Sound propagation requires a material medium such as air, water, steel, and so on. It is unable to travel in a vacuum. The Bell Jar experiment demonstrates that sound cannot travel in a vacuum.
We take an electric bell and an airtight glass jar. Inside the airtight jar, an electric bell is suspended. As shown in the above figure. The jar is connected to a vacuum pump. We will be able to hear the sound of the bell if it is made to ring.
When the vacuum pump is used to evacuate the jar, the air in the jar is gradually pumped out, and the sound becomes feebler and feebler. If the air is completely removed, there will be no sound (if the jar has a vacuum). This proves that sound needs a medium to travel.
Sound is a wave:
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Sound as wave
Sound travels through a medium from the point of origin to the listener's ear. When an object vibrates, it causes the medium's particles to vibrate as well. However, the vibrating particles do not travel all the way to the ear from the vibrating object.
The equilibrium position of a medium particle in contact with a vibrating object is displaced. It then applies a force to a nearby particle. As a result, the adjacent particle is shifted from its resting position. The first particle returns to its original position after displacing the adjacent particle. Until the sound reaches our ears, this process continues in the medium.
It's worth noting that only the disturbance caused by a sound source travels through the medium, not the medium's particles.
The medium's particles all restrict themselves to a small to and fro motion known as vibration, allowing the disturbance to be carried forward. A wave is defined as a disturbance that propagates through a medium.