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Lecture 1 - Conductive Mechanisms - the Outer and Middle Ear - Coggle…
Lecture 1 - Conductive Mechanisms - the Outer and Middle Ear
Describe the basic structure of the auditory system
Explain the role of the pinna as sound collectors and for vertical localisation
The pinna is a funnel that serves to collect and direct sound into the external auditory meatus
The various shapes of the pinna modify the sounds, particularly at high frequencies.
The sound waves can take different pathways into the ear canal with reflections off different parts of the pinna.
The pattern of modification of the sound is different depending on the direction that the sound comes from, causing resonances.
These resonances implant direction-specific patterns into the frequency responses of the ears, which can be evaluated by the auditory system for vertical sound localisation.
Explain the role of the middle ear in transmitting sound from the outer ear to the cochlea
As the sound hits the TM it causes it to vibrate at different speeds depending on the frequency of the sound
These vibrations are then transferred to the ossicles
The middle ear matches the acoustic impedance of air to that of the fluid within the cochlea.
TM to stapes footplate area ratio = 56.7mm2:3.2mm2
20 x log (17.7) = 25dB
Middle ear ossicle lever ratio
Tympanic membrane buckling (another lever effect)
TM moves less further way from the area where the footplate is attached causing another lever effects
2:1 increase in pressure (or +6dB)
Describe what is meant by the acoustic reflex
When muscles in the middle ear contract in response a high level stimulus.
This stiffens the ossicular chain, and reducing low-frequency sound transmission through the middle ear. This protects the inner ear from damage due to sound over-exposure.
Muscles do not contract fast enough to protect against sounds that start suddenly (e.g. a gun shot) - 10ms latency.
This reflex also fatigues for long sounds that last for more than a few seconds. Approx decay time=10s
Primarily attenuates sounds below 1-2kHz by up to 10dB
The stapedius muscle contracts just before and relaxes just after we speak. This reduces transmission of the sound of our own voice to the inner ear.
Have a basic understanding of the cochlea works
The stapes/footplate is attached to the oval window, which converts the mechanical pressure from the middle air to the fluid within the cochlea
This movement of the fluid between the vestibul scala and the tympani scala via the helicotrema excites the basilar membrane
The movement of the BM triggers the hair cells within the organ of Corti (located in the scala media) to depolarise and causes auditory nerve to fire
Explained in depth in lecture 2
