ARC 507: Environmental Control III (Acoustics and Noise Control) Department of Architecture, Federal...

ARC 507: Environmental Control III (Acoustics and Noise Control)Department of Architecture, Federal University of Technology, Akure, Nigeria

Lesson 2: Hearing mechanism

Environmental control iii (arc 507)

Department of architecture

Federal university of technology, akure


The Hearing Mechanism 1. Introduction. 2. Hearing. 3. The Ear. 4. The External Ear. 5. The Inner Ear. 6. Frequency Discrimination. 7. The Organ of Corti. 8. Tests and Exercises. 9. References.


1. Introduction.

The importance to humans of the sense of hearing.

Details the hearing mechanism.

The role played by the different parts of the ear and the brain.

Hearing is needed for protection from danger, communication and enjoyment of surroundings.

Noise is unwanted or damaging sound.

The three main parts of the ear - the external, middle and inner ear - and their roles in hearing sounds.

Damage to the ear by excessive noise.


2. Hearing.Hearing is essential for the location of

sounds, development of speech and language for communication.

Figure x: The ear.


2.1 The Importance of Hearing

"...after a lifetime in silence and darkness that to be deaf is a greater affliction than to be blind ... I have imagination, the power of association, the sense of touch, smell and taste, and I never feel blind, but how can I replace the loss of hearing?“ – Helen Keller (Gasaway, 1997)

Helen Keller, the well-known campaigner for the blind once said:


2.2 Hearing System Is Easily Overloaded.

The hearing mechanism receives input from all directions.Never shuts off. Hearing can only be partially protected from continual high intensity noise by complex Central Nervous System (CNS) mechanisms.Ears cannot cope with some of the noises introduced to our environment since the industrial revolution.

This results for many people in partial deafness. Overloading the system by noise hinders the understanding of what we are hearing.


2.3 Sensitivity of the Ear.

The ear detects sounds over a wide range of frequencies and sound intensities.

Figure x: Threshold of hearing.

The minimum sound pressure level perceptible to the ear at a particular frequency is the threshold of hearing.

A young, healthy ear can respond over a frequency range of 20 Hz to 20 000 Hz.

A progressive loss in sensitivity at the high frequencies occurs with increasing age (presbyacusis).


3. The EarThe ear is situated in the temporal bone of the skull.

The external ear - collects the airborne sound waves.

The middle ear - transmits the sound waves as a vibration.

The inner ear - changes the sound waves to electrical impulses and sends them via the auditory nerve to the brain.

Figure x: The ear structure.


4. The External Ear.

It consists of the outer ear uricle, ear canal and the eardrum.

Outer Ear collects the sound waves.

The sound waves then travel through the ear canal.The eardrum is situated at the end of the ear canal.It vibrates in synchrony with the sound pressure.

.1 Figure x: The external ear

It detects the direction of the sound source.


4.1 Outer Ear.

By modifying the frequency spectrum of the impinging sound.

Is the visible part, collects the sound waves.

Is located on each side of the head.

It detects the direction of the sound source.


4.2 Ear Canal

Sound waves travel through the ear canal.The outer third consists of cartilage.

These allow the ear canal to clean itself.

Wax build-up blocks the ear canal.

It protects the eardrum from mechanical damage.It has an effect on the transmitted sound magnitude.


4.3 Eardrum.The eardrum is situated at the end of the ear canal.It is a very thin cone shaped membrane.

Perforation of the eardrum causes a hearing loss. The eardrum vibrates in synchrony with the sound pressure changes.The first of the three middle ear bones is attached to the eardrum.

Figure x: The ear structure.

About 7 mm in diameter at an angle of 55° to the canal floor.


4.4 Ossicles.The middle ear, a box-like cavity is about the size of the nail of the index finger. In the upper part lie three small bones collectively called the ossicles.These are: the hammer (malleus); the anvil (incus); and the stirrup (stapes).

The energy transferred is most efficient in the 1 to 4 kHz frequency range.

The vibration is amplified about 25 times due to the mechanical advantage of the lever action.

Interruption of the ossicle chain can result in a hearing loss of 60 dB.

Figure x: The middle ear.


4.5 Tensor Tympani & Stapedius Muscles.

The ossicles are suspended by ligaments and two small muscles.The tensor tympani muscle is attached to the hammer and the stapedius muscle to the stirrup. These muscles contract in the presence of intense sound.Sound intensity is reduced by about 10 to 30 dB, partially protecting the inner ear from damage.It takes approximately 25 milliseconds for the muscles to respond.

Figure x: The middle ear.

Impulsive noises (hammering) will not be attenuated.


4.6 Eustachian Tube.

The middle ear opens to the eustachian tube which connects with the back of the nose.

It maintains equal air pressure on both sides of the eardrum.

It is normally closed, but opens with swallowing or yawning.

Inflammation or infection of the nose or throat may cause blockages of the eustachian tube.

Fall in pressure or infection of the middle ear can lead to hearing loss.


5. The Inner Ear.

Figure x: The inner ear.

It is the part of the ear for hearing, with structures which are damaged by excessive noise.

