When the needle on your radio set is exactly on a transmitting station you can see a number on the dial (for example, the needle indicates 700 kHz). Each transmitter sends out electromagnetic waves of a determined length. The frequenby (or number of vibrations per second) of this carrier wave is measured in Hertz(Hz), from the name of a German physicist who made this discovery. Music and speech are vibrations in the air. These are transformed via the membrance of a microphone into a signal which is superimposed on the carrier wave. This ten undergoes a change called 'modulation'. For the low frequencies, the amplitude is changed; for the high frequencies, the frequency modulation (FM) intervences. Inside the aerial of the receiver, the different transmitters produce vibrations. If you want to obtain a specific transmitting station, you turn the needle of the selector to the length of the chosen wave which is then amplified. Then, the modulated vibration is again separated from the wave by ta detector, the signal received is once again amplified then sent towards the loud-speaker. IN fact, the movement is opposite to that in the microphone. The rest is simple; the vibrations make the membrance in the loud-speaker terrible which is turn makes the air terrible, and you hear the orginal sound.