Question

Describe an activity to show that sound requires a material medium for its propagation.

Answer 1

Sound is a mechanical wave that needs a material medium like air, water, steel, etc., for its propagation. A sound wave travels in the form of a longitudinal wave and it requires a material medium for its propagation. Sound always originates from some vibrating body. These vibrations are produced by tuning forks, drums, bells, the strings of a guitar, etc. The bell jar experiment is a common experiment used to demonstrate that sound needs a medium to travel. A bell jar is a laboratory equipment used for creating a vacuum. It is so named as its shape is similar to that of a bell. A bell jar is placed on a base which is vented to a hose fitting that can be connected via a hose to a vacuum pump. By pumping the air out of the bell jar, the air pressure inside the jar can be varied. The experiment is done by placing an electrical bell in the bell jar. As the air is pumped out of the sealed bell jar, the sound from the bell jar fades. At a particular vacuum, no more sound is heard from the bell, but we can see that the hammer continues hitting the gong and sound is produced. However, the sound is not audible to our ears because of the vacuum inside the jar. This demonstrates that the sound wave cannot travel through vacuum. That is, a sound wave needs a material medium for its propagation.