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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?

Also, please don't go about placing your ears on the track.

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The steel rail does emit sound conducted from the wheels of an approaching train, but the degree of acoustic coupling between the rail and the air is poor so you have to be very close to the rail to hear that sound.

The coupling between your ear and the rail is greatly improved by touching the rail with a bar of substance with an acoustic impedance intermediate between that of steel and that of air, and sticking the other end of the bar in your ear. That's why the stick of wood helps.

In my experience, though, if the train is close enough to produce audible sound waves in the rail, it is also close enough to hear the noise of the engine just using your ear in the usual manner.

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  • $\begingroup$ Thank you for the answer. I am in grade 12 and do not have much idea about these terms (acoustic coupling, acoustic impedance etc.). I tried searching online but it's not very clear. Could you suggest me any resource or text for the theory regarding the topic? $\endgroup$
    – AbsoluteZero
    Commented Apr 7, 2020 at 5:37
  • $\begingroup$ 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). $\endgroup$
    – taciteloquence
    Commented Apr 7, 2020 at 7:21
  • $\begingroup$ @AbsoluteZero, sorry, acoustic impedance is a college-level topic. simply put, if the speeds of sound in two media are different, so will be their impedances, and a sound wave trying to propagate from one medium into the other will partially reflect off the boundary between them. this is called an impedance mismatch. for a prefect match, all the sound propagates through and there are no reflections. for a bad mismatch, most of the sound bounces off and little is transmitted through. more in next comment. $\endgroup$
    – niels nielsen
    Commented Apr 7, 2020 at 7:48
  • $\begingroup$ @AbsoluteZero, in the case of a bad mismatch, you can improve things by inserting a piece of "matching section" with an impedance that is inbetween the two extremes (like the wood piece mentioned above); this is called an impedance transformer and minimizes reflections- thereby maximizing the signal strength propagating through the system. $\endgroup$
    – niels nielsen
    Commented Apr 7, 2020 at 7:52

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