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عرض

Dr Hadi Al -Sagur

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Definition

 The ear is the organ that detects sound. It not only receives sound,

but also aids in balance and body position. The ear is part of the
auditory system.

 The ear is a cleverly designed converter of very weak mechanical

waves in air into electrical pulses in the auditory nerve.
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3 Fig 1: Cross -section of the ear. pharynx

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 The auditory system of the body is structured into a :
 Mechanical system to catch and to amplify acoustical
information (ear)
 Sensory (electrical) system which converts mechanical
pulses into electrical signals which are passed on by the
auditory nerves to the brain .
 Auditory system to decode and analyze the electrical
nerve signal in the auditory cortex (brain)
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 The ear is the body's main receiver system for acoustic
wave information . The main objective of the ear is to
receive the acoustic waves, amplify the intensity, analyze
the frequency and intensity structure of the wave and to
reject random background noise .
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The ear is divided into three parts :

1. The outer ear ≈ 2.5 cm long ear canal terminated by the eardrum.

2 . Middle ear cavity section containing by three small bones
(ossicles ) they are: malleus, incus, and stapes with connecting tube
to the mouth cavity ( Eustachain tube )

3 . Inner ear spiral -shaped, fluid -filled tube system ( chochlea ) with

internal organ of corti .

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Fig 2: Cross -section of the ear.

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 Hair cells in the organ of corti convert vibrations of sound
waves hitting the eardrum into coded nerve that inform the brain
of these sound waves.
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Fig 3: Organ of corti

1 . The outer ear

Consists of two parts:
a - Pinna.
b - External auditory canal.

a - Pinna:

 It is the curved part and the tip of the ear outside
the head, which is free from the bones, and
consists mainly of a strong fabric and elastic
called: cartilage, which is covered with a thin
layer of skin.
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b . External auditory canal:

 It is the slot that you see given direct to the ear,
which is a road leading to the eardrum, a thin
membrane taut strongly and has a radius of about
10 mm which separates the outer ear and the
middle, and also called the tympanic membrane.

 The eardrum or tympanic membrane is about mm

thick (paper thin).and has an area about 65 mm 2.
It couples the vibration in the air to the small
bones in the middle ear.
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 The main purpose of the membrane is to absorb and
transmit the pressure variations caused by acoustical waves
in the outer ear canal.
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 It is possible for sound pressures above 160 db to rupture
the eardrum. A ruptured eardrum normally heals just as
other living tissue does. .

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 It contains the skin in the outer third of them on the hair,
and sweat glands, and other glands secrete
(ear wax) is a liquid secreted by glands greasy wax job and
ear protection from bacteria and fungi. Which helps protect
the drum captures dirt that may accumulate around.
 It takes two -thirds of the temporal bone tunnel external
auditory, and this bone is more bone stiffness in the body,
which also surrounds the middle and inner ear, which thus
protects the precision components for those parts of the ear .
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2 . The middle ear

 Dominant feature of the middle ear are three small bones, the

ossicles , malleus (hammer), incus (anvil), and stapes
(stirrup) . Purposes of the bones are to serve as mechanical
impedance matching and amplifying system for the
transmission of the eardrum vibrations towards the inner ear .
 These bones are full adult size before birth . The fetus can
hear while it is still in the womb .
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 The malleus is attached directly to the eardrum membrane
to absorb the vibration ; the incus couples the malleus with
stapes which in turn is attached to the oval window
membrane which separates the middle ear cavity from the
inner ear .
 The ossicles amplify the pressure of the sound waves at the
entrance to the inner ear . The lever action of the ossicles is
such that the motion of the plate of the stapes at the oval
window of the inner ear is about 0 .7 that of the malleus at
the eardrum .

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 The middle ear contains air, and it is important for the air
pressures on both sides of the thin eardrum to be essentially
the same, the Eustachian tube serves to equalize the
pressures .

3 . The inner ear

 The inner ear, hidden deep within the hard bone of the skull, is
man's best -protected sense organ. The inner ear consists of a
small spiral -shaped, fluid -filled structure called the cochlea. The
ossicles of the middle ear communicate with the cochlea via a
flexible membrane (the oval window) ; the stapes its pressure
variations of incoming sound waves across this membrane to the
cochlea.

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 The cochlea communicates with the brain via the auditory
nerve a bundle of about 8000 conductors that inform the
brain via coded electrical pulses which parts of the cochlea
are being stimulated on both the frequency and the
intensity of the sounds that we hear.

