Inner Ear
The inner ear is a complex system of fluid-filled cavities in the temporal bone. Among these cavities are the three semicircular canals, whose function is not hearing, but rather the detection of movements of the head. The organ of the inner ear concerned with hearing is the cochlea, a tube of about 3.5 cm coiled in a tight spiral. The tube is divided lengthwise into three adjoining ducts, separated by two membranes, Reissner’s membrane, and the basilar membrane. The sensory receptor of the inner ear is the organ of Corti, consisting of thousands of hair cells, which sit on the basilar membrane. The vibrations of the oval window excite a wave motion in the fluid of the cochlea, which shakes the basilar membrane. The hair cells detect this motion of the basilar membrane and convert the mechanical energy into electric nerve impulses. The basilar membrane is stiff at the end near the oval window, and soft at the distant end. Because of this, the near part of the membrane responds most readily to high-frequency disturbances. Thus, there is a correlation between the frequency of the incident sound and the regional pattern of motion of the basilar membrane. The hair cells, and the brain, recognize these regional patterns and thereby discriminate among frequencies.
The range of frequency audible to the human ear extends from 20Hz to 20,000 Hz. These limits are somewhat variable; for instance, the ears of older people are less sensitive to high frequencies. Sound waves above 20,000 Hz are called ultrasound; some animal's dogs, cats, bats, and dolphins can hear these frequencies.
Ultrasonic waves of very high frequency do not propagate well in air. They are rapidly dissipated and absorbed by air molecules. However, these waves propagate readily through liquids and solids. In recent years, this property has been exploited in the development of some interesting practical applications of ultrasound. For instance, such waves are now used in place of X-rays to take pictures of the interior of the human body. This technique, called sonography, permits examination of the fetus in the body of a pregnant woman and avoids the damage that X-rays might do to the very sensitive tissues of the fetus. The ultrasonic “cameras” that take such pictures employ sound waves of a frequency of about 106 Hz. Further development of this technique has led to the construction of acoustic microscopes. The most powerful of these devices employ ultrasound waves of a frequency in excess of 109 Hz to make highly magnified pictures of small samples of materials. The wavelength of sound waves of such extremely high frequency is about 10-6 m, roughly the same as the wavelengths of ordinary light waves. The micrographs made by experimental acoustic microscopes compare favorably with micrographs made by ordinary optical microscopes.
Ultrasound waves are being increasingly used in medical and surgical diagnosis by a technique based on the fact that different types of tissue, e.g. bone, fat, and muscle have different properties for very high-frequency waves.
