Sound Needs a Medium to Travel

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The origin of sound is always some vibrating body. In some cases the vibrations of the source may be very small or very large that it may not be possible to detect them. This type of vibrations is produced by tuning fork, drum, bell, the string of a guitar etc. Human voice originates from the vibrations of the vocal chords and the sound from the musical instruments is due to the vibrations of the air columns. Sound travels in the form of longitudinal wave and it requires a material medium for its propagation.

Experiment to show that sound waves (mechanical waves) require a material medium for its propagation

Electric bell suspended inside an airtight glass bell jar

An electric bell is suspended inside an airtight glass bell jar connected to a vacuum pump. As the electric bell circuit is completed, the sound is heard. Now if the air is slowly removed from the bell jar by using a vacuum pump, the intensity of sound goes on decreasing and finally no sound is heard when all the air is drawn out. We would be seeing the hammer striking the gong repeatedly. This clearly proves that sound requires a material for its propagation.

Sound can propagate not only through gases but also through solids and liquids. Some materials like air, water, iron etc can easily transmit sound energy from one place to another. On the other hand materials like blanket and thick curtains absorb most of the sound energy.