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Sound Transmission

Sound travels through the air or another material.

Sound cannot travel through a vacuum.   

 
  The alarm clock cannot be heard because sound cannot travel through a vacuum. 
         
    Before the air is sucked out of the bell jar, the clock can be heard ringing.    The vacuum pump sucks out the air and even though the hammer can be seen striking the bell, the ringing cannot be heard, because there is no air to carry the vibrations.
         
         
Sound travels through all sorts of materials. It travels very well in solids.  

 
  A distant train can be heard more easily if you are near the track because the metal track is a good conductor of sound. You notice this on a station platform.
    NEVER GO TOