Physiology of Hearing

Conduction of sound

• Impedance matching mechanism or Transformer action of middle ear: conversion of sound of greater amplitude with lesser force to that of lesser amplitude but greater force
• It is accomplished by
• vLever action of ossicles
• Handle of malleus is 1.3 times longer than the long process of incur
• Mechanical advantage  = 1.3
• Lever ratio = 1.3 : 1

Hydraulic action of TM

• Effective vibratory area of TM = 55 sq.mm.
• Area of Foot Plate of stapes = 3.2 sq.mm.
• Hydraulic ratio  = 17: 1
• The product of Lever ratio and Hydraulic ratio (17 x 1.3) gives the Transformer ratio of 22: 1

Natural resonance and efficiency of auditory apparatus

• External auditory canal  — 3000 Hz
• Tympanic membrane — 800 to 1600 Hz
• Middle ear  — 800 Hz
• Ossicular chain — 500 to 2000 Hz

Intensity of various sound at a distance of 1 m

• Whisper — 30 dB
• Normal conversation  — 60 dB
• Shout  — 90 dB
• Discomfort of the ear  — 120 dB
• Pain in the ear  — 130 dB

?    Frequency range in normal hearing — 20 Hz to 20000 Hz
?    In routine audiometric testing — 125 to 8000 Hz are evaluated

PHYSIOLOGY OF HEARING

Conduction of sound

• Impedance matching mechanism or Transformer action of middle ear: conversion of sound of greater amplitude with lesser force to that of lesser amplitude but greater force
  
• It is accomplished by
  
• Lever action of ossicles
  
• Handle of malleus is 1.3 times longer than the long process of incur
  
• Mechanical advantage     = 1.3
  
• Lever ratio    = 1.3 : 1
  

Hydraulic action of TM

• Effective vibratory area of TM     = 55 sq.mm.
  
• Area of Foot Plate of stapes    = 3.2 sq.mm.
  
• Hydraulic ratio    = 17: 1
  
• The product of Lever ratio and Hydraulic ratio (17 x 1.3) gives the Transformer ratio of 22: 1
  

Natural resonance and efficiency of auditory apparatus

• External auditory canal — 3000 Hz
  
• Tympanic membrane  — 800 to 1600 Hz
  
• Middle ear  — 800 Hz
  
• Ossicular chain  — 500 to 2000 Hz
  

Intensity of various sound at a distance of 1 m

• Whisper  — 30 dB
  
• Normal conversation  — 60 dB
  
• Shout  — 90 dB
  
• Discomfort of the ear   — 120 dB
  
• Pain in the ear  — 130 dB
  

?    Frequency range in normal hearing — 20 Hz to 20000 Hz

?    In routine audiometric testing — 125 to 8000 Hz are evaluated