Experiment 13.1: Arrange the apparatus as shown in fig. 13.4 where a bell jar is placed on the platform of a vacuum pump. An electric bell is suspended in the bell jar with the help of two wires passing through a cork fixed in the mouth. The electric bell will begin to ring on connecting the free ends of the two wires to a battery.
The sound of the bell can be heard. Now start pumping out air from the jar by means of a vacuum pump. The sound of the bell starts becoming feeble. At last the sound of the bell becomes hardly audible inspite of the fact that we see the hammer striking the bell. When we let air into the jar we can hear the sound of the bell again. We can conclude from this experiment that the presence of air is necessary for the propagation of sound.
Ring the bell inside the water near the bank of a pond. At some distance from the bell, suspend the stethoscope in water such that its detector is directed towards the bell (Fig. 13.5). The sound of the bell can be clearly heard through the stethoscope. We can infer from this experiment that sound can travel easily even through water.
Strike one end of long railing with a hammer (fig. 13.6). Your friend standing near the other end of the railing, can hear the sound on bringing his ear near the railing. If the railing is very long, your friend hears two sounds, one through the railing and a moment later a second one through air. This shows that the speed of sound in solids is greater than in air.
We conclude from the above experiments that a material medium is necessary for the propagation of sound from one place to another. This material medium may be a gas, a liquid or a solid.