What is Vacuum?
A vacuum is a space void of any matter. It means that it is a space that does not have any matter. For humans, it is practically impossible to find or create one hundred percent vacuum.
Outer space is the closest approximation of a perfect vacuum. This is the reason why we are unable to hear any explosions from stars or planets here on earth.
Let us perform an experiment to see that sound cannot travel through a vacuum.
Experiment: Sound cannot propagate in a vacuum
We will need a glass jar in which an electric bell can be fixed with the help of a cork. The bell is then connected to an external battery and a key. The glass jar also has a vacuum attached at the bottom. When we close the key, the current starts to flow and the hammer starts to strike the gong and continues to do so. We can clearly hear the sound of the bell.
Now, we will start pumping the air out from the glass jar through the vacuum pump. You will observe that the sound starts to faint. As the amount of air in the jar reduces the intensity of the sound decreases. This is because we create a vacuum inside of the glass jar when we remove the air out.
Electric Bell Jar
Note that we cannot create a hundred percent vacuum. Hence we cannot remove all the air from the glass jar. So there will still be some soft sound that we will be able to hear. Thus, somehow if we are able to create a perfect vacuum then we won’t be able to hear the sound of the bell.