Instructor
Wannabe
Joined: Sep 27, 2004
Posts: 2
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A Definition of Sound
I will take a crack at a comprehensive definition of sound based on what I have studied over the years.
Sound is what we call the sensation that is perceived by our brains when the eardrum responds to changes in air pressure. When the air pressure in the ear canal is greater than the air pressure behind the eardrum, the ear drum moves inward. If the air pressure is lower on the outide of the eardrum than within the chamber of the middle ear, the eardrum will move outward. Attached to the eardrum are three tiny bones arranged like a series of levers that transmit and amplify this inward and outward motion of the eardrum to a snail-shaped, liquid-filled chamber called the cochlea. The vibrations from the eardrum are transmitted to the liquid which causes hair-like cells lining the cochlea to vibrate. These hair cells then transmit electro-chemical impulses to the brain via the audiotory nerve and that is when we actually hear a sound.
In order to get the hair cells to vibrate, the changes in air pressure must be relatively quick. They must generally occur between 20 times to 20,000 times per second. This corresponds to a frequency range of 20 Hertz (Hz) to 20 kiloHertz (kHz). Any thing that vibrates in the air in this frequency range will make a sound that a human with excellent hearing could hear. The average adult would have a somewhat dimished range. Also, our ears are much better at hearing frequencies in the middle of this range that at the extremes.
All musical instruments, human vocal cords, speaker cones, etc. are simply creating vibrations in the air molecules surrounding them. These vibrations result in changes in air pressure that correspond to how quickly the object is vibrating. The vibrating object sends out waves of fluctuating air pressure much like a dripping faucet sends out ripples in a tub of water. When the wave of fluctuating pressure reaches your eardrum it causes it to move in and out at the same frequency as the vibrating object. If the changes are sustained and regular, you will hear a pitch or a note. If the vibrations also contain more complex harmonically and non-harmonically related frequencies, you will perceive a specific instrument or a unique voice.
Remember the age old question: "If a tree falls in the forest and there is no one there to hear it, does it make a sound?" If you take the question very literally the answer is no. The tree disturbs the air and makes the ground shake. But if there are no ears to pick up the vibrations and transmit them to a brain that can interpret the stimulus, then it isn't really a sound. It is just vibrations in air.
Another way to think of it is like this: Consider a very low frequency vibration like the tremor of a distant earthquake. You may feel its effects with your feet if you are standing on the ground, but you won't hear it. You would say it is not making any sound, but the shaking earth is disturbing air molecules. It is just not doing so at a rate that is quick enough to cause your hair cells to react and send impulses to the brain.
However, an animal like an elephant for instance, may indeed be able to hear low frequencies of which a human may be totally unaware. In fact, it has been documented that elephants call to each other at frequencies we humans consider subsonic or infrasonic (below the range of hearing). These "sounds" are not subsonic to elephants because their ears are able to pick them up and there brains are able to interpret them. In other words, sound is in the ear of the beholder!
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