MadSci Network: Physics

Re: How do sounds travel to your radio?

Date: Mon Oct 26 12:40:31 1998
Posted By: Bob Novak, Other (pls. specify below), Sr Process Research Engineer, Carpenter Technology
Area of science: Physics
ID: 908242389.Ph

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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