hearing structure and function. function of auditory system analyse discriminate -sounds -widely...
TRANSCRIPT
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HEARING
Structure and Function
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Function of auditory system• Analyse
• Discriminate-sounds - widely varying intensity
widely varying frequenciesany spatial direction
complex varying patterns of pressure and frequency which differ at the two ears
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Action of human ears is to interpret sound input by
perceiving: intensity, frequency,
timbre, localisation
& masking of one sound by another.
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Definitions(subjective correlations)
• Intensity = Loudness
• Frequency = Pitch
• Timbre = Musical quality
• Localisation = Spatial position
• Masking = One sound is
selectively heard in
preference to others
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Measurement
• Sound intensity is normally measured in N/m² or Pascals
• The vast range of sound pressures perceived by humans is more conveniently described by a smaller range expressed in a logarithmic scale as decibels as a ratio of a reference intensity (2 x 10 ⁵ N/m⁻ ²)
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Measurement
• Human range is:
0 db - 130 dB
“absolute” threshold pain threshold
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0 dB is a reference point for the “average”
Step sizes less than 1 dB are rarely detectable
Changes in intensity of equal numbers of dB correspond to approximately equal steps in
loudness
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0 dB is a reference point for the “average”
Step sizes less than 1 dB are rarely detectable
Changes in intensity of equal numbers of dB correspond to approximately equal steps in
loudness
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0 dB is a reference point for the “average”
Step sizes less than 1 dB are rarely detectable
Changes in intensity of equal numbers of dB correspond to approximately equal steps in
loudness
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Some levels
• From 1 m - whisper - 30 dB
- conversation - 60 dB
- shout - 90 db
- discomfort - 120 dB
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Definitions(subjective correlations)
• Intensity = Loudness
• Frequency = Pitch
• Timbre = Musical quality
• Localisation = Spatial position
• Masking = One sound is
selectively heard in
preference to others
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Measurement
• Frequency is measured in Hz
• Frequency has the subjective correlate of pitch which is how the ear perceives changes in frequency
• However, complex sound has no clear specific pitch and the hearing apparatus has to sort the complex input and produce an intelligible interpretation
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Measurement
• Human ear can appreciate frequencies between 12Hz and 20 000Hz
(some people have greater range)modern piano keyboard is 25Hz – 4000Hz
• “Speech range” is 200Hz - 10 000Hz
(mostly utilise 500-4000 Hz)
• Music 50Hz - 20 000Hz
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Definitions(subjective correlations)
• Intensity = Loudness
• Frequency = Pitch
• Timbre = (musical) quality
• Localisation = Spatial position
• Masking = One sound is
selectively heard in
preference to others
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Timbre
• The fundamental frequency is the lowest note in a complex sound
• The overtones or harmonics are simple multiples of the fundamental frequency and are responsible for the quality of the sound
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Definitions(subjective correlations)
• Intensity = Loudness
• Frequency = Pitch
• Timbre = Musical quality
• Localisation = Spatial position
• Masking = One sound is
selectively heard in
preference to others
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Ear
• Conducting apparatus
- auricle - EAC - TM - ossicular chain - IE fluids -
• Perceiving apparatus
- end-organ (Corti) - VIII - cerebral cortex -
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External ear
• Pinna / auricle
• Skin covered cartilage
• “Directs” sound into EAC
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External ear canal
• Skin covered cartilage and bony canal
• Channels acoustic energy to TM
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Tympanic membrane
• 3 layers:
– Squamous epithelium
fibrous tissue
– Respiratory mucous membrane
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Middle Ear
• Contains:
malleus - hammer
incus - anvil
stapes - stirrup
• Small “auditory muscles”
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Transformer
• Acoustic energy arrives at TM & is transmitted to the stapes footplate
- area ratio TM : footplate = 14:1
- lever effect of ossicles = 1,3:1
- overall “transformer ratio” = 18:1
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• Amplitude of vibration at TM is reduced by the time it reaches the footplate
