 Let's take a closer look at just what sound is and how fast it travels. An elastic substance is one that returns to its original shape after having been disturbed, like this ball. And an elastic medium doesn't, like this pizza dough. A disturbance in an elastic medium will propagate through the medium. Air is an elastic medium. And sound is a disturbance that moves through it. Sound waves are compression waves. These are waves where the disturbance moves along the line that the wave moves. In this animation, each dot represents an air molecule. As the surface on the left moves in, the nearest molecules are compressed. When it moves back, the compression area becomes rarefied. The compression, followed by a rarefaction cycle, moves to the right. How fast the wave moves depends on the characteristics of the medium. In particular, the higher the resistance to compression, its compressibility, the faster the movement. And the closer the molecules are to each other, its density, the slower the movement. In dry air, at 20 degrees centigrade, that's 68 degrees Fahrenheit, the speed of sound is 1,236 kilometers per hour.