The Physics of the Ear - Essay Example

The Physics of the Ear The ear is the organ in the body which deals with the sense of hearing. It is a part of the auditory system, and acts as a receptor for sound waves. Actual ‘hearing’, however, is done by the human brain. When sound waves—a form of energy moving through the medium of air, water or other matter—exert pressure on the ear, they are transmitted as nerve impulses to the brain and the central nervous system, which perceive and make sense of them. Vertebrates have a pair of ears placed symmetrically on either side of the head, which helps in localizing the source of sounds. The ear also plays a role in maintaining physical stability and balance of the body. The size of the outer ear differs considerably among mammals—from the large floppy ears of the hare to the smaller ones for humans—though there is not much difference in its internal functioning across species. The size of the ear canal is determined by the skin, cartilage and bone formation at this location. Since there is no growth of cartilage in adulthood, the size of the ear canal remains unchanged. However, the outer ear grows in size through adulthood. Ferrario VF. Sforza C. et al., (19(4):226-33, 1999.) calculated the length of the left exterior ear from apex to the lowest part with a transparent ruler; the result (in mms) as well as the patients age, were noted. The results established that as humans grew older the outer ear correspondingly increased in size. The ear consists of three sections—the outer ear, the middle ear and the inner ear. The outer ear (pinna or auricle) is the part visible to us. Its main function is to collect sound waves and filter them; so the larger it is, it collects more sound waves and focuses them in the right direction. However, this part is not vital to hearing. Besides, the pinna plays no role in the ear’s function of providing the sense of balance. The outer ear also consists of the ear canal, where wax is produced. Earwax protects the canal by means of the chemicals it contains that keep infections at bay, and also by acting as a barrier to collect dirt, which keeps the ear canal clean. (3 D Ear-Website) The middle ear receives sound waves that move through the ear canal to reach it. The middle ear converts sound waves into vibrations delivered to the inner ear. It does this through the eardrum (tympanum) which is a thin piece of skin stretched tight. The eardrum separates the outer ear from the middle ear and the ossicles, which are the three smallest and most delicately crafted bones in the body. The ossicles consist of the malleus (hammer), which is attached to the eardrum, the incus (anvil) which is attached to the malleus, and the stapes (stirrup)—the smallest bone in the body—which is attached to the incus. Sound waves reach the eardrum, causing it to vibrate, which in turn causes these vibrations to pass through the ossicles—from the hammer to the anvil and on to the stirrup. (3 D Ear-Website) Sound passing through the inner ear as vibrations enters the cochlea, which is a tiny curled tube located in the inner ear. The cochlea is filled with a liquid, in which waves are caused by the vibrations of the ossicles. It is here that sound waves pass from an airy medium to a liquid one. Remember, air surrounds the head and is also present in the ear canal as well as in the middle ear, but not in the inner ear. The hollow canals of the cochlea contain liquid and the sensory epithelium which is also covered with hair cells. The hair components, which are visible through the aid of a microscope, are protein filaments that protrude into the fluid. When sound reaches the cochlea, the vibrations cause the hairs on the cells to move. The multiple hair cells serve as mechanoreceptors that discharge a chemical neurotransmitter after stimulation. Sound waves are converted into nerve impulses, and the eighth cranial nerve facilitates the movement of these nerve impulses from both ears to that portion of the cerebral cortex dealing with the auditory function, which is located in the temporal lobe. The impulses concerning balance are also transmitted to the brain through this nerve. (3D Ear- Website) Diagram Depicting Parts of the Ear (The New World Encyclopedia) Towards the bottom, on the right side of the diagram above, we can see the eustachian tube. This connects the middle ear to the back of the nose. In a human adult In the diagram—at the bottom to the right—note the placement of the eustachian tube. This tube is around 35 mms long and 3 mms in diameter. The support for the tube is provided by cartilage for two-thirds of its length near the nose, and the remaining third, near the middle ear, is made of bone. The main function of the eustachian tube is to ventilate middle ear space to ensure that the air pressure there remains the same as that outside. The eustachian also drains accumulated secretions and debris from the middle ear space. In the absence of this tube the middle ear cavity would be an isolated air pocket inside the head, vulnerable to every change in ambient air pressure. Normally this tube is closed to prevent contamination of the middle ear space from the secretions at the back of the nose. A