Sound is something we cannot live with. At the same time, noise is something we cannot live with easily. Yet most of us have little understanding of sound!

Definition And Introduction To Sound: I am sure everyone has thrown stones into calm water. The moment the stone hits water, circular waves form and spread outwards. The process of forming the concentric circles continues for some time, and then dies out because the energy imparted by the stone is used up. Waves similar to what is seen in the pond can be produced in any material -- solid, liquid or gas. All what is needed is a source of energy that can produce a disturbance in that material. Since you are able to "hear" them, they are called "sound" or "noise".

In other words, if matter vibrates with a frequency between 20 to 20,000 cycles per second, you are able to recognize those vibrations with your "ears" and those vibrations are called sound. If the vibrations have a frequency higher than 20,000 cycles per second, they are still sound but your ears cannot pick them up any longer. They are called ultra-sound and can be picked up only with special-purpose electronic devices. If the vibrations have a frequency lower than 20-cycles, they are still sound, but your ears cannot pick them up. They are called infra-sonic sound that can be picked up only with the help of special-purpose electronic devices.

However, if the frequency of the infra-sonic vibrations is so low that the body vibrates only once or twice per second, you are suddenly able to "see" it. That is why you see the waves in water. When the frequency becomes high, your eyes no longer have the sensitivity to see them and that is why you fail to see them. As soon as the frequency crosses 20 and goes to a higher level, your ears begin to pick them up.

Every wave produced this way has a "frequency". We notice that waves are produced by the up and down motion of water. The number of times this up and down motion takes place every second is called the "frequency" of the wave.Obvioulsy, the waves that you see in water have a low frequency because they oscillate up and down only about once a second or once in two seconds. However, if the waves had a high frequency such as 1000 oscillations per second, the concentric circles would form so close to each other (1000 circles in 1 second) that they would touch each other closely and you would no longer be able to "see" the waves. At the same time, your ears would suddenly be able to "pick up" the presence of the waves.

It is simply amazing, but whenever a solid, liquid or gas vibrates with a frequency that is between 20 to 20,000 vibrations per second, you are able to "hear" those vibrations.

So What Is Sound: When matter vibrates with a frequency between 20 to 20,000 cycles per second, our ears are able to pick up those vibrations. This is called sound. The major sound produced by all musical instruments lie between 20 to 20,000 cycles per second. Vibrations of higher or lower frequencies are also "sound" but our ears cannot "hear" them. When the frequency is is so low that there is only one or two vibration per second, our eyes can pick them up. That is why we see concentric circles in a calm pond disturbed by a stone.

Sound can be produced only in matter and therefore they can travel only through matter (solid, liquid, or gas). Since there is no matter in vacuum, sound cannot travel through vacuum. This is the reason why sound cannot travel through space, most of which is vacuum.

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