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2019.04.11.
1
Physiology of the hearing (I)
(Learning objective: 102)
Dr. Attila Nagy
2019
Hearing
• Hearing is a mechanical sense
• A compression wave in air is changed to a vibration of a solid then a wave in a fluid, a displacement of a membrane, a bending of a hair cell and the generation of an action potential
• humans hear sounds 16 to 20000 cycles/second (Hz)
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Phylogenesis of the inner ear
The Hearing System
Basic schematic diagram of the entire auditory system
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Sound System
Source– Any vibrating object
Medium– Any gas, liquid or solid
Receiver– anything designed to
detect the vibrations within the medium originating from the source
A Common Sound System
Illustration of the distribution of molecules surrounding a sound source
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Condensation and Rarefaction
Bands of compression and rarefaction emanating from a sound source
Important Physical Characteristics of Sound
Frequency
• Rate of pressure change as a function of time
• Measured as cycles/sec or Hertz
• The primary determiner of pitch
Intensity
• Magnitude of the pressure change
• Measured as decibel (dB)
• The primary determiner of loudness
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Frequency and Intensity
Sounds a and c share the same frequency and sounds b and c share the same intensity
How to quantify loudness: What is a
decibel?
• 20 lg (p1/p0) dB Sound Pressure Level (SPL)
• (p1: sound of interest, p0: threshold of human hearing at 2 kHz)
• hearing protection: longer exposure of levels above 85 dB
• p0= hearing threshold (2 x 10-5 Pa)
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Loudness and Intensity
Here are some common sounds and their decibel equivalents
Pure (basic) ton: it could be characterized by a single frequency
Sound (musical) tone: a basic pitch tone and and its integral
multiples the harmonics
Noise: it contains several frequencies of the hearable frequency range
(white noise)
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Equal loudness curves, unit (phons)
Psychophysical Measurement
dBPain theshold
Area of speech
hearing theshold
phon
Path of Sound
(air conduction, bone conduction)
AIR CONDUCTION:
• External canal
• Vibrates eardrum
• Vibration moves though ossicles
– Malleus, incus, stapes
• Stapes vibrates oval window of cochlea
• Creates pressure wave in the fluid inside
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The Outer Ear
Major Landmarks:
Pinna
External Auditory Meatus
Tympanic Membrane
Function of the Outer Ear
• Collect and funnel sound to the eardrum
• Protection
• Resonance
• (more esthetic than functional role in
human)
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The Middle Ear
Major Landmarks:
• Eustachian tube
• Oval and round windows
• Ossicles
• Middle ear space
• Middle ear muscles
Muscles of the middle ear
• Stapedius muscle (facial nerve) and Tensor
tympani (trigeminal nerv) may influence of
the movement of the ossicles.
• These muscles contract in response to loud
sounds, thereby reducing the transmission of
sound to the inner ear. This is called Tympanic
reflex.
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incus
stapes
maleus
Thympanic m.
50 mm2
4 mm2
Middle ear amplification (impedance matching)
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BONE CONDUCTION
- Bypasses the outer and the middle ear
- The bones of the scull conduct the vibrations
directly to the inner ear
- Its function in recognition of own woice
•CONDUCTIVE HEARING LOSS
•SENSORY HEARING LOSS
•NEURAL HEARING LOSS
•MIXED HEARING LOSS
Classification of hearing loss
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Located in external ormiddle ear
Temporary
CONDUCTION
DEAFNESS
NERVE
DEAFNESS
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Rinne test –
Weber test
Tuning fork investigations
Audiometry
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Audiometry
Ears closed Failing Eardrum
and ossiclesCochlear or/and
nerval damage
Audiometry
Bone conduction
Air conduction
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Presbyacusis
Presbyacusis
Men Women
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Investigation of pressure relation in the middle ear: Tympanometry
Investigation of pressure relation in the middle ear: Tympanometry
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Investigation of pressure relation in the middle ear: Tympanometry
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