The vibration of the ossicles against a window at its base generates waves that ripple through this fluid, pushing against a membrane that is lined with thousands of tiny (about a thousandth of an inch long) hair cells-the receptors of the auditory system. The cochlea is shaped like a spiral snail shell and contains fluid-filled canals. These ossicles, the smallest bones in the body, magnify the motion of the eardrum some twentyfold.Īt their other end, the ossicles transmit their tiny movements to the cochlea, the organ of the inner ear that actually translates the energy of sound waves into nerve signals. In the middle ear, the vibrations are transmitted to three linked bones. Sound waves enter the hearing system through the outer ear, traveling down through the inch-long ear canal to strike the tympanic membrane, or eardrum, and making it vibrate. The sounds we hear represent a richly layered mix of frequencies and amplitudes, faithfully transmitted through a finely engineered apparatus. The loudest we can handle without immediately damaging our hearing (for example, standing 100 feet from a jet at takeoff) carries about a million million times more energy than the barely audible. The diversity of sounds we can hear typically ranges from 20Hz (cycles/second) to 20,000Hz. It turns physical movement into the electrical signals that make up the language of the brain, translating these vibrations into what we experience as the world of sound. Amplitude-how wide the pressure variations-determines volume. Frequency-how many cycles per second-determines the pitch of the sound. Such motions as objects striking or rubbing against each other, air agitated by vocal cords, or gases rushing through a car’s muffler produce cyclical pressure variations in the air: sound waves. Words of love, or wisdom the timeless murmur of wind in the trees the warning blare of a car horn the sublime harmonies of Mozart-our sense of hearing informs, enriches, and all too often disrupts our lives.
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