The Human Ear Structure And Function

By Martinez - November 01, 2017

Human Ear

Your Ear—The Great Communicator


YOU can close your eyes when you do not want to see. You can hold your breath when you do not want to smell. But you cannot really shut down your ears when you do not want to hear. The saying "to turn a deaf ear" is only a metaphor. Your hearing, like your heartbeat, goes on working even when you sleep.
Indeed, our ears are working all the time to keep us in touch with the world around us. They select, analyze, and decipher what we hear and communicate it to the brain. Within the confines of about one cubic inch [16 cu cm], our ears utilize principles of acoustics, mechanics, hydraulics, electronics, and higher mathematics to accomplish what they do. If our hearing is not impaired, consider just a few of the things the ears can do.

? From the softest whisper to the thunderous roar of a jet plane taking off, our ears can cope with a 10,000,000,000,000-fold difference in loudness. In scientific terms, this is a range of about 130 decibels.

? Our ears can pick out and focus on one conversation across a room full of people or detect a wrong note played by one instrument in an orchestra of a hundred.

? The Human ear can detect a change of just two degrees in the direction of a sound source. They do this by sensing the minute difference in the arrival time and the intensity at the two ears. The time difference may be as little as ten millionths of a second, but the ears can detect this and convey it to the brain.
? Our ears can recognize and distinguish between some 400,000 sounds. Mechanisms in the ear automatically analyze the sound wave and match it with those stored in our memory bank. That is how you can tell if a musical note is played by a violin or a flute, or who is calling you on the phone.

Parts Of The Ear And Thier Functions
The "ear" we see at the side of our head is really only a portion, the most visible portion, of our ear. Most of us probably still remember from our school days that the ear is made up of three sections: outer, middle, and inner ears, as they are called. The outer ear consists of the familiar "ear" of skin and cartilage and the ear canal leading inward to the eardrum. In the middle ear, the three smallest bones in the human body—the malleus, incus, and stapes, commonly called hammer, anvil, and stirrup—form a bridge linking the eardrum with the oval window, the portal to the inner ear. And the inner ear is made up of two strange-looking parts: the cluster of three semicircular canals and the snail-shaped cochlea.

Outer Ear—The Tuned Receiver
Obviously, the external ear serves to collect sound waves in the air and channel them to the inner parts of the ear. But it does much more than that.
Outer ear

Have you ever wondered if the convoluted shape (having many twists and curves) of the external ear serves any specific purpose? Scientists find that the cavity at the center of the external ear and the ear canal are so shaped that they enhance sounds, or resonate, within a certain frequency range. How does that benefit us? It so happens that most of the important characteristics of human speech sounds fall in about the same range. As these sounds travel through the external ear and the ear canal, they are boosted to about twice their original intensity. This is acoustical engineering of the highest order!
Parts of the outer ear also plays an important role in our ability to locate the source of sound. As mentioned, sounds coming from the left or right of the head are identified by the difference in intensity and arrival time at the two ears. But what about sounds that come from behind? Again, the shape of the ear comes into play. The edge of our ear is shaped in such a way that it interacts with sounds coming from behind, causing a loss in the 3,000- to 6,000-Hz range. This alters the character of the sound, and the brain interprets it as coming from behind. Sounds from above the head are also altered but in a different frequency band.

Middle Ear—A Mechanic's Dream
The job of the middle ear is to transform the acoustical vibration of the sound wave into mechanical vibration and pass it on to the inner ear. What takes place in this pea-sized chamber is truly a mechanic's dream.
Contrary to the notion that loud sounds cause significant movement of the eardrum, sound waves actually do so by only microscopic amounts. Such minuscule movement is hardly enough to cause the fluid-filled inner ear to react. The way this obstacle is overcome demonstrates once again the ingenious design of the ear.
The linkage of the three little bones of the middle ear is not only sensitive but also efficient. Functioning as a lever system, it magnifies any incoming forces by about 30 percent. 
Middle ear

Furthermore, the eardrum is about 20 times larger in area than the footplate of the stirrup (stapes). Thus, the force exerted on the eardrum is concentrated on a much smaller area at the oval window. These two factors together amplify the pressure at the vibrating eardrum to 25 to 30 times as much at the oval window, just enough to set the fluid in the cochlea in motion.
Do you find that a head cold sometimes affects your hearing? This is because proper operation of the eardrum requires that the pressure on either side of it be equal. Normally this is maintained by a small vent, called the Eustachian tube, that connects the middle ear with the back of the nasal passage. This tube opens every time we swallow and relieves any pressure build-up in the middle ear.

