Slinky spring

Let us try to see if a slinky spring could provide us clues on how does sound travels.
Consider a slinky spring kept on a table. You compress it by giving it a sudden jerk from one side.

Watch the slow motion video given below.  You can also try it out in your classroom.
 

                                          Video 4.2 (a): A single compression traveling along a slinky
 
Now let us move our hand back and forth.
 

                         Video 4.2 (b): Many compressions and rarefactions traveling along a slinky

You can see that there are alternate regions where the slinky spring is compressed and where it is elongated. The diaphragm of a speaker also moves back and forth. It compresses the air when it moves forward and makes the air rarer/sparser/less dense when it moves backward. Just as these successive compressions and rarefactions that travel along the slinky, sound travels away from a speaker through air. Sound travels in the same way through liquids and solids too.