Download Hearing part I

‫بسم هللا الرحمن الرحيم‬
‫﴿و ما أوتيتم من العلم إال قليال﴾‬
‫صدق هللا العظيم‬
‫االسراء اية ‪58‬‬
By
Dr. Abdel Aziz M. Hussein
Lecturer of Physiology
Member of American Society of Physiology
• Audition or hearing is the sense that allows us to
communicate and hence interact with other organisms
throughout the world
• Sound waves are the adequate stimulus for hearing
• The ear is the organ of hearing
• They are longitudinal pressure waves consisting of
alternating phases of compression and rarefaction that
requires an elastic medium for its transmission (usually air
or water not vacuum)
Sounds waves have frequency, intensity, and wave form
Frequency or Pitch
•It is the number of waves
that pass a certain point in a
second.
•It is measured in
cycles/second or Hertz (Hz)
•Male voice 125 Hz
•Female voice 250 Hz
Intensity (amplitude) of sound
It is the amount of sound energy passing through a
given unit of area per unit of time (dyne/cm2)
The decibel scale is a logarithmic scale that is usually used to
describe the sound intensity
•The intensity of standard sound is zero decibels (dB)
Normal speech has an intensity of about 60-65 dB
Intensity of Sound
Threshold of audibility
It is the lowest intensity just
gives a sound sensation
It equals the intensity of
standard sound or zero
decibel
Sounds intensities of about
120dB produce discomfort
140dB are painful
Wave form (quality or timbre)
The timbre or quality of
the sound is determined
by its wave form
• The organ of hearing is the ear that consists of;
1. External ear
2. Middle ear
3. Cochlea of the inner ear.
dr Abdelaziz Hussein, Mansoura Faculty
of Medicine
• The external ear consists of
1. Ear pinna or auricle
2. External auditory meatus
• It is the cartilaginous
external flap and its
extension (the tragus)
Function
1. It collects and directs
sound waves
2. It plays an important role
in sound localization
• It is an oblique tortuous skin-lined canal of about 2.5
cm length in the temporal bone
Functions
1. It conducts and concentrates the sound waves
2. It produces resonance which amplifies the sound
Functions
3. It protects the tympanic membrane
4. It maintains the proper temperature and humidity of
the air inside
• It is an air-filled cavity
in the temporal bone.
•3 bony ossicles; the malleus
(hammer), incus (anvil), and
stapes (stirrup).
•2 skeletal muscles tensor
tympani, and stapedius ms
•bounded by the tympanic
membrane on lateral side and
the oval and round windows on
the medial side
Functions of Middle Ear
•Transfer sound from the external ear to the inner ear
• Its surface area is about 55 mm2 and its thickness is
about 0.1 mm.
It acts as a resonator
It is highly damped
It is aperiodic
• The fluid has great inertia than air, therefore,
the transmission of sound waves from air in
the external ear to endolymph in the inner ear
results in a great loss of energy
• More than 97% of a sound's energy would be
reflected at the surface of water
This impedance or resistance
to sound transmission can be
matched by 2 mechanisms
1. Areal ratio between
tympanic membrane and
oval window
And
2. Level system of bony
ossicles
The areal ratio of the tympanic membrane and the oval
window
The surface area of the drum (55
mm2) is about 17 times greater than
that of the foot plate of the stapes
(3.2 mm2)
It produces an amplification
sound waves about 20-fold
at the oval window
The lever system of the bony ossicle
ossicles are arranged in such
a manner that they function
as a series of levers
handle of the malleus is about
1.3 times that of long process of
the incus
increases the force of movement of
the stapes about 1.3 times
Both mechanisms amplify the sound
pressure at the oval window about 22
times
The efficiency the impedance
matching device is not
100%, but it is about 50 to
75% for sound frequencies
between 300 and 3000Hz
THANKS