Why is it possible to hear the train sound so far from the ears against the tracks?

To know if a train is coming from afar, railway workers or passengers often put their ears on the listening tracks. If the sound were heard, it would not be long before the train began to run. That is why? It turned out to be related to the speed of sound transmission.

We know that the propagation of sound has a certain speed. But in everyday life, it sounds like you hear it right away. For example, you talk face to face with family members, enjoy the show on TV, etc. That’s because the sound source (an object that emits sound) is so close to us. If the sound source is a bit away from us, for example, watching the pile driver working far away, it is not difficult that you do not notice, after the air hammer has dropped in a blink of an eye, you will hear the sound of the air hammer to the wooden stake.

The propagation of sound not only has a certain speed, but in different environments, the speed is also different. For example, in the air, sound can travel about 340 m / s; in the country, it reaches 1440 m / s, and in rails, it is even faster, about 5000 m / s. The train speed is generally 100 – 200 km / h, which means that it is in the range of 60 m / s, which is much slower than the speed of sound transmission in the tracks. If a train is within 5 km of us, it will take more than 80 seconds for the train to come before us; if standing, it would take nearly 15 seconds to hear the train; If you put your ear to the railroad track, it only takes about a second to hear the train’s murmur.

Moreover, the sound intensity is reduced during transmission. When transmitted in the air, the sound emanates from the surrounding, reducing rapidly. When you hear the sound of a train, it comes near; rushing quickly often causes disaster. Due to the directional effect of the track on the sound, the sound’s attenuation in the tracks is relatively slow. When you put your ear to the rail and hear the sound of the train, you know the train is coming. Then it’s far away from us; you will be safe.

So what if I used my eyes to look? Light travels much faster than sound. With a distance of 5 km, the light will only need 0.000017 seconds to arrive! But due to causes such as the obstruction of the horizon, the cover of the fog and the dust curtain, the meandering of the tracks, the obstruction of buildings, mountains, trees, etc. so we can’t see the train well beyond 5 km. So, to judge whether a train is coming from afar, the easiest and easiest method is to apply your ears to the listening tracks.