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| Propagation of Sound |
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| Ordinarily, we hear sound, transmitted through the air. Unlike light, sound cannot pass through vacuum. This was discovered in 1654 by Otto Von Guericke. Let an electric bell be enclosed within a bell jar which is placed over a disc. Through a hole in the disc, air can be removed by using a vacuum pump. On gradually removing the air, the sound of the bell becomes feebler and becomes almost inaudible when the limit of exhaustion is reached. The reason why absolute silence is not attained, is that the conducting wires transmit some sound. |
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| In air, sound is propagated in the form of compressions and rarefactions i.e., longitudinal waves. |
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| In earlier days, doctors used stethoscopes consisting of thin wooden rods with broadened ends. By placing one end to his ear and placing the other end on the patient's chest, he could hear the sound of the heart beats transmitted through the wood. Motor mechanics sometimes, use wooden rods as stethoscopes to assist in tracing the source of the knocking noises in engines. Cotton, wool and felt are poor conductors of sound. A piece of thread does not conduct sound when slack, but will conduct it well when stretched. |
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| We know that water transmits sound. This
can be shown by clapping two pieces of stone or metal against each other
under water, when the sound of the clapping can be heard above the water. In 1654, Otto Von Guericke found that fish were attracted by the sound of a ringing bell underwater and therefore, concluded that sound could travel through water as well as air. |
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| Sound waves can undergo reflection, refraction, interference and diffraction. But they cannot undergo
polarization. |
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| Reflection of sound is evident in the environment in the form of echoes. The rolling of thunder is largely due to successive reflections from the clouds and land surfaces. For appreciable reflection of a wave from any surface, the surface area should be fairly large in comparison to the wavelength of the waves incident on it. Consequently, larger surfaces are required for complete reflection of sound waves. Sound waves being larger than light waves, do not require the reflecting surface to be smooth. For this reason, a brick wall, a wooden board and a row of trees serve as reflectors of sound waves. The reflection of ultrasonic Sound pulses in water is the principle of 'Sonar'. |
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| Refraction of sound waves in the atmosphere occurs when sound waves travel through regions of varying air density. The density of a gas is inversely proportional to its temperature. Thus, if there is a temperature variation in the air, sound waves are refracted as they pass through the layer boundaries. Dispersion is of negligible importance for ordinary sound waves. Like light waves (in air), the velocity of sound waves is independent of the frequency. Sound can also undergo total internal reflection. |
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| When sound waves meet at a common point, the resultant effect may be more or less loud depending on how the waves overlap with each other. The redistribution of energy in the regions of overlap results in the interference of sound. Sound waves produced in one direction can be heard from other directions. The voice of a person on the other side of the wall can be heard but the person cannot be seen. This bending of sound around corners is referred to as diffraction.
Polarization is a phenomenon which transverse wave, alone can exhibit and therefore, longitudinal sound waves do not exhibit
polarization. |
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| The following examples show that sound takes an appreciable time to travel from one place to another. |
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 Though lightning and thunder are produced simultaneously, the flash of the lightning is seen much before the sound of the thunder. |
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 When a gun is fired at some distance, the flash is seen before the sound is heard. |
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 The puff of steam issuing from the whistle of a distant locomotive engine is seen before the sound is heard. |
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 In a cricket match, the striking of the ball by the batsman is seen before hearing the sound. |
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