Scottish Sensory Centre

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 auditory pathways."
Bamford and Saunders (1992)

Binaural listening provides:

1. Localization - Information provided by time of arrival and intensity differences at the two ears
   
2. Binaural summation - Binaural threshold 3dB better than monaural. may be significant for users of binaural aids
   
3. 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.
   
4. 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

Anatomy of the Ear

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.

The Outer Ear

Part of ear visible from outside of body

Made up if Pinna and Ear Canal

The Pinna:

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:

The ear canal reinforces sound as it is directed towards the eardrum in two ways:

1. Intensity of sound at eardrum is increased by 20 dB because of resonances from the external ear (concha, meatus, canal and eardrum).
2. 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:

1. The curved nature of the canal makes it difficult for foreign bodies to make their way to the eardrum.
2. 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.
3. 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 eardrum

the middle ear cavity

the ossicles

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.

The Eardrum:

• 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 Ossicles:

• 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.

The rolled out Cochlea

• The functioning of the cochlea can be better understood if it is pictured as though it were unrolled.
• The footplate of the stapes connects to the cochlea at the oval window
• The cochlea is filled with fluid and partitioned by the Basilar membrane
• Sitting on the basilar mambrane is the Organ of Corti

The Organ of Corti:

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 functions

• When the footplate of the stapes moves back and forth in the oval window the fluid in the cochlea is displaced
• The round window is flexible to allow for the displacement of the fluid
• The motion of the cochlear fluids cause the basilar membrane to bend slightly
• Movement of the basilar membrane causes the hair cells embedded in the tectorial membrane to be distorted
• The hair cells are connected to nerve fibres
• When the hair cells are bent the nerve fibre sends an electrical signal to the auditory nerve
• This nerve impulse is then carried to the brain where it is perceived as sensory sound.

Two major functions of the cochlea

1. A frequency analyser
2. An amplifier
3. The cochlea as a frequency analyser:

• The basilar membrane is narrower at its base (oval window) than at its apex
• The base of the basilar membrane responds to high frequency sounds. As move towards the apex it responds more to lower frequency sounds.
• Therefore, freqency analysis of the incoming signal takes place at the auditory periphery, within the cochlea.

The cochlea as a sound amplifier:

• Inner hair cells are connected to auditory nerve and send signals to the auditory nerve when stimulated
• Outer hair cells contract and expand when stimulated
• Outer hair cells (3 rows) are embedded into tectorial
• Inner hair cells (1 row) are not touching tectorial membrane.

The theory of amplification in the cochlea

• Low intensity sound enters cochlea, the fluids are displaced but insufficient movement to move inner hair cells and send signal to brain
• However, minimal displacement is picked up by outer hair cells and they react by expanding and contracting
• Since tips of outer hair cells are embedded in tectorial membrane it begins to move up and down
• The movement of the tectorial membrane moves the inner hair cells
• The inner hair cells move and send a signal via auditory nerve to the brain
• The low intensity soud is heard

The cochlea is an active mechanism not a passive one.

Summary

• Various forms of energy are involved in the function of hearing
• Sound in the environment (acoustic energy) is enhanced by the resonances in the outer ear and converted to mechanical energy by the eardrum.
• The mechanical energy is enhanced by the eardrum and ossicles and transmitted to the cochlea fluids through the oval window
• The motion of the fluids within the cochlea is a hydraulic form of energy
• The fluid movement distorts the hair cells which sit on the basilar membrane
• The hair cells convert the mechanical/hydraulic energy into electrical impulses
• The electrical impulses are transmitted through the nervous system to the brain where they are finally perceived and interpreted as sound