Sound can only travel through a solid, liquid or gas and not through vacuum. As there is near complete vacuum in space, sound cannot travel and be heard through the ears like on Earth. Other forms of electromagnetic radiation including radio waves, however, can travel through vacuum. When astronauts are in a space shuttle or a space station, they can speak normally there are enough air particles to vibrate and take the sound to their ear drum. But when they are conducting a spacewalk, they need a special device to communicate with each other The helmets of astronauts are fitted with a device which converts the sound waves generated by their speech into radio waves and transmits them to other astronauts. When the headset of another astronaut receives the radio waves, it translates the signal into the sound form. The same principle is used to send and receive messages from Earth.
Picture Credit : Google
