Sound

Sound

Sound is a form of energy propagated from one point to another as a wave. It is produced by vibrating sources in a medium. This medium can be solid, liquid or gas. 

Sound travels fastest in solid, then liquid and slowest in gas because the particles in solid are closely packed.
Sound cannot travel in vacuum. It requires a medium to travel from one point to another.

Sound is a longitudinal wave and consists of a series of compressions (C) and rarefactions (R) in the medium.
In air, compression is a region in which air particles are pushed towards each other and the air is at a pressure higher than surrounding air pressure;
Rarefaction is a region in which air particles are pulled from each other and the air is at a pressure lower than surrounding air pressure.

Propagation of sound

In Diagram (a), the tuning fork is stationary on the left.
When the tuning fork is struck, the prongs vibrate towards and away from each other to produce sound. 

In Diagram (b), when the prongs move away from each other, the air particles nearest to the prongs are pushed away to the right. This causes a compression in the layers of air particles in Region A.
The displaced air particles collide with and transfer energy to the other air particles in Region A.

In Diagram (c), as the prongs move towards each other, the air particles nearest to the prongs are pulled back to the left. This causes a rarefaction in the layers of air particles in Region A and compression passes from Region A to Region B due to energy transfer. This cycle then repeats. 
After a while, a series of compression and rarefaction is set up in the air.

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