - 20,000 Hz, (corresponds with pitch)

  Intensity: 10-12 - 10 watts/m2 (0 to 130 decibels)


  Pressure: 2 x 10-5 - 60 Newtons/m2 (2 x 10-10 - .0006
  atmospheres



For an air temperature of 20°C where the sound speed is 344 m/s,
the audible sound waves have wavelengths from 0.0172 m (0.68
inches) to 17.2 meters (56.4 feet).
 Relation between sound intensity
        and ear‘s response
   The ear’s response to an increase in intensity
   has at least three parts:
1. A greater movement of the basilar membrane,
   producing more stimulation of the nerve
   endings by the hair cells.
2. Additional hair cells are activated to stimulate
   nerve endings, in the particular location for the
   frequency of sound.
3. Nerves are stimulated beyond the part of the
   membrane as a result of his greater
   movement.
   Frequency discrimination
Is the ability to distinguish one frequency from
another.
The ear response is also frequency dependent.
It is greatest at low frequencies: in the range 60-
100 Hz a difference of about 3 Hz can be
distinguished, but above 10 KHz it is very poor.
The behavior of the ear has resulted in the
development of musical intervals, which depend
on the ratio on the ratio of upper and lower
frequencies of 2:1.
DEFECTS OF
 HEARING
               Hearing Loss
Hearing loss is typically described as being conductive,
sensorineural, or mixed.

Conductive hearing loss refers to an impairment of
one's ability to conduct airborne sound through the middle
ear to the inner ear. Scar tissue or otosclerosis, the
abnormal growth of bone within the middle ear, can lead
to restricted movement of the ossicles. Recently it has
been shown that there can also be conductive problems
with the basilar membrane of the inner ear that reduce the
efficiency of energy transfer to the hair cells (Holt).

Sensorineural hearing loss refers to impairment of the
sensory unit consisting of the auditory nerve and the hair
cells that excite it.
             Hearing Loss
Sometimes the distinction between these two
types of hearing loss can be made with a simple
tuning fork test. If the tuning fork cannot be
heard when sounded in air, then the base of the
tuning fork is placed against the hard bone
behind the ear.
If the person can now hear it by conduction
through the bone, then conductive hearing loss
is indicated. It in cannot be heard by either air or
bone conduction, then sensorineural loss is
indicated.
          Hearing Loss
 0 to -15 Db Normal range
-16 to -40 dB Minimal loss
-26 to -15 dB Mild loss
-41 to -55 dB Moderate loss
-56 to -70 dB Moderate/severe loss
-71 to -90 dB Severe loss
> -91 dB Profound loss
            Hearing Loss
The "power of ten" or logarithmic nature of
hearing response is evident in the fact that a
loss in sensitivity by a factor of 10,000, or -40
decibels, is still at the edge of "minimal loss".
By the admittedly simplistic "rule of thumb" for
loudness, this -40dB sound would still be 1/16 as
loud as the 0 dB reference.
0 dB in this table represents the normal hearing
threshold, or 0 dB Hearing Level. The categories
of hearing loss are based on measurements at
500, 1000 and 2000 Hz.
       Pure Tone Audiometry
The testing of hearing is most often carried out by
establishing the threshold of hearing, the softest sound
which can be perceived in a controlled environment.
It is typical to do this testing with pure tones by providing
calibrated tones to a person via earphones, allowing that
person to increase the level until it can just be heard.
Various strategies are used, but pure tone audiometry
with tones starting at about 125 Hz and increasing by
octaves, half-octaves, or third-octaves to about 8000 Hz
is typical.
Hearing tests of right and left ears are generally done
independently. The results of such tests are summarized
in audiograms.
Audiograms compare hearing to the normal threshold of
hearing, which varies with frequency as illustrated by the
hearing curves. The audiogram is normalized to the
hearing curve so that a straight horizontal line at 0
represents normal hearing
The progressive loss of high frequency
sensitivity with aging is typical, and is
called presbycusis.
The loss of the high frequencies can make
it difficult to understand speech, since the
intelligible differences in speech sounds
are often in the range above 2000 Hz.
Audiograms Showing Hearing Loss

Audiograms can help with the diagnosis of
various types of hearing disorders.
Specific geometries of curves are found to
be typical of presbycusis, and a
characteristic notch in the hearing curve
may be the signature of damage by a
sudden loud sound like a gunshot or a
firecracker explosion close to the ear.
The curves are normalized so that a
straight horizontal line represents
equal loudness.
                Hearing Aids

 Sometimes a satisfactory level of hearing can be
 restored by a hearing aid - a combination of a
 microphone to sense ambient sound, an amplifier, and a
 tiny speaker that projects the amplified sound into the
 ear canal.
 A typical modern hearing aid would employ an electret
 condenser microphone - small and rugged with a high
 signal-to-noise ratio.
 The frequency range of application is typically
100-10,000 Hz. While some assistance may be rendered
 by bone conduction, this discussion will be limited to
 hearing aids that operate by sounds produced in the air.
Hearing Aids
Military aircraft F/A-18 and sonic boom. The pressure produced by
the aircraft's speed caused the water vapor around it to condense into
a cloud.
The term sonic boom is commonly used
to refer to the shocks caused by the
supersonic flight of an aircraft.
Sonic booms generate enormous amounts
of sound energy, sounding much like an
explosion.
Thunder is a type of natural sonic boom,
created by the rapid heating and
expansion of air in a lightning discharge.

				
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