Radio waves travel at 186,000 miles per second through air. In contrast, sound waves travel at only 1/5 of a mile per second. If a modulation is made of the radio wave that exactly reproduces the amplitude and frequency characteristics of the original sound wave, then sound can be transmitted rapidly over long distances. This leads to a very interesting phenomena. During a live broadcast in New York, the music will reach listeners in California a fraction of a second before it can be heard by the New York audience sitting in the back of the concert hall.
How radio waves carry sound
|Radio transmissions are a combination of two kinds of waves: audio frequency waves that represent the sounds being transmitted and radio frequency waves that "carry" the audio information. All waves have a wavelength, an amplitude and a frequency as shown in the figure. These properties of the wave allow it to be modified to carry sound information.|
|In AM (amplitude modulation) radio transmissions, the amplitude of the combined audio frequency and radio frequency waves varies to match the audio signal. AM radio is subject to problems with static interference. Electromagnetic waves (like radio waves) are produced by the spark discharges in car ignition systems, brushes of electric motors and in all sorts of electrical appliances, as well as in thunderstorms. There is considerable background noise that changes the amplitude of the radio wave signal adding random crackling noises called static.|
|In FM (frequency modulation) radio transmissions, the frequency of the combined waves changes to reproduce the audio signal. For example, higher frequency is associated with the peak amplitude in the audio wave. FM waves do not have a problem with interference because the noise background does not modify the radio wave frequency. In addition FM waves give better sound reproduction.|