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 It is divided into three small fluid -filled chambers that run
its full length the oval window is on the end of the
Vestibular chamber, the middle chamber is the cochlea
duct, and the third chamber is that Tympanic chamber, the
vestibular and tympanic chamber are interconnected at the
tip of the spiral.
 A sound wave entering at the oval window produces a wave -like
ripple in the basilar membrane of the cochlear duct. This duct contains
the sensors that convert the sound into nerve signals. The motions of
this membrane are about 10 times smaller in amplitude than the
motion of the eardrum. Stimulation of nerves in the cochlear duct near
the oval window indicates high -frequency sounds. Low -frequency
sounds cause large motions in the basilar membrane and stimulation of
nerves in the cochlear duct near the tip of the spiral
 The transducers that convert the mechanical vibrations into electrical
signals are located in the bases of the fine hair cells in the organ of Croti

The auditory system of the body is structured into :

• Mechanical system to catch and to amplify acoustical
information (ear )

• Sensory (electrical) system which converts mechanical

pulses into electrical signals which are passed on by the
auditory nerves to the brain

• Auditory system to decode and analyze the electrical nerve

signal in the auditory cortex (brain)
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The commonly stated range of human hearing is

20 Hz to 20 kHz
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Sensitivity of the ears

The ear is not uniformly sensitivity over the entire hearing range.
Its best sensitivity is in the region of 2 to 5 KHz . this sensitivity
changes with age. The highest frequency you can hear will
decrease as you get older, and the level of sound will need to be
greater for you to detect them.
Hearing test
The tests are normally done in a specially constructed soundproof
testing room. Each ear is tested separately ; test sounds can be
sent to either ear through a comfortable headset. The subject is
asked to give a sign when he/she hears the test sound. Selected
frequency from 250 to 8000 Hz are used. At each frequency the
operator raises and lower the volume unit a consistent hearing
threshold is obtained.
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Deafness and hearing aids

The frequency range most important for understanding
conversational speech is from about 300 to 3000 Hz. A person who
is deaf above 4000 Hz but who has normal hearing in the speech
frequencies is not considered deaf or even hard of hearing. Hearing
handicap are classified according to the average hearing threshold
at 500 ,1000 and 2000 Hz in the better ear.

A person with a hearing threshold 30 dB above normal would

probably not have hearing problems.
People with hearing threshold of 90 dB are considered deaf or
stone deaf.
The average sound level of speech is about 60 dB. But this average
become more or less according quiet place.
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Decibel (dB), unit for expressing the ratio between two physical

quantities, usually amounts of acoustic or electric power, or for
measuring the relative loudness of sounds. One decibel ( 0.1 bel)
equals 10 times the common logarithm of the power ratio.
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Source Intensity (W/m 2) Intensity Level
Sound that is barely perceptible 1*10 -12 0 dB
Whisper 1*10 -10 20 dB
Average dwelling 1*10 -9 30 dB
Business office 1*10 -7 50 dB
Normal Conversation 1*10 -6 60 dB
Busy Street Traffic 1*10 -5 70 dB
Subway or automobile 1*10 -3 90 dB
Sound that produces pain 1*10 0 120 dB
Jet aircraft 1*10 1 130 dB
On rocket launch pad 1*10 5 170 dB
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Sound levels for nonlinear (decibel) and linear (intensity) scales

There are tow common causes of reduced hearing :

1. Conduction hearing loss : in which the sound vibrations do

not reach the inner ear. it may be temporary due to a plug of
wax blocking the eardrum or fluid in the middle ear. It may be
due to a solidification of small bones in the middle ear.

This condition can be corrected by an operation in which the

stapes, which pushes on the oval window . it is replaced with
a piece of plastic. If a conduction hearing loss is not curable ,
a hearing aid can be used to transmit the sound through the
bones of the skull to the inner ear .
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2 . Nerve hearing loss : in which the sound reaches the inner

ear but no nerve signals are sent to brain . It affect only a
narrow band of frequency or it may affect all frequency .
The hearing threshold that requires a person to use a hearing
aid is quite variable according to :
Reflection :the sound reflects about 6 -8 dB of additional
sound into ear canal .
The earliest artificial hearing aid was the ear trumpet . The size
and shape of the ear trumpet affect its efficiency .
Electronic hearing aids : it is like a small public address
system . It consist of a microphone to detect sound, an
amplifier to increase its energy , and loudspeaker to deliver
the increased energy to the ear .
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Thank you

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Questions

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رفعت المحاضرة من قبل: Haider Abdullah
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