Audiology Refresher 2
Presented in November, 2002
- The anatomy of the ear
- The outer ear
- The ear canal
- The middle ear
- The inner ear
- How the cochlea functions
We listen with two ears
"A listener's ability to perceive and organize his auditory environment
depends partly on the use of two ears and the resulting neural interactions
that occur between the binaural signals as they progress through the
Bamford and Saunders (1992)
Binaural listening provides:
- Localization - Information provided by time of arrival and intensity
differences at the two ears
- Binaural summation - Binaural threshold 3dB better than monaural.
may be significant for users of binaural aids
- Precedence effect - This is the way we fuse similar sounds together,
provided they are close together in time and same spectral pattern.
One way normally hearing listeners cope with signals and their echoes
in reverberant environments.
- Binaural masking level differences (BMLD) - Also called 'binaural release from masking', occurs whenever the phase or intensity differences of the signal at the two ears are not the same as the phase and intensity differences of the masker at the two ears. Analogous to the 'cocktail party' phenomenon where normal listeners are able to squelch out the noise and hear the speech signal they are attending to.
To find out more read: Bamford J and Saunders E, 1992. Hearing Impairment, Auditory Perception and language Disability. London, Whurr Publishers Ltd, 216-222
The ear is divided into three sections:
the outer ear
the middle ear
the inner ear
Each part of the ear plays a different role in the transmission of sound from the environment to the brain where the sound is perceived.
Part of ear visible from outside of body
Made up if Pinna and Ear Canal
The pinna is made of cartilage and is attached to the side of the head
The pinna has three main purposes:
It protects the ear canal and eardrum by repelling any objects that strike it.
It collects sound from the environment, directing it down into the ear canal. Up to 6 dB intensity increase.
Its presence on both sides of the head allows us to localise the source of sound, using intensity and phase differences of signal at two ears
The ear canal reinforces sound as it is directed towards the eardrum in two ways:
- Intensity of sound at eardrum is increased by 20 dB because of resonances from the external ear (concha, meatus, canal and eardrum).
- This increase in sound intensity is for high frequency sounds. In adults the peak freqeuncies are found around 2500 Hz. For children the peak intensities are at higher frequencies.
- Low frequencies get little gain
- High frequencies more gain
- Children have smaller narrower tubes therefore higher resonant frequencies
- for given input, smaller the volume then higher the intensity
It provides protection for the eardrum in two ways:
- The curved nature of the canal makes it difficult for foreign bodies to make their way to the eardrum.
- EarWax produced by the ear canal and small hairs also impede progress of small objects down the canal and ensure that they do not get down to the eardrum.
- The lenght of the canal also controls the environment for the eardrum and middle ear. The ear canal will stay warm and moist so that outside conditions will not adversely affect the middle ear and eardrum.
- The outer two thirds of the ear canal is made of cartilage.
- The innermost one third is surrounded by the bone of the skull. This bony portion is highly sensititve and injury to this area is painful.
The Middle Ear
The middle ear consists of
the middle ear cavity
The Ossicles are three small bones suspended in the middle ear cavity
The primary function of the middle ear is to transmit sound to the inner ear.
- Sound is sent down the earcanal and it strikes the eardrum
- The eardrum sits at an angle at the end of the ear canal
- A healthy eardrum is pearly white in colour
- The changes in pressure produced by the vibrating air particles will cause the eardrum to also vibrate
- The acosutic energy in the sound is changed to mechanical energy in the eardrum.
- The malleus or hammer is attched to the back of the eardrum
- As the eardrum vibrates back and forth it moves the malleus which is attached to the Incus (anvil) and Stapes (stirrup).
- This produces a piston like motion which connects the movement of the eardrum to the movement of the oval window attached to the footplate Stapes.
The Middle ear as an Impedence Transformer
- The process of hearing requires that there is a transfer of acoustic energy from air-borne sound waves (in the environment) to fluid borne vibrations (in the cochlea)
- This is a problem because the transfer of energy from air to fluid is very inefficient
- The middle ear ats to address this problem
- The surface area of the eardrum is many times larger than the surface area of the footplate of the stapes
- The effect of this difference is to focus the energy at the eardrum to a smaller space
- Similar principle to pushing a tack into a wall with your thumb - pressure collected over large surface and focused onto the point of the tack
- The eardrum and ossicles provide a 30 dB increase in signal as sound is passes from eardrum (air) to oval window (fluid).
Air Pressure in the Middle Ear Cavity
- The eardrum moves back and forth in response to small changes in air presure (sound)
- The eardrum is most efficient at transferring sound when the air pressure on both sides of the eardrum is the same
- Changes in environmental pressure can deform the eardrum if the air pressure is unequal on both sides of the eardrum
- To work properly, the pressure must change in the middle ear cavity at the same rate it changes in the atmosphere outside
- For this reason the eustachian tube connects the middle ear cavity to the mouth cavity
- Sometime when we have a cold the eustachian tube can become blocked. This causes a lowering in pressure in the middle ear. The eardrum is pulled in and the efficiency of hearing is reduced.
The Inner Ear
The inner ear consists of an inticate system of cavities within the bones of the skull.
This is called the Cochlea
It is a small snail like structure encased in bone. It contains the structures responsible for converting
the mechanical energy from the middle ear to electrical impulses which are
sent to the brain. This is the structure that generates nerve impulses
It is made up of The tectorial membrane - a jelly like substance Three rows of outer hair cells One row of inner hair cells The basilar membrane Supporting cells
How the Cochlea
The rolled out Cochlea
The Organ of Corti:
Two major functions
of the cochlea
The cochlea as a
of amplification in the cochlea
is an active mechanism not a passive one.
It is a small snail like structure encased in bone.
It contains the structures responsible for converting the mechanical energy from the middle ear to electrical impulses which are sent to the brain.
This is the structure that generates nerve impulses It is made up of
The tectorial membrane - a jelly like substance
Three rows of outer hair cells
One row of inner hair cells
The basilar membrane
How the Cochlea functions