Physiology of Hearing By Dr Abdel Aziz M

Physiology of Hearing By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology

Hearing • 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

Sound Waves • 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)

Physics of Sounds waves have frequency, intensity, and wave form

Physics of Sound 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

Physics of Sound Intensity (amplitude) of sound It is the amount of sound energy passing through a given unit of area per unit of time (dyne/cm 2) The decibel scale is a logarithmic scale that is usually used to describe the sound intensity • The intensity of standard sound is zero decibels (d. B) Normal speech has an intensity of about 60 -65 d. B

Physics of Sound Intensity of Sound

Physics 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 120 d. B produce discomfort 140 d. B are painful

Physics of Sound Wave form (quality or timbre) The timbre or quality of the sound is determined by its wave form

The Ear

The Ear • 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

External Ear • The external ear consists of 1. Ear pinna or auricle 2. External auditory meatus

Ear Pinna • 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

External Auditory Meatus • 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

External Auditory Meatus Functions 3. It protects the tympanic membrane 4. It maintains the proper temperature and humidity of the air inside

Middle Ear

Middle Ear • 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

Middle Ear Functions of Middle Ear • Transfer sound from the external ear to the inner ear

Tympanic Membrane • Its surface area is about 55 mm 2 and its thickness is about 0. 1 mm. It acts as a resonator It is highly damped It is aperiodic

Function of the Ossicular System (Bony ossicles)

Function of the Ossicular System • 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

Impedance Matching 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

Impedance Matching The areal ratio of the tympanic membrane and the oval window The surface area of the drum (55 mm 2) is about 17 times greater than that of the foot plate of the stapes (3. 2 mm 2) It produces an amplification sound waves about 20 -fold at the oval window

Function of the Ossicular System The lever system of the bony 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

Function of the Ossicular System The efficiency the impedance matching device is not 100%, but it is about 50 to 75% for sound frequencies between 300 and 3000 Hz

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