Sounds are vibrating waves of energy that travel through a material medium. When a baby cries, the sound wave it produces spreads out in all directions, forming an expanding sphere in air. This is the result of molecules that collide with one another, pushing their neighbors forward, then rebounding, causing the wavefront to move forward at the speed of sound. The full range of sound includes very small to very large wave disturbances such as atomic stress waves in solids, ocean waves, seismic waves, solar and galactic waves. Sound waves travel at speeds ranging from subsonic velocities of several meters per second, to hypersonic velocities that approach the speed of light, depending on the medium in which the sound is traveling. Sound waves vibrate at frequencies that range from billions of cycles per second, to a single cycle within a period of several days, months, or years. Sounds are produced wherever there are free moving bodies in close proximity.