Inner Ear—The Business End of the Ear
From the oval window, we come to the inner ear. The three mutually perpendicular loops, called the semicircular canals, enable us to maintain balance and coordination. It is in the cochlea, however, that the business of hearing really begins.
The cochlea (from Greek ko·khli'as, snail) is basically a bundle of three fluid-filled ducts, or canals, coiled up in a spiral like the shell of a snail. Two of the ducts are connected at the apex of the spiral. When the oval window, at the base of the spiral, is set in motion by the stirrup, it moves in and out like a piston, setting up hydraulic pressure waves in the fluid. As these waves travel to and from the apex, they cause the walls separating the ducts to undulate.
The inner ear

Along one of these walls, known as the basilar membrane, is the highly sensitive organ of Corti, named after Alfonso Corti, who in 1851 discovered this true center of hearing. Its key part consists of rows of sensory hair cells, some 15,000 or more. From these hair cells, thousands of nerve fibers carry information about the frequency, intensity, and timbre of the sound to the brain, where the sensation of hearing occurs.

The Mystery Unraveled
How the organ of Corti communicates this complicated information to the brain remained a mystery for a long time. One thing scientists did know was that the brain does not respond to mechanical vibrations but only to electro-chemical changes. The organ of Corti must in some way convert the undulating movement of the basilar membrane into matching electrical impulses and send these to the brain.
Organ of corti

It took the Hungarian scientist Georg von Békésy some 25 years to unravel the mystery of this tiny organ. One thing he discovered was that as the hydraulic pressure waves travel along the ducts in the cochlea, they reach a peak somewhere along the way and push on the basilar membrane. Waves generated by high-frequency sounds push on the membrane near the base of the cochlea, and waves from low-frequency sounds push on the membrane near the apex. Thus, Békésy concluded that sound of a specific frequency produces waves that flex the basilar membrane at a specific spot, causing the hair cells there to react and send signals to the brain. The location of hair cells would correspond to the frequency, and the number of hair cells triggered would correspond to the intensity.
This explanation holds good for simple tones. Sounds occurring in nature, however, are rarely simple. A bullfrog's croak sounds quite different from a drumbeat even though they may be of the same frequency. This is because each sound is made up of a fundamental tone and many overtones. The number of overtones and their relative strength give each sound its distinctive timbre, or character. This is how we recognize the sounds we hear.
The basilar membrane can respond to all the overtones of a sound simultaneously and detect how many and what overtones are present, thus identifying the sound. Mathematicians call this process Fourier analysis, naming it after the brilliant 19th-century French mathematician Jean-Baptiste-Joseph Fourier. Yet, the ear has been using this advanced mathematical technique all along to analyze the sounds heard and communicate the information to the brain.
Even now, scientists are still not sure what sort of signals the inner ear sends to the brain. Investigations reveal that the signals sent by all the hair cells are about the same in duration and strength. Thus, scientists believe that it is not the content of the signals but the simple signals themselves that convey a message to the brain.
To appreciate the significance of this, recall the children's game in which a story is relayed from one child to another down the line. What the child at the other end hears often bears no resemblance to the original. If a code, such as a number, is passed along instead of the complicated story, it will likely not be distorted. And that, apparently, is what the inner ear does.
Interestingly, a technique used in today's advanced communications systems, called pulse code modulation, works on the same principle. Rather than sending the details of an event, a code representing that event is sent. This was the way pictures of Mars were sent to earth, in binary bits, or the way sounds may be converted into bits for recording and playback. But, again, the ear had it first!

Our ears may not be the most acute or most sensitive among ears, but they are eminently suited to fulfill one of our greatest needs—the need to communicate. They are designed to respond especially well to the characteristics of human speech sounds. Infants need to hear the sound of their mother's voice to grow properly. And as they grow, they need to hear the sounds of other humans if they are to develop their faculties of speech. Their ears allow them to discern the subtle tonal inflections of each language so precisely that they grow up speaking it as only a native can.
Most of the distinguishing features of human speech sounds fall in the range of from 2,000 to 5,000 Hz (cycles per second), and these are approximately the frequencies at which the ear canal and the central cavity of the external ear resonate. Really, it's a masterpiece.

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