Hearing is the ability of the human ear to collect, process, and interpret sound.
Description
Sound vibrations travel through air, water, or solids in the form of pressure waves. When a sound wave hits a flexible object, such as the eardrum, it causes it to vibrate, which begins the process of hearing. The process of hearing involves the conversion of acoustical energy (sound waves) to mechanical, hydraulic, chemical, and finally, electrical energy where the signal reaches the brain and is interpreted.
The basis of sound is simple: there is a vibrating source, a medium in which sound travels, and a receiver. For humans the most important sounds are those which carry meaning, for example, speech and environmental sounds. Sounds can be described in two ways, by their frequency (pitch) or by their intensity (loudness).
Frequency (the number of vibrations or sound waves per second) is measured in Hertz (Hz). A sound that is 4,000 Hz (like the sound the letter 'F' makes) has 4,000 waves per second. Healthy young adults can hear frequencies between 20 and 20,000 Hz. However, the frequencies most important for understanding speech are between 2,000 and 8,000 Hz. As adults age, the ability to hear frequency sounds decreases. An example of a high frequency sound is a bird chirping, while a drum beating is a low frequency sound.
Intensity (loudness) is the amount of energy of a vibration, and is measured in decibels (dB). A zero decibel sound (like leaves rustling in the wind), can barely be heard by young healthy adults. In contrast, a 120 dB sound (like a jet engine at 20 ft [7 m]) is perceived as very loud and/or painful. Extremes in both loudness or pitch may seriously damage the human ear and should be avoided.
The difference between frequency or pitch and intensity or loudness can be illustrated using the piano as an analogy. The piano keyboard contains 88 keys that represent different frequencies or notes. The low frequencies or bass notes are on the left, the higher frequencies or treble notes are on the right. Middle C on the keyboard represents approximately 256 Hz. The intensity or loudness of a note depends on how hard you hit the key. A light touch on middle C may produce a 30 dB, 256 Hz note, while a hard strike on middle C may produce a 55 dB, 256 Hz note. The frequency, or note, stays the same, but the intensity of loudness varies as the pressure on the key varies.
Function
Human hearing involves a complicated process of energy conversion. This process begins with two ears located at opposite sides of the human head. The ability to use two ears for hearing is called binaural hearing. The primary advantages of binaural hearing are the increased ability to localize sounds and the increased ease of listening to a particular sound while having other noises in the background. Sound waves from the world around us enter the ear and are processed and relayed to the brain.
The actual process of sound transmission differs in each of the three parts of the human ear. The three parts of the human ear are the outer ear, middle ear, and inner ear.
Role in human health
The outer ear plays an important role in hearing. The pinna of the outer ear gathers sound waves from the environment and transmits them through the external auditory canal and eardrum to the middle ear. In the process of collecting sounds, the outer ear also modifies the sound. The external ear, or pinna, in combination with the head, can slightly amplify or increase as well as attenuate or decrease certain frequencies. The amplification or attenuation is due to individual differences in the dimensions and contours of the head and pinna.
The external auditory canal can also modify sound. This tube-like canal is able to amplify specific frequencies in the 3,000 Hz region. An analogy would be an opened, half-filled soda bottle. If you empty some of the fluid and blow into the bottle again, the frequency of the sound will change. Since the size of the human ear canal is consistent, the specific frequency it amplifies is also constant. Sound waves travel through the ear canal until they reach the tympanic membrane or eardrum. Together, the head, pinna, and external auditory canal amplify sounds in the 2,000–4,000 Hz range by 10–15 dB. This boost is needed since the process of transmitting sound from the outer to the middle ear requires added energy.
The middle ear is separated from the outer ear by the tympanic membrane or eardrum. The membrane vibrates in response to pressure from sound waves traveling through the external auditory canal. The initial vibration causes the membrane to be displaced (pushed) inward by an amount equal to the intensity of the sound, so that loud sounds push the eardrum more than soft sounds. Once the eardrum is pushed inwards, the pressure within the middle ear causes the eardrum to be pulled outward, setting up a back-and-forth motion that begins the conversion and transmission of acoustical energy (sound waves) to mechanical energy (bone movement).
The three small, connected bones of the middle ear, together called the ossicle, are: the hammer or malleus, the anvil or incus, and the stapes or stirrup. The tiny, interconnected bones move as a unit in a type of lever-like action. The first bone, the malleus, is attached to the tympanic membrane and the back-and-forth motion of the tympanic membrane sets all three bones in motion. The final result of this bone movement is pressure of the foot plate of the last and smallest bone, the stapes, on the oval window. The window is one of two small membranes that allow communication between the middle ear and the inner ear. The lever-like action of the bones amplifies the mechanical energy from the eardrum to the oval window. The energy in the middle ear is also amplified due to the difference in surface size between the tympanic membrane and the oval window, which has been calculated to be a difference of about 14 to one.