change in outside air pressure puts pressure on the eardrum, which is immediately equalized by a normally functioning eustachian tube, which opens up to let in or let out air. However, with defective functioning of the tube, additional air pressure can damage the tympanum or the ear drum. Typically, we experience the effect of the change in air pressure during the descent of an airplane, when outside air pressure increases, and we feel our eardrums being pushed inward (retraction). Such pressure or pain is heightened if we suffer from a blocked nose or cold symptoms. Scuba diving also causes us to experience this sensation, as outside pressure increases in proportion to the depth of our dive. Swallowing or yawning activates the muscles at the back of the throat that opens the eustachian tube. Taking a deep breath, and then blowing out while simultaneously pinching your nose and keeping your mouth tightly shut, forces open the eustachian tube. When we experience a popping of our ears while doing this, we can be sure that the eustachian tube has opened. (Hoffman, Douglas) To understand what constitutes a hearing defect, we need to know what ‘normal hearing’ entails. Sounds are categorized in terms of their frequency (pitch) and intensity (loudness). Frequency is measured in hertz (Hz), while intensity is computed in decibels (dB). A person with normal hearing can hear frequencies between 20 and 20,000 Hz. (20 kHz) “The most important sounds we hear every day are in the 250 to 6000 Hz range.”(Disease Control-Website) Vowel sounds like ‘u’ have low frequencies (250 to 1000 Hz), while consonants like ‘s’, ‘h’ and ‘f’ have higher frequencies (1500 to 6000 Hz). Since consonants convey most of the meaning in our daily speech, a person who cannot hear high frequency sound will not be able to understand speech well. At normal hearing ability a person hears sounds from 0 to 140 dB. A whisper is 30 dB, while conversations are carried on usually at 45 to 50 dB. Sounds above 90 dB are uncomfortable to the ear. (A rock concert may be as loud as 110dB.) Hearing impairment can happen in either frequency or intensity or as a combination. Severity of hearing loss is defined on the basis of how well a person can register frequencies/ intensities most often associated with speech. On the basis of this, severity can be mild, moderate, severe, or profound. The term ‘deaf’ is used to describe someone who has a hearing loss of approximately 90 dB or greater, or “who cannot use hearing to process speech and language information, even with the use of hearing aids.”(Disease Control-Website) ‘Hard of hearing’ is used to describe people who have a hearing loss less severe than deafness. Hearing loss that affects one ear is termed as unilateral loss, while the term ‘bilateral loss’ is used to describe a loss that affects both ears. Hearing loss may also be classified as 1) Conductive loss caused by a problem in the outer/ middle ear. This affects all frequencies to the same extent. A conductive loss is not usually severe. 2) Sensori-neural loss which is due to a problem in the inner ear or auditory nerve. This affects a person’s ability to hear certain frequencies, which results in sound distortion even with the use of an aid. Sensori-neural loss can range from slight to profound. 3) Mixed loss is a combination of conductive and sensori-neural loss. 4) Central loss is “caused by a problem along the pathway from the inner ear to the auditory region of the brain or in the brain itself.” (Disease Control-Website) The threshold of human hearing (20 to 20,000 Hz) decreases with age, and as such, many adults may not be capable of detecting sounds beyond 16,000 Hz. Trigg DJ. Applebaum (1998) notes that the ear in its own right cannot tune to frequencies that are below 20Hz, though these can be detected through the sense of touch. The hearing capabilities of dogs are determined by their age and breed. The general range of sound detection for dogs is around 4 to 60,000 Hz. which represents a greater range than that for humans. Certain breeds of dog like the German Shepard, and the Miniature Poodle go deaf with age just like humans. The superior hearing ability of the dog is evident from the use of whistles of high frequency sometimes used in their training. Use of such a whistle would, to an observer, appear as if the dog were responding to a non-existent sound. The sense organ of the ear along with the whole auditory system is indeed a wonderful construction of nature, a part of the human anatomical and physiological system, which we normally take for granted. It opens our world to language and music and the sounds of nature—the roar of thunder, the crashing of waves on rocks, the pitter patter of rain—and the laughter of children. Works Cited Centre for Disease Control and Prevention Website, Hearing Loss Ferrario VF. Sforza C. et al., Morphometry of the Normal Human Ear: A Cross-sectional Study from Adolescence to Mid-adulthood in Craniofacial Genetics Developmental Biol. 19(4):226-33, 1999.) Hoffman Douglas, Weather Changes Cause Ear Pain in Your Total Health , retrieved 27th Nov, 2008. New World Encyclopedia, Ear , retrieved 27th Nov, 2008 3 D Ear , retrieved 27th Nov, 2008 Read More