you turn on a radio you hear sounds because the transmitter at the radio station has converted the sound waves into electromagnetic waves, which are then encoded onto an electromagnetic wave in the radio frequency range (generally in the range of 500-1600 kHz for AM stations, or 86-107 MHz for FM stations). Radio electromagnetic waves are used because they can travel very large distances through the atmosphere without being greatly attenuated due to scattering or absorption. Your radio receives the radio waves, decodes this information, and uses a speaker to change it back into a sound wave. An animated illustration of this process is given below (mouse-over the images for animations).
- A sound wave is produced with a frequency of 5 Hz - 20 kHz.
- The sound wave is equivalent to a pressure wave traveling through the air.
- A microphone converts the sound wave into an electrical signal.
- The electrical wave traveling through the microphone wire is analogous to the original sound wave.
- The electrical wave is used to encode or modulate a high-frequency "carrier" radio wave. The carrier wave itself does not include any of the sound information until it has been modulated.
- The carrier wave can either be amplitude modulated by the electrical signal, or frequency modulated.
- The signal is transmitted by a radio broadcast tower.
- Your radio contains an antennato detect the transmitted signal, a tuner to pick out the desired frequency, a demodulator to extract the original sound wave from the transmitted signal, and an amplifier which sends the signal to the speakers. The speakers convert the electrical signal into physical vibrations (sound).