It consists of three cavities: the vestibule; the three semi-circular canals and the cochlea Each division contains an incompressible fluid called perilymph.


5.1 CochleaA bony spiral organ,sss about 35 mm long, shaped like a snail shell of 2 1/2 turns. The cochlea is itself divided lengthwise into three chambers:The scala vestibuli - which has the oval window at its base;The scala tympani - which ends in the round window ; andThe scala media - which contains the true hearing sensory structure.The scala media is at a slightly higher electrical potential than the other two chambers (+80 mV).

This potential difference is important for the correct functioning of the cochlea.

Figure x: Cross-Section of the Cochlea.


5.2 Organ of Corti.

Vibration of the stirrup and oval window sends a travelling wave through the perilymph fluid in the scala vestibuli and scala tympani.

The amount of displacement of the membrane depends on the amplitude of the wave at a particular point.

The organ of Corti is a complicated system of cells extending along the basilar membrane.

There are about 30,000 hair cells placed in four rows.The hair cells transform the movement into nerve impulses.

This movement is detected by the sensory hair cells of the organ of Corti.

Causing both the round window and the basilar membrane to move.


5.3 Auditory Nerve.

Nerve fibres carry the impulses from the hair cells.

They pass through the spiral ganglia, to join together to become the auditory nerve.

This connects to the cochlea nuclei in the brain stem.

Here the messages, received and analysed by the ear, are interpreted.

Hence to the higher auditory centres in the temporal lobe of the brain.


6. Frequency Discrimination.

6.1 BASILAR MEMBRANE.

The ear detects different frequencies in sound due to the basilar membrane.The basilar membrane is one of the most important structures in the cochlea.

The elasticity interacts with the inertia of the fluids in the cochlea to support a wave-like motion travelling from the basal end to the apex.The ratio of stiffness to mass varies.

This frequency discrimination is essential to good hearing.ss

The basilar membrane acts to sort the incoming sound waves into different frequency components.

It has the mechanical properties of elasticity, damping and mass.


7. The Organ of Corti.

7.1 BASILAR MEMBRANEThe organ of Corti contains the sensory hair cells.

There are two types of hair cells - inner and outer.

The inner hair cells form a single row along the inside spiral of the cochlea.The outer hair cells are in three parallel rows towards the outside of the spiral.

Figure x: Section of basilar membrane with organ of corti

The sensory cells are embedded in supporting cells attached to the basilar membrane.


7.2 Stereocilia.

Stereocilia are a cluster of hair like structures.

They are arranged in "w" or "v" formations.

They are rigid and composed of actin enclosed in a plasma membrane.They vary in length depending on their position along the basilar membrane.

The tectorial membrane is attached to the outer hair cell stereocilia.They function like a microphone.


7.3 Function of the Inner and Outer Hair Cells.

The inner hair cells are the primary sensory cells.They directly connect to individual nerve fibresof the auditory nerve.The sound-induced voltage changes within the inner hair cells.

Its amplitude increases as the wave slows to a halt.

This lead to electrical activity in the nerve, which is sent to the brain.

Figure x: Stereocilia of outer hair cells.

Figure x:movement of the basilar membrane and stereocilia


7.3 (cont.) The feedback loop process

Acoustic energy enters the cochlea via the motion of the stirrup.The vibration induces the travelling wave.It slows down as each frequencycomponent approaches the cut-off point.The outer hair cells sense the basilar membrane motion.The wave vibrations reach a peak andthen fall away.

Figure x: Feedback loop.


7.4 Damage to Hair Cells.

Hearing loss is acquired loud noise, certain drugs and the ageing process.

It results to an inability to hear softer sounds except louder ones.

The damaged outer hair cells result to a greatly reduced amplitude of vibration.

The outer hair cells inject a limited amount of energy.

They have little influence on large amplitude vibrations.


7.5 the nerve fibres

The nerve fibres are attached to both the inner and outer hair cells.

More than 90% of the afferent fibres are connected to the inner hair Cells.

The numerous outer hair cells connect with about 9% of the afferent fibres.

There is only a weak connection probably passing control information.Most efferent fibres terminate on outer hair cells, with fewer attached to inner hair cells.

They are either afferent (to the brain) or efferent (from the brain).


9. References.

Callender, J.H. (1974). Time-Saver Standards for Architectural Design Data. McGraw-Hill BookCompany.Callender, J.H. (1974). Time-Saver Standards for Architectural Design Data. McGraw-Hill BookCompany.Givoni, B. (1976). Man, Climate And Architecture. Second Edition. Applied SciencePublishers Ltd., London.Koenigsberger, O.H., Ingersoll, T.G., Mayhew, A. and Szokolay, S.V. (1974). Manual of TropicalHousing And Building, Part I, Climatic Design. Longman, London.Markus, T.A. and Morris, E.N. (1980). Buildings, Climate and Energy. Pitman International,London. National Universities Commission (1977). Standards Guide for Universities. NationalUniversities Commission, Lagos.Olgyay, V. (1963). Design With Climate - Bioclimatic Approach To Architectural Regionalism. Princeton University Press, Princeton,NewJersey.

United Nations (1971). Design of Low Cost

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