I Can Hear You Whisper Read Online Free PDF

hard depends on whether I’ve whispered, yelled, or spoken in a conversational voice. Either way, I’ve created a sound wave, a form of energy that can move through air, water, metal, or wood, carrying detailed information.
    Traveling through the air, the sound waves are acoustic energy. The outer ear is designed to catch that energy in the folds of the earlobe (the pinna). It does a slightly better job with sounds coming from the front. Cats and deer and some other animals have the ability to turn their pinna to a sound like radar dishes, but humans must turn their heads. Once collected, the waves are funneled into the ear canal, which acts as a resonance chamber.
    At the eardrum (tympanum), the waves have reached the middle ear. When they hit the eardrum, it vibrates. Those vibrations in the membrane of the eardrum are carried across the little pocket of the middle ear by a set of tiny bones—the smallest in the body—called the ear ossicles, but more commonly known as the hammer (malleus), anvil (incus), and stirrup (stapes). Converting the original acoustic energy to mechanical energy, the hammer hits the anvil, the anvil hits the stirrup, and the stirrup, piston-like, hits a membrane-covered opening called the oval window, which marks a new boundary.
    On the far side lies the fluid-filled cochlea, the nautilus-like heart of the inner ear. The vibrations transmitted from the stapes through the oval window send pressure waves through the cochlear fluid; mechanical energy has become hydro energy. Outside, the cochlea is protected by hard, bony walls. Inside, the basilar membrane runs along its length like a ribbon. Thin as cellophane, the basilar membrane is stiff and narrow at one end, broad and flexible at the other. As sound waves wash through, the basilar membrane acts as a frequency analyzer. Higher-pitched sounds, like hissing, excite the stretch of membrane closest to the oval window; lower pitches, like rumbling, stimulate the farther reaches. Like inhabitants of a long curving residential street, specific sounds always come home to the same location, a particular 1.3 millimeters of membrane and the thirteen hundred neurons that live there, representing a “critical band” of frequencies.
    Sitting on top of the basilar membrane is the romantically named organ of Corti. Known as the seat of hearing, it holds thousands of hair cells. Quite recently, scientists discovered a distinction between the functions of inner and outer cells. Twelve thousand outer cells, organized in three neat rows, amplify weak sounds and sharpen up tuning. Another four thousand inner hair cells, in one row, take on the work of sending signals to the auditory nerve fibers. Like microscopic glow sticks that light up when you snap them, the tiny stereocilia on each hair cell bend under the pressure of the movement of fluid caused by the sound wave and trigger an electrical impulse that travels up the nerve to the brain.
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    The most obvious way to assess hearing is to test how loud a sound has to be to be audible—its threshold. Decibels, a logarithmic scale that compares sound intensity levels, were invented in Bell Laboratories, the source of most things sound-related into the mid-twentieth century. Named for Alexander Graham Bell, decibels (dB) provide a means of measuring sound relative to human hearing. Zero decibels doesn’t mean that no sound is occurring, only that most people can’t hear it. With normal hearing, a person can distinguish everything from the rustling of leaves in a slight breeze (ten decibels) to a jet engine taking off (130 dB). The leaves will be barely noticeable, the airplane intolerable and damage-inducing. Urban street noise registers about eighty decibels; a bedroom at night is closer to thirty. A baby crying can reach a surprising 110 dB, a harmful level with prolonged exposure. (Thank goodness they grow up.) A whisper hovers around thirty