Question

Why can the sound of a train be heard by putting your ear on the track well before you could hear its sound in air?
(A) Because sound travels faster through a solid
(B) Because speed of sound in air is highest
(C) Because speed of sound through steel is minimum
(D) None

Answer 1

Hint: The speed of sound depends on a lot of factors which basically tell us about the ability of the material to transfer a pressure wave. If the particles are closely packed, it means that the vibration of one particle will be transferred to the next much quickly. Metals are an example of closely packed materials.

Complete Answer:
The sound of a train can be heard by putting your ear on the track well before you could hear its sound in air. The energy is given to one particle in the metal in the form of vibrations. The adjacent particle is very close to it; hence the energy will be transmitted to the next particle much quickly. Hence sound travels faster through a solid than in air where the particles are far apart. Sound is actually just a pressure wave, a disturbance that pushes into its neighboring atoms to propagate this disturbance wave. As the train is touching the tracks, it’s sending a pressure wave down the rail. If one has to compare the speed of sound in different types of solids then one will have to look for other parameters that affect the transfer of vibrations.

Additional information: Dimensions play an important role in the propagation of a sound wave in various mediums. A speaker produces sound that propagates in three dimensions-like a sphere and so the intensity drops (according to the area getting bigger). In 3D the area gets four times bigger when the distance gets doubled. The intensity is lost by \[\dfrac{1}{{{r^2}}}\]. In 2D, like throwing a stone in a lake, the wave intensity only drops by $\dfrac{1}{r}$. But in 1D, like the railway tracks, the intensity never drops. The area of which the intensity should be distributed around never grows with the distance. That is why we can spot a train from far away- the only thing stopping the sound wave in the railway tracks are dampening, such as absorption of the wave or friction. The sound wave in air just dissipates because it’s being distributed in three dimensions causing the intensity to drop very fast. But this doesn’t happen in a steel track.

Note:
 Sound travels faster in solids, slower in liquids and slowest in gases. This is because solids are much denser. There are many more molecules in solids. There are fewer molecules in liquids and there are even fewer molecules in gases. Because the molecules are very close together in solids they can collide quickly and therefore the wave travels faster. For example, in air the pressure wave is \[343\dfrac{m}{s}\] (that’s equal to the speed of sound) but in water it’s actually \[1482\dfrac{m}{s}\] and in steel it’s almost \[6000\dfrac{m}{s}\] so, in our setup the sound wave would arrive a much sooner in the tracks than in the air. Because the pressure wave in steel moves 17 times faster than air.