• Force entering fluids is increased in same proportion
• Widely differing acoustic resistances between air & fluid are matched
• Transfers maximum energy from air to fluid
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• Stapedius & tensor tympani muscles reflexly contract above 90 dB
• This attenuates loud sound to protect IE against acoustic trauma
Impact / explosion noise reaches cochlea before reflexes can act – damage is worse than with steady state noise
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Physiology• Sound is transmitted to IE in three ways:
1. Via the ossicular chain - most important
2. Bone conduction – through bones of the skull, sound energy taken up and transmitted to cochlea
3. Directly across ME – on to round window when there is TM perforation
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Physiology• Airborne sound – vibrations of atmosphere –
pass along the EAC to the TM - set in motion
• Transmitted to ossicles – which rotate around fulcrum & transmit vibrations to oval window
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Physiology
• Transmitted through oval window to perilymph in scala vestibuli around the helicotrema to round window
(reciprocal movement)
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Physiology
• This sets up a travelling wave starting from the cochlea base & progressing toward the helicotrema with increasing amplitude
• There is a sharply defined region of maximum displacement – position depends on frequency
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Physiology
• High frequency – maximum displacement at basal turn of cochlea
• Low frequency – longer travelling wave with maximum amplitude nearer the helicotrema
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Physiology
• Vibrations displace basilar membrane – shearing movement between hair cells and tectorial membrane – hairs displaced in relation to their bodies
• Not fully understood but this displacement results in neuronal stimulation - nerve impulses in VIII
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Neuronal stimulation
• 3 main theories
• Attempt to explain conversion of all varied dimensions of sound
• Possible variations in nerve action potentials depend upon
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Neuronal stimulation
• Particular nerve fibre being activated – place along basilar membrane
• Total numbers of fibres activated
louder sound – more activated
• Threshold of individual fibres – majority of fibres have threshold 10-15 dB only a few above 80 dB
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Internal Auditory Canal
• ± 1 cm long
• In petrous temporal bone
• Transmits - VII
- VIII
- internal auditory art. & vein
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Audiometry
• Pure Tone Audiogram (PTA) is the cornerstone of assessment of hearing
• Establishes subject’s pure tone hearing threshold – the minimum sound level at which a specific response can be obtained
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Audiometry
• Test 125, 250, 500, 1000, 2000, 4000, 8000 Hz
(3000 & 6000 for noise-induced loss claims )
• Air conduction
• Bone conduction - not 125 & above 4000 as
unreliable
- indication of cochlea
function
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Audiometry
• Soundproof room
• Earphones / bone conductor
• Subject signals by pressing button
• Pure tones produced by calibrated audiometer
• Intensities - - 20 dB
+ 110 dB
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Normal audiogram
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Speech vs Pure tone
• Main function of human ear is perception of spoken word
• Speech consists of very complex sound• Varying frequencies – fundamentals & harmonics
• Vowels & consonants
• Accents
• Languages
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Speech vs Pure tone
• PTA is not always good assessment of speech discrimination
• Speech Audiometry measures actual disability produced by any hearing loss
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Definitions(subjective correlations)
• Intensity = Loudness
• Frequency = Pitch
• Timbre = Musical quality
• Localisation = Spatial position
• Masking = One sound is
selectively heard in
preference to others
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Localisation - Binaural hearing
- differences in time of arrival (10-20μs)
- differences in intensity at two ears
- phase differences ( at less than 1500Hz)
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Localisation - Pinnae
- spectrum of sound entering ear
modified by pinna in a way that
depends on direction of sound source
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Localisation - Precedence effect
• Many different paths - direct
- reflected (echoes)
Several sounds in close succession are the direct sound & its echoes. These are perceptually fused and the location of the total sound is determined by the combination of the direct sound and its echoes.
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Complex?
Fascinating?
Want to know more?