The ear converts pressure variations in the air into electrical signals which it sends to the brain where the signals are interpreted as sound.
The ear senses pressure changes not motion of air molecules. (Even though the motion of air molecules produces the pressure changes.) We can discover this by going into a large tube closed at both ends which is resonating with sound, like the Sound Column exhibit at the Exploratorium. Here you can place your ear at a node of motion and yet hear a loud sound then place your ear at an antinode of motion and hear almost no sound at all. The antinode of motion is the node of pressure change. Sound Column Experiment.
Your eardrum is a thin membrane separating the outside air from an air space inside your head. It separates the outer ear canal from the middle ear. (Yes you do have holes in your head. This inner air cavity is connected to the outside air by a eustachian tube which can be opened or sealed to allow the pressure inside the ear to change to match the slowly changes of outside air pressure. If you rapidly change altitude in an airplane or by driving a car in the mountains you can feel the pressures across your ear drum until you open your Eustachian tube and equalize the pressure) When the pressure outside the ear drum increases, the eardrum is pushed inward.
The conversion of motion to electricity is done by sensory hairs within the inner ear, inside the cochlea. The cochlea is full of fluid and it is difficult for sound to go from air into a liquid. (Just think of how hard it is to hear someone standing on land shouting when you are under water, their sounds simply bounce off the surface of the water.) To get the signal from the outside air into the liquid of the inner ear the body uses bones as levers. There is a series of three bone-levers which connect your ear drum to the oval window. The oval window is an eardrum like membrane separating the fluid filled inner ear from the air of the middle ear. these levers reduce the distance that the ear drum moves while at the same time increasing the force of the push. This allows 100 times more energy to penetrate into the liquid of the inner ear. When your head is stuffed up, your middle ear becomes packed with fluid which reduces the effectiveness of these ear bone-levers at carrying sound and so your ability to detect sounds is greatly reduced.
Once the vibration enters the fluid of the cochlea it moves down along an elastic membrane, the basilar membrane that divides the cochlea tube into a top and bottom half. At the far end it passes through a hole and returns along the bottom of the membrane. The passage of the sound wave excites the membrane into oscillation.
The basilar membrane has sensory hairs along its length. The tension and thickness of the membrane varies along its length so that different parts of the membrane resonate with different frequencies of motion. The end nearest the inner ear bones resonates with high frequencies, the far end resonates with low frequencies. When one part of the membrane resonates with one frequency, the hairs on that portion of the membrane are excited into motion. The motion causes these hair cells to trigger nerve impulses. These nerve impulses are interpreted as sound by the brain.
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Scientific Explorations with Paul Doherty |
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22 Feb 99 |