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How We Hear: A Look at the Ear and How it Processes Sound Brittany O’Shea Written For: Communication Sciences & Disorders Majors taking Introduction to Audiology February 26, 2012 Audience and Scope This document was created to inform students majoring in Communication Sciences and Disorders about the hearing process and how the ears works. Specifically this document will be used in an Introduction to Audiology class where hearing and hearing disorders are discussed. Before one can learn about hearing disorders however, they must understand the basic process of hearing in a healthy, normal functioning ear. This document could be used in a class lecture or as supplementary notes to a lecture. Introduction The ear is the portion of the body that converts sound waves into electrical and chemical energy making it possible for sound to be perceived by the brain. The three major portions of the ear (outer, middle and inner) are all used in this process. This document will take you through the steps of how all sounds travel from the environment and are processed by your brain. How the Ear Works The process of hearing can be quite a complex task, however when broken down into basic steps it is much easier to understand. Sounds travel through the ear and to the brain in the following manner: 1. Sound is funneled in by the outer ear into the ear canal. 2. Sounds waves cause the tympanic membrane (the ear drum) to vibrate. 3. The ossicles of the middle ear vibrate. 4. The vibrations cause fluid waves to be sent up the cochlea. 5. Hair cells in the cochlea convert the chemical energy to electrical energy 6. The electrical impulse is sent up the auditory nerve to the brain where the sound is perceived and processed. Let’s Take a Closer Look… The Human Ear Figure 1 http://www.hearingcentral.com/howtheearworks.asp 1. Sound is funneled in by the outer ear into the ear canal As you can see in Figure 1 the shape of the outer ear resembles a funnel. This is not just for cosmetic purposes, but rather to “funnel” in sound waves as a way to direct the sound where it needs to go. The sounds waves are channeled from the outer ear and the ear canal and directed to the tympanic membrane, which is more commonly known as the eardrum. 2. Sound waves cause the tympanic membrane to vibrate. The energy from the sound waves causes the tympanic membrane to vibrate. The tympanic membrane is the barrier that separates the outer and middle ear. This vibration is then continued into the middle ear. 3. The ossicles of the middle ear vibrate The energy from the vibration of the tympanic membrane causes the ossicles of the middle ear to vibrate as well. On the other side of the tympanic membrane (in the middle ear) there are three tiny bones known as ossicles. These bones are called the malleus, incus and stapes ( see Figure 1) and when the vibrations hit them they begin to rock back and forth which triggers fluid in the inner ear to be set in motion. 4. The vibrations cause fluid waves to be sent up the cochlea The vibrations on the middle ear can be felt in the inner ear, which causes fluid there to create wave like motions in the cochlea (the inner ear). The cochlea, which is snail-shaped, is known as the organ of hearing and is lined with tiny hair cells. When the fluid is in motion the hair cells are activated. The cochlea is known to be tonotopically organized, meaning that each area on the cochlea responds to different frequencies of sound. Figure 2 http://www.clarkaudiology.com/tinnitus.html 5. Hair cells in the cochlea convert the chemical energy to electrical energy As you can see from Figure 2 the hair cells in the cochlea lay on what is known as the basal membrane or the base of the cochlea. The chemical energy activates the hairs cells causing them to move in a back and forth motion. When the hair cells become activated they then transform this chemical energy into electric impulses. The sound energy is now ready to be sent to the brain. The loss or damage of hair cells which can be seen in Figure 2 causes sounds in those certain frequency areas to not be heard, this is known as sensorineural hearing loss. 6. The electrical impulse is sent up the auditory nerve to the brain where the sound is perceived and processed. The electrical impulse that is created in the cochlea contains information about the sound and its frequencies. This information carrying impulse is sent from the cochlea and up the auditory nerve. From the auditory nerve the impulse goes to the brain. The area known for comprehension of sound is the temporal lobe, here the sound is perceived and processed and the hearing process is complete. Conclusion While it is possible to function without one’s hearing, it presents a major communication barrier. If one step in the hearing process goes wrong sound will not be transmitted correctly and the message could be misinterpreted. In order for the correct message to be relayed from the environment to the brain the sound needs to first be funneled into the ear canal, then cause vibrations in the tympanic membrane and the ossicles of the middle ear. The vibrations must then be strong enough to create fluid waves up into the cochlea. From there the activated hair cells will convert the energy into an electric impulse, allowing the message to be sent to the interpreting center in the brain. If these steps are not followed or there is damage to one or more areas of the ear, a hearing loss will be the result. Figure Citations: Figure 1: Hearing Central, LLC, 2012 http://www.hearingcentral.com/howtheearworks.asp Figure 2: Clark Audiology, 2011 http://www.clarkaudiology.com/tinnitus.html