The document summarizes the mechanism of hearing in three main steps: 1) Mechanical conduction of sound through the pinna, ear canal, and middle ear bones, which function as an acoustic transformer to increase sound pressure; 2) Transduction of mechanical energy into electrical impulses by hair cells in the cochlea; 3) Conduction of electrical impulses through the auditory nerve pathway to the brain. It then describes four historical theories of hearing: Helmholtz's place theory in which different frequencies excite different regions of the cochlea, Rutherford's frequency theory in which all frequencies activate the entire cochlea, Wever's volley resonance theory involving asynchronous discharges, and von

1. Dr Ajay Manickam
JR – DEPT OF ENT AND HNS
RG KAR MEDICAL COLLEGE
The Mechanism of Hearing

2. The Mechanism of hearing

3. Broad classification
Mechanical
conduction of sound
Transduction of
mechanical energy
to electrical
impulses
Conduction of
electrical impulses
to brain

4. Mechanical conduction
 Pinna - determine origin of
sound – concha acts like a
megaphone to concentrate
sound – 6dB increase in
sound pressure
 EAC - in concert with pinna
increase sound pressure by
15 to 22 dB at 4000Hz
 Middle ear transformer
mechanism
1. Catenary lever
2. Ossicular lever
3. Hydraulic lever

5. Mechanical conduction (Acoustic
Transformer)
Middle ear transformer
mechanism
1. Catenary lever – ear drum –
2times gain in sound
pressure
2. Ossicular lever – ossicles –
handle of malleus 1.3 times
longer than long process of
incus – mechanical
advantage of 1.3
3. Hydraulic lever – diff in
surface between TM (55 sq
mm) & Stapes (3.2 sq mm) –
areal ratio – 14: 1

6. Acoustic transduction
 By the lever effect
sound waves enter the
liquid medium from air
medium
 Thus by transduction
sound waves are
converted to electrical
waves by hair cells,
basilar membrane.

7. Conduction of electrical impulses to brain
1
• Hair cells peripheral / spiral ganglia central
2
• Cochlear nerve
3
• Ventral & dorsal Cochlear nucleus
4
• Superior olivary nucleus
5
• Lateral lemniscus
6
• Inferior colliculus
7
• Medial geniculate body
8
• Auditory cortex

8. Theories of hearing
 Helmhotz’s place theory (1883)
 Rutherford’s frequency theory (1886)
 Wever’s volley resonance theory (1949)
 Von bekesy’s travelling wave theory (1960)

9. The PLACE theory (1883)
 High frequency will
excite BASAL region
 Low frequency will
excite APICAL
region

10. The FREQUENCY theory (1886)
 All frequencies activate
entire length of basilar
membrane
 All vibrations are
portrayed to the brain
without complex
vibrations of cochlea

11. VOLLEY RESONANCE theory (1949)
 High frequencies –
perceived in basal turn
 Low frequency – frequency
stimulation
 Intermediate frequencies –
asnchronous discharges
which then combine
actively to represent the
frequency of stimulus

12. TRAVELLING WAVE theory (1960)
 Begins from base move to
apex
 Independent of frequencies
 Region of maximum
displacement varies
according to frequency
 High pitched – short
travelling wave
 Low pitched – long wave
maximum displacement
near apex

13. Thank you