Acoustics, Physiological
Physiological acoustics is the study of the transmission of sound and how it is heard by the human ear. Sound travels in waves, vibrations that cause compression and rarefaction of molecules in the air. The organ of hearing, the ear, has three basic parts that collect and transmit these vibrations: the outer, middle and inner ear. The outer ear is made of the pinna, the external part of the ear that can be seen, which acts to funnel sound through the ear canal toward the eardrum or tympanic membrane. The membrane is highly sensitive to vibrations and also protects the middle and inner ear. When the eardrum vibrates it sets up vibrations in the three tiny bones of the middle ear, the malleus, incus and stapes, which are often called the hammer, anvil and stirrup because of their resemblance to those objects. These bones amplify the sound. The stapes is connected to the oval window --the entrance to the inner ear which contains a spiral-shaped, fluid-filled chamber called the cochlea. When vibrations are transmitted from the stapes to the oval window, the fluid within the cochlea is put into motion. Tiny hairs that line the basilar membrane of the cochlea, a membrane that divides the cochlea lengthwise, move in accordance with the wave pattern. The hair cells convert the mechanical energy of the wave form into nerve signals that reach the auditory nerve and then the brain. In the brain, sound is interpreted.
Early research into the physiology of hearing was conducted by Hermann von Helmholtz, a German physician who enjoyed the study of physics and made a close study of the function of both the eyes and ears. He theorized that the ear detected differences in pitch through the action of the cochlea, the snail-shaped organ of the inner ear. As a physicist he understood sound waves and their properties, such as pitch (the highness or lowness of a sound) and amplitude or loudness. He proposed that certain notes sounded pleasing together because their pitches had a mathematical relationship. However, the human ear can distinguish between two instruments playing the same pitch. He contended that the quality of a tone depended on the intensities of other pitches known as overtones which combine to give a sound a particular tone or timbre.
In 1857, Helmholtz proposed his resonance theory of hearing in which he suggested that the fibers along the basilar membrane of the cochlea were of different lengths and thus had their own natural vibration or frequency. When a sound of that same frequency entered the cochlea, that fiber would resonate and sense the sound. He also suggested that the cochlea's structure resonated at particular frequencies to enable both pitch and tone to be perceived.
Although many of Helmholtz's ideas were right, his grasp of what occurs inside the cochlea was incorrect. Many years later, Georg von Békésy, a Hungarian-American physicist, studied the cochlea by placing it in a fluid bath and thus could see in more detail what occurred. He also studied the cochlea indirectly by making mechanical models to observe what happened when the fluid in the cochlea begins to move.
For nearly a quarter of a century, Békésy worked for the Hungarian telephone system doing research on acoustics. He began research in physiological acoustics in 1923, first studying the eardrum, then the basilar membrane. He constructed a mechanical model of a cochlea, first made of a rubber membrane stretch over a metal frame and later one containing fluid. He found that vibrations transmitted to the fluid in the cochlea set up traveling waves in the basilar membrane. When the frequency (pitch) of the stimulus was increased, the section of sensed vibration moved toward the end of his model that was closest to the middle ear. When the frequency was decreased, the section of sensed vibration moved toward the inner ear.
When he came to the United States in 1947, Békésy suggested a different theory of hearing to replace that of Helmholtz. The basilar membrane that separates the chambers of the cochlea is made up of about 24,000 fibers that stretch across its width. The fibers are progressively wider moving along the cochlea. Helmholtz thought that each fiber would have its own natural vibration and thus respond to sounds with that vibration. Békésy, using his artificial model to mimic the cochlea, found that sound waves passing through the fluid in the cochlea set up a wave in the membrane, and it is the shape of that wave that goes to the brain and is interpreted as sound. The hair cells along the wave transform the mechanical energy of the vibration into nerve impulses that can be sent to the brain and interpreted as sound. The wave travels from the stiffer basal part of the cochlea the more flexible upper part of the cochlea. Because of the shape of the cochlea, the resulting wave form is quite complex. Békésy likened the cochlea to a frequency analyzer, an electronic device that measures and interprets the frequency of waves. For his work on physiological acoustics, Békésy was awarded the Nobel Prize in medicine or physiology in 1961, the first time a physicist ever won in that category.
The understanding of the function of the inner ear, particularly the cochlea, has undergone a revolution in the last two decades. For example, scientists had believed that the cochlear tuning process was passive and mechanical. However, recent studies have shown that one group of cochlear hair cells have an active motion that enhances hearing. Research focused on the physiology of acoustics also laid the ground work for such advances as hearing aids and the cochlear implant, which involves surgically implanting electrodes in the cochlea to help stimulate the nerves involved in hearing. The implant helps people with hearing defects due to injury or loss of cochlear hair cells, which accounts for the most incurable forms of deafness.
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