Three observers hear the sound of an explosion from the same distance but through different media.

Observer A hears the sound through air, B hears it through water, and C hears it through steel.

(a) Arrange the observers in the order in which they would hear the sound first.

(b) In which medium will the sound travel the fastest and why?

(c) On a cold morning, sound from a distant train is heard more clearly than during noon. Give reason.

(d) "Solids are heavier, therefore sound should travel slower in them." Analyse this statement.

(e) If sound could travel in vacuum, what property of sound would have to change?

(a) Observer C hears it first, followed by B, then A. Hence, the order is : C → B → A

Reason — The speed of sound is:

• maximum in solids
• less in liquids
• least in gases

(b) Sound travels fastest in steel because steel is a solid in which the particles are very closely packed and the elasticity is high. Therefore, vibrations are transferred more quickly from one particle to another.

(c) On a cold morning, the air usually contains more moisture (humidity) than at noon. Humid air is less dense than dry air, so sound travels faster through it. Therefore, the sound of a distant train reaches us more easily and is heard more clearly in the morning than during noon.

(d) The statement is incorrect.
The speed of sound in a medium is given by the relation:

$V = \sqrt{\dfrac{E}{\rho}}$

where $E$ is the modulus of elasticity and $\rho$ is the density of the medium.
Although solids are denser, sound travels faster in them because of their high elasticity (rigidity). The effect of elasticity dominates in solids.

(e) If sound could travel in vacuum, then its basic nature as a mechanical wave would have to change.
Reason — Sound is a mechanical wave and needs a material medium because it is produced and transmitted by the vibration of particles. In vacuum, there are no particles to vibrate. So, for sound to travel in vacuum, it would have to become a wave that does not require a medium for propagation.