Radio

“…Radio is the transmission of signals through free space by electromagnetic waves with frequencies significantly below visible light, in the radio frequency range, from about 3 kHz to 300 GHz.[1] These waves are called radio waves. Electromagnetic radiation travels by means of oscillating electromagnetic fieldsthat pass through the air and the vacuum of space.”

Information, such as sound, is carried by systematically changing (modulating) some property of the radiated waves, such as their amplitude, frequency, phase, or pulse width. When radio waves strike an electrical conductor, the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form…” (Wikipedia)