|MadSci Network: Physics|
Hi Melissa, I think you are asking two questions. How does sound get to and from the radio and how is it transmitted by the radio? The sounds which humans hear result from the periodic compression of the air around us. The speed at which sound travels varies with the temperature and density of the air. At sea level in the standard atmosphere sound travels approximately 340 meters per second. Sound waves tend to expand in area as they propagate. As the pressure wave gets farther from the source the energy is distributed over a larger area until it is so small that it can no longer be heard. Radio uses electromagnetic radiation to transmit voice and other information. Electromagnetic radiation consists of oscillating electric and magnetic fields that propagate at the speed of light (300,000,000 meters per second). So how does a pressure wave (sound) become electromagnetic radiation? A microphone converts the changes in air pressure into a changing electrical signal. The electrical signal is amplified and used to modulate an oscillating electrical signal. AM radio uses alternating current oscillating in the frequency range of 500 Kilohertz to 1 Megahertz. The alternating current is then amplified and sent to an antenna. The antenna converts the alternating current into electromagnetic radiation of the same frequency. The energy of the electromagnetic radiation also diminishes as it spreads out into a larger area. With amplification, very weak electromagnetic fields can be detected. The inverse process occurs at the receiving end of the system. The antenna converts the electromagnetic radiation back into an alternating electrical current. The alternating current is amplified and filtered to reduce noise. The alternating current drives a speaker which produces pressure waves in the air. The sound then propagates through the air to the ear. If everything works right, the sound produced by the radio is pretty close to the sound which originally entered the microphone at the transmitter. A good way to learn how a radio works is by building a crystal radio. The kits are inexpensive and available in a lot of toy stores. The science is usually more fun to play with than just reading or thinking about it. Have fun with that project, Bob Novak
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