Hearing
V. Vemuri
Compared to vision, the sense of hearing always suffered from a second class status. Many I know show sympathy toward blind people but, alas, they poke fun at the deaf. If you think I am too harsh and judgmental, just look at a sample of Indian cinemas and count the number of times deaf people are made the butt end of jokes. You want another example. Wearing corrective lenses is considered fashionable (among men, in this male chauvinistic world); but many prefer to find a way to hide their hearing aid.
Helen Keller felt that the loss of hearing is more debilitating, more painful, and more isolating than blindness. Loss of hearing means loss of the most vital stimulus, namely the sound of voice that brings language, the one attribute that separates man from animal. Still not convinced? Look at our language. In Latin, surdus means "deaf and mute" which in Arabic is ab surd which in turn gave birth to the English absurd. All of this means that it is impossible to make sense of this world if one is deaf. Yet, despite Keats' observation that "Heard melodies are sweet, but those unheard/ Are sweeter," most people say that given a harsh choice, they would rather lose their hearing than their sight.
Our eyes have about 300 million light sensitive cells, whereas our ears have a paltry 32, 000 hearing receptors - or "hair cells" as they are called - that are sensitive to sound. Adding insult to injury, we begin to lose these cells from the moment we are born.
The phenomenon which we call hearing begins when sound waves enters the outer ear, which is comprised of the pinna, the fleshy projecting part, and the ear canal. The canal funnels the sound waves toward the tympannic membrane, or the eardrum. The vibrations thus induced on the ear drum tickles the three little articulated bones in the middle ear: the anvil, hammer and stirrup. The last of these, whose scientific name is stapes, flexes the membrane of a small window in the fluid filled cochlea, the organ resembling the shell of a snail, located in the inner ear. This flexing causes the tiny hair cells in the cochlea to move. This in turn causes an electrical signal to be sent to the brain. Because the pea-sized, spiral shaped cochlea is buried deep in the thickest part of the skull, its immense complexity is difficult to study in live subjects.
We can tell where a sound comes from because of the way our ears are constructed. First of all we need two ears for stereo location of sound, just as we need two eyes for depth perception. How can we tell if the sound is coming from right or left? Because the speed of sound is about 344 meters per second, our ears can detect a delay when a sound is presented to one ear six to ten millionth of a second after it is presented to the other. That difference is enough for the ear to tell if a sound is coming from the right or the left. Also the sound is slightly louder in the ear that is closer to the source.
Surprisingly, the pinna, which had been dismissed as a useless appendage of evolution, helps us locate the source if it is above, below, front or back. Part of this ability comes from the structure of the pinna itself, with its ridges and valleys and all. As waves of sound roll toward the ear they hit the pinna like ocean waves beating against a rocky shore line. Depending on the direction of the wave and the position of the ear, the waves pile up in different patterns before they enter the ear canal. This causes the pitch of the sound waves that actually enter the ear canal to differ depending on where the sound source is. To understand how this happens, let us conduct a simple thought experiment. Think about a sound to your right, may be above the eye level and at some 45 degrees. Instead of using a tuning fork which produces pure notes, think of a complex sound such as the snapping of fingers, which produces a whole mixture of frequencies. If you record the sound with a tiny microphone in the ear canal - that is somewhere past the pinna - you will notice that some of the original frequencies are amplified. If you snap your finger from another position a different set of frequencies are amplified. With this ability of pinna to amplify different frequencies, depending on the location of the source, we can pin point the source of sound. (This probably is the reason Lakshmana cut the ears and nose of Surpanakha - to disorient the Rakshasi.)
Typical human voice has a frequency range of about 100 cycles per second for males and 150 cycles per second for females. Middle C in music is 256 cycles per second. At the peak of our hearing we can hear from sixteen to 20, 000 cycles per second - almost ten octaves. Mercifully, we cannot hear low frequencies well; if we did, sounds of own bodies would drive us mad.With aging we lose this ability because the cells that are sensitive to high-frequency (high pitch) sounds wear out and perish easily. By middle age an average male loses 40 percent of the hair cells at birth. When we abuse our ears, we damage the hair cells even more. Look at how we abuse our ears. I have seen many who carelessly put a sharpened pencil in their ears to extract ear wax. Many teenagers in Western countries go to hard rock music concerts where the loudness of the music can exceed 115 decibels, a deafening sound indeed.
About ten percent of the people suffer from some type of hearing loss. The most common type of loss is the inability to hear high-frequency signals. Causes of this loss include the declining number of hair cells in the cochlea and the thickening of the ear drum. Although there is nothing available to overcome this problem in the short run, hope is rising on the horizon.