Question
A bell is suspended inside a glass jar. When the jar is sealed and air is slowly pumped out, the sound of the bell becomes fainter and eventually inaudible — even though we can still see the bell ringing. Why does this happen? What does this tell us about sound?
Solution — Step by Step
Sound is not a thing that travels by itself — it is a disturbance. When the bell strikes, it vibrates, and those vibrations push and pull the surrounding air molecules, creating alternating regions of compression and rarefaction. This pattern of disturbance moves outward from the source.
As the pump removes air from the jar, there are fewer and fewer air molecules to carry these compressions and rarefactions. The disturbance has nothing to pass through. The bell is still vibrating — you can see it — but there is no medium to relay the message to your ear.
When the jar reaches near-vacuum, you hear nothing at all. This proves the key conclusion: sound requires a material medium to travel. It cannot travel through vacuum. Light can (that’s why we see the bell), but sound cannot.
Sound needs a medium — solid, liquid, or gas — because it is a mechanical wave. It travels by making the particles of the medium vibrate. No particles, no vibration, no sound. This is fundamentally different from electromagnetic waves like light or radio waves, which need no medium.
Why This Works
Sound belongs to the category of mechanical waves — waves that exist only as disturbances in matter. Think of it like passing a message along a line of students by tapping the shoulder of the next person. If there’s no one in between, the message never reaches the end.
In the bell jar experiment, we are essentially removing those “students” one by one. As the air thins out, the tapping gets weaker until there’s no one left to pass the message to your ear. The bell jar makes this abstract idea physically visible: you can watch the bell vibrate while hearing nothing — two separate phenomena happening simultaneously.
This also explains why astronauts in space cannot talk to each other without radio communication. Space is (nearly) a vacuum. Two astronauts floating side by side with no suits on would hear absolute silence from each other, even if they were shouting.
Alternative Method
Instead of the bell jar, we can demonstrate the same principle using two bricks underwater. Strike them together — the sound carries beautifully through the water to a swimmer nearby. Now try the same in air from a distance: the sound is much fainter. This shows sound travels through liquids too, and often faster through denser media.
Sound actually travels faster in solids than liquids, and faster in liquids than gases. Iron transmits sound at roughly 5000 m/s; air manages only about 340 m/s at room temperature. Denser, more elastic media transmit the disturbance more efficiently — this is a favourite conceptual question in board exams.
The bell jar is a subtraction experiment (remove the medium, sound disappears). The underwater brick is an addition experiment (add a different medium, sound travels fine). Both lead to the same conclusion from opposite directions.
Common Mistake
Students often write “sound travels faster in vacuum because there is no resistance.” This is completely wrong — sound does not travel in vacuum at all. Speed of zero, not infinite. The confusion comes from mixing up sound (mechanical wave) with light (electromagnetic wave). Light travels fastest in vacuum; sound cannot travel in vacuum. Keep these two absolutely separate in your head.
A related slip: students say “the bell jar proves sound needs air.” That’s too narrow. The correct statement is sound needs a medium — solid, liquid, or gas all work. Air is just the medium we removed in this particular experiment. NCERT questions often test this exact distinction, so write “material medium” not just “air.”