Propagation of Sound
Energy has various forms like heat, light, sound, etc. A sound is a vibration which propagates as an audible mechanical wave of displacement and pressure. Propagation of sound require. The sound moves through a medium by alternatively compressing and expanding parts of the medium it is traveling through. Thus, sound waves consist of regions of high pressure called compressions and low pressures called rarefactions.
Sound waves require mechanical medium to oscillate for its propagation. In case there is no medium, compressions, and rarefactions cannot take place and as a result, sound cannot propagate. Thus sound is a mechanical wave and for the propagation of sound, a medium is required.
Bell Jar Experiment
Bell jar experiment can be conducted to prove that propagation of sound requires a medium. For this experiment, we need a bell jar and an electronic bell. When the electronic bell is turned on, the sound can be easily heard due to the presence of air inside the jar. Gradually, the air is pumped out using a vacuum pump and it is noticed that the sound starts fading. Finally, when there is almost no air inside the jar no sound comes out of the jar.
Hence, it can be concluded that propagation of sound needs a medium and it cannot travel through a vacuum.
Let us consider plucking of a string of a guitar. All the air molecules near it start vibrating and regions of compressions and rarefactions are formed which travel along the medium longitudinally. But the important point to be noticed here is whether sound always travels longitudinally? The answer is no. Sound doesn’t always travel longitudinally. In case of liquids and gases, the sound will travel longitudinally but in case of solids, on creating a vibration, the molecules of solid have to compress and expanded to pass sound through it. Since solids have much higher elasticity as compared to liquids or gases, they resist a change in their shape. Thus, liquids and gases take the shape of the container but solids resist any change in its form. Therefore in solids, sound can travel both as a longitudinal and as a transverse wave owing to elasticity.
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