Question

When we say ‘sound travels in a medium’ we mean:
A. the particles of the medium travel
B. the source travels
C. the disturbance travels
D. the medium travels

Answer 1

Sound waves take air as a material for propagation and sound propagates faster in more elastic medium than the less elastic medium. Eg air propagates faster in solids when compared to liquids and gases because of the elasticity of solids.

Complete answer:
Sound travels through the air by forming compressions and rarefactions i.e., it travels as a longitudinal wave and it requires air for propagation. Sound is a form of energy that needs a material to travel, the material is air. Sound is travelling as a wave or the disturbance of the air particles. Sound is produced by vibrations. Each layer of air is made up of tiny air particles. Consider the example of a speaker when it is off the layers of air are stationary and when the music is on the vibrations are coming out of the speaker and these vibrations go ahead and disturb the layers of air. The vibrations disturb the first layer, which touches the second and so on and this disturbance will continue through the layers of air as air is an elastic medium i.e., the first layer after disturbing the second layer will comes back to the original position and this process continues and the last layer goes and vibrates the eardrum of the person which causes the sensation of listening or hearing.
Thus the correct answer is D.

Additional information:
Sound travels faster in solids than liquids and slowest in gases because solids have much higher elasticity as compared to liquids and gases. Elasticity is defined as the property by virtue in which a material is able to deform when a force is applied and is able to get back to its original shape when the force is removed. Anything that is easy to deform is not the only measure of elasticity, when compared to rubber string and steel wire where steel wire is more elastic than rubber string when rubber string is pulled it is merely more stretchable but it is not very elastic thus elasticity is the resistance to change. Thus more the elasticity of the medium faster will be the speed of sound in that particular medium. In case of solids they are characterised with molecules with strong forces of attraction for each other and the molecular force of attractions can be considered as springs that control how faster the particles comes back to their original positions. So the particles that return to their resting positions quickly are ready to move again more quickly thus they can vibrate at much higher speeds thus sound can travel faster in the medium with higher elastic properties like that of steel than in rubber which has low elasticity. Another reason why sound travels faster in solids is that in solids the molecules are closely packed and they are tightly bonded so being so close to each other they can collide very quickly. Thus it takes less time for a molecule of the solid to bump into its neighbouring molecules. Thus causing the disturbance to move forward in very less amount of time

Note:
During sound propagation the energy is transferred from one particle to another and the particle begins to oscillate on its position until the energy is transferred to another particle. Once it is done the particle comes back to its original position.