Question

Explain the experiment, with neat diagram, to prove the following:
'Sound needs material medium for propagation.'

Answer 1

Bell jar experiment can prove that sound needs material medium for its propagation.
Construction

• A bell jar
• A vacuum pump connected to the jar through a tube
• An electric bell inside the jar and is connected to the electric supply through the lid of the bell jar

Working
Initially the vacuum pump is 'OFF' so that air is present inside the jar. Now, if electric supply is switched on, the electric bell will start ringing and this ringing can be heard outside the bell jar.
Now, switch on the vacuum pump such that it starts sucking the air inside the jar. It will be observed that, with the decrease in quantity of air inside the jar, the level of ringing sound heard outside also decreases. If the pump is used for sufficiently long period, then the quantity of air inside the jar will be so low that the ringing sound outside it will not be audible. This proves that sound requires medium for its propagation.