The Auditory Syatem and the Hearing Mechanism
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[pic 1]FACULTY OF SCIENCE AND TECHNOLOGY[pic 2]SBFS1103THINKING SKILLS AND PROBLEM SOLVING[pic 3]THE AUDITORY SYSTEM AND THE HEARING PROCESS AND SOUNDSNITHIA SHARMILEE9009261462860019009261462860173549572syamanthra@yahoo.comA.MANIMARAN A/L K.ANJAN Petaling Jaya Learning CentreMAY 2014 SemesterTable of Contents1.0. iNTRODUCTION32.0. AUDITORY SYSTEM 32.1. Outer Ear42.2. Middle Ear42.3. Inner Ear52.4. Central Auditory System73.0. HEARING PROCESS74.0. CLASSIFICATIONS OF SOUND9 3.3. Loudness9 3.4. Pitch 10 3.5. Timbre115.0. CONCLUSION115.0. CITATION13IntroductionWe experience the world we live in through our senses that allow us to make sense of our environment and effectively navigate it. We see, hear, smell, taste, and touch the stimulus in the environment through our sensory organs. The auditory system provides us with vital environmental and emotional information. Our ability to sense and perceive different types of sounds in their varied forms plays a significant part in our lives. For example, it warns of potential dangers (e.g. hearing a car approaching from behind) and facilitates the expression of thoughts, feeling and intentions as well as learning through listening. Moreover, it serves for relaxation and entertainment (i.e. listening to music). Nature has evolved the human ear into an organ able to attend to specific sounds and detect the type, source and changes in these sounds. To understand the complex working of the ear, we will discuss both the periphery and central auditory system and hearing process and the types of sounds the ear can differentiate. Auditory System The auditory system encompasses the peripheral and central auditory system. As you can see in Figure 1, the auditory periphery can be further divided into three parts; the outer ear, middle ear and inner ear. The central auditory system encompasses the cerebral cortex. [pic 4]Outer EarThe main components of the outer ear are the auricle (i.e. pinna) and the ear canal. The auricle is a concave cartilaginous structure that laterally extends from the sides of our head. Its deep shell-like part that gives it its shape is called the concha. The auricle works like a funnel and selects sound waves and amplifies or sometimes attenuates it before directing it into the auditory canal. The change of spectrum act as cues to azimuth, elevation, distance, and velocity. For instance, Rohan can tell that the woman screaming his name is far away from him because of the low volume (amplitude) of her shrill voice (high pitch).
The external auditory meatus adjacent to the concha serves as the entrance to the auditory canal which is a long and narrow canal leading to the eardrum (i.e. tympanic membrane). The average ear canal is about 1 inch long and 5 to 7mm in diameter, making the middle and inner ear less accessible to foreign objects. Moreover, tiny hairs and ceruminous glands that excrete cerumen (i.e. earwax which is noxious to insects) are lined along its tortuous wall to deter dust, insects and foreign particles from entering the ear and damaging the eardrum. The canal also maintains optimum temperature and humidity to preserve the elasticity of the tympanic membrane. The eardrum marks the end of the outer ear and the beginning of the middle ear.Middle EarThe middle ear begins with the tympanic membrane which is a greyish pink cone-shaped membrane, about 10 millimetres wide, that separates the outer ear and middle ear. It is formed by three fine layers of tissue. The outer layer is consistent with the skin of the outer ear, whereas the inner layer is consistent with the mucous membrane that lines the tympanic membrane. The middle layer is made of fibrous tissues that make the eardrum stiff. The eardrum vibrates when sound waves hit it and transforms them into vibrations of the ear. The tensor tympani muscle connected to it keeps the eardrum constantly tugged inward, thus maintaining its tautness so that every square-inch of it is sensitive to even a small sound-wave. The middle ear encompasses a small (2-cm3) air cavity that houses the ossicular chain. This cavity receives air from the Eustachian tube which connects the middle ear to the nasopharynx through an opening at the bottom of the anterior wall. It equalizes the air pressure on both sides of the eardrum, thus maintaining the rigidity and thereby enabling transmission of sound waves through the tympanum membrane. Moreover, it prevents damage to the eardrum, aids head orientation and drains middle ear of mucus, thereby reducing infection. The Eustachian tube is normally closed but opens during the act of chewing or yawning. The ossicular chain consists of the three lever-teletype bones; hammer (malleus), anvil (incus), and stirrup (stapes). The malleus is attached to the tympanic membrane whereas the footplate of the stapes in tucked into the fluid-filled oval window of the inner ear. The incus links the stapes to the malleus with a synovial joint, thus forming the ossicular chain. Without the middle cavity, the acoustic energy would strike the fluid-filled cochlea, and 99% of the energy will be reflected away due to the greater impedance of the water. The middle cavity overcomes the impedance mismatch by letting the vibrations travel an approximate 55 more in area before exiting the stapes. Furthermore, the difference in size of the eardrum and stapes, results in increase of pressure at the exit, thereby ensuring the vibrations enter the fluid cochlea in addition to being amplified. Apart from that, the ossicular chain also protects the inner ear from loud noises. The tensor tympani muscle at the eardrum and the stapedius muscle at the stapes, contract in reflex to high-intensity sound vibrations, thereby stiffening the ossicular chain and halting the transmission of vibrations. This protects the inner ear from damage. However, the ear is still vulnerable to sudden loud noises (e.g. thunder) as it has a 10-millisecond reaction speed.[pic 5]