Elementary though it might sound, the problem of how sound (and any longitudinal wave, for that matter) travels had been haunting my mind for years. For example, when a tuning fork vibrates, I was taught, it alternately moves forward and backward, so alternately it compresses and rarefies the medium (makes it more dense and less dense at certain areas).
Now, when a tuning fork moves backwards, it does rarefy air; but, when it moves forward again, it sends the same amount of ‘air particles’ forward, and even more. So, the rarefaction effect should get cancelled, and a compression should be formed instead. Now, when it moves backward again, the compression is also cancelled, and rarefaction rules again. Like this, the tuning fork should continually self-cancel and there should be no net effect. This had been my argument ‘against’ sound propagation(!), and I could not initially see any flaw in it.
Now, let us take a closer look at the situation: Let’s take up the same tuning fork again, let us vibrate it again. Now, let us see the slow motion version of the vibration. First the fork moves forward, pushes the nearby air particles in front of it away from it. These particles in turn, push the particles in front of them away from themselves. In this way, the energy that the tuning fork gave continually moves away from it, through air particles. First, the nearby air particles were compressed against their neighbours, then they were compressed against their neighbours, and so this ‘compression’ energy goes on. Now, what about the tuning fork’s backward movement? This pulls air particles towards the fork (since the pressure decreases near the fork). They in turn pull their neighbours, and they in turn pull their … well, you know the story! Now, why doesn’t the compression energy cancel out the rarefaction energy? The answer is, the rarefaction energy is a little too late for that. Wherever it goes, the compression energy has already crossed that point, and has moved forward. These diagrams would probably give a clearer picture:
Tuning fork(the slash: '/' ) moving forward:
-------> / ~
While the tuning fork(the backslash: '\' ) is moving backwards, the compression energy(shown by ~ ) is moving forward, and the rarefaction energy(shown as ^ ) has only started its journey:
<------- \ ^ ~
The ‘next’ compression energy is being given:
-------> / ~ ^ ~
Thus we can see that while the first rarefaction energy is sort of trying to catch up with the first compression energy, the second compression is trying the same at the first rarefaction energy. This goes on and on, and finally, after some time, you have alternate columns of compressions and rarefactions in the air: you have Sound!