Question

In the experiment shown below, the sound of the ringing bell is initially heard. As the air is gradually pumped out of the jar, the sound starts to become less loud and after some time, it can no longer be heard. What is the reason behind this?

• A. The amplitude of sound made by the bell decreases with time.
• B. The frequency of the sound made by the bell increases with time.
• C. The sound waves from the bell become transverse.
• D. Sound waves need a material medium to travel through.

Answer 1

The correct option is D Sound waves need a material medium to travel through.

Sound waves always need a material medium to be propagated. Initially when the bell is full of air, its ringing can be heard. But as the air is sucked out of the jar using the air pump, no material medium is available for the sound of the bell. Since, vacuum is not a material medium, sound cannot travel through it. Hence, the sound of the bell is no longer heard as long as there is no air in the jar.