# Behavior of sound on interface of 2 media

We say that sound travels faster in steel than in air. Thus if we want to know whether a train is approaching us we can put our ear on the rail track as we would be able to hear that quickly.

Now that I have established the premise I would like to ask...do the sound waves travelling in a medium like a steel bar placed in air also emerge simultaneously into the surrounding air from the steel bar. It seems like they should...sound travels in all directions and the surface particles would certainly interact with the nearby air particles. So do they? If yes, why do we have to put the ear on the rail to hear the sound? Wouldn't it automatically emerge out from the track near me?

Also I read somewhere that if you had a solid piece of wood or metal you could place one end firmly onto the track and your ear to the other end. So on which factors does the fact that a wave spreads out to other media depend?

• This is a great answer! I would add that the difference in the speed of sound is not the important part here. Instead, if you can hear the sound through the rail before the air, it's because the rail is a more efficient conductor (sigh) of sound. In air, the sound does spread out in all directions and falls off like $1/r^2$, but the rail is effectively a 1D object, so there is no $1/r^2$ rule (although there is still damping). Commented Apr 7, 2020 at 7:21