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.

□ Human ears 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.

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.

Have you ever wondered if the convoluted shape 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!

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. Furthermore, the eardrum is about 20 times larger in area than the footplate of the stirrup. 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.

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.

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.