The relationship of the eardrum or tympanic membrane to the oval window can be compared to that of a thumbtack. The eardrum would be the head of the thumb-tack and the oval window would be the pin point of the thumbtack. The eardrum or the head of the tack would collect and apply pressure and then focus it on the oval window or the pin point, driving it into the surface. The overall amplification in the middle ear is approximately 25 dB. The conversion from mechanical energy or bone movement to hydraulic energy or fluid movement requires added energy since sound does not travel easily through fluids.
The inner ear is the site where hydraulic energy or fluid movement is converted first to chemical energy or hair cell activity and finally to electrical energy or nerve transmission. Once the signal is transmitted to the nerve, it will travel up to the brain to be interpreted.
The bone movements in the middle ear cause movement of the stapes foot plate in the membrane of the oval window. This pressure causes fluid waves or hydraulic energy throughout the entire two-and-a-half turns of the cochlea. The design of the cochlea allows for very little fluid movement, therefore the pressure at the oval window is released by the interaction between the oval and round windows. When the oval window is pushed forward by the stapes foot plate, the round window bulges outward and vice versa. This action permits the fluid wave motion in the cochlea. The cochlea is the fluid-filled, snailshell-shaped, coiled organ in the inner ear that contains the actual sense receptors for hearing. The fluid motion causes a corresponding, but not equal, wave-like motion of the basilar membrane. Internally, the cochlea consists of three fluid filled chambers: the scola vestibuli, the scola timpani, and the scala media. The basilar membrane is located in the scala media portion of the cochlea, and separates the scala media from the scala timpani. The basilar membrane holds the key structure for hearing, the organ of Corti.
The physical characteristics of the basilar membrane are important, as is its wave-like movement, from its base or originating point to its apex or tip. The basilar wave motion slowly builds to a peak and then quickly dies out. The distance the wave takes to reach the peak depends on the speed at which the oval window is moved. For example, high frequency sounds have short wavelengths, causing rapid movements of the oval window, and peak movements on the basilar membrane near the base of the cochlea. In contrast, low frequency sounds have long wavelengths and cause slower movements of the oval window, and peak movements of the basilar membrane near the apex. The place of the peak membrane movements corresponds to the frequency of the sounds. Sounds can located or "mapped" on the basilar membrane. High frequency sounds are near the base, middle frequency sounds are in the middle, and low frequency sounds are near the apex. In addition to the location on the basilar membrane, the frequency of sounds can be identified based on the number of nerve impulses sent to the brain.
The organ of Corti lies upon the basilar membrane and contains three to five outer rows (12,000 to 15,000) of hair cells and one inner row (3,000) of hair cells. The influence of the inner and outer hair cells has been widely researched. The common view is that the numerous outer hair cells respond to low intensity sounds below 60 dB. The inner hair cells act as a booster, by responding to high intensity, louder sounds. When the basilar membrane moves, it causes the small hairs on the top of the hair cells or stereo cilia to bend against the overhanging tectorial membrane. The bending of the hair cells causes chemical actions within the cell itself creating electrical impulses in the nerve fibers attached to the bottom of the hair cells. The nerve impulses travel up the nerve to the temporal lobe of the brain. The intensity of a sound can be identified based on the number of hair cells affected and number of impulses sent to the brain. Loud sounds cause a large number of hair cells to be moved, and many nerve impulses to be transmitted to the brain.
Each of the separate nerve fibers join and travel to the lowest portion of the brain, the brain stem. Nerves from the vestibular, or balance, part of the inner ear combine with the cochlear nerves to form the VIII cranial nerve (auditory or vestibulocochlear nerve). Once the nerve impulses enter the brain stem, they follow an established pathway, known as the auditory pathway. The organization within the auditory pathway allows for a large amount of crossover. "Cross-over" means that the sound information or nerve impulses from one ear do not travel exclusively to one side of the brain. Some of the
nerve impulses cross over to the opposite side of the brain. The impulses travel bilaterally on both sides of the brain up the auditory pathway until they reach a specific point in the temporal lobe called Heschl's gyrus. Crossovers act like a safety net. If one side of the auditory pathway is blocked or damaged, the impulses can still reach Heschl's gyrus to be interpreted as sound.
Common diseases and disorders
There are several common diseases, disorders, and conditions that occur in the external ear, middle ear, eardrum, and inner ear that can affect the sense of hearing in humans.
External otitis (swimmer's ear), an inflammation or infection of the external ear.
National Institute on Deafness and Other Communication Disorders. National Institutes of Health. 31 Center Dr., MSC 2320, Bethesda, MD 20892-2320. <http://www.nidcd.nih.gov>.
