Sound waves can travel through solids, liquids, and gases. The thicker the medium, the faster it goes, but they cannot travel through a vacuum (a place with nothing in it). This is why astronauts cannot talk to each other in space: they need a radio to hear each other. Sound can travel through water faster than through air; and even faster in solids like stone, iron, and steel. Sound travels at 335 metres (1,100 feet) per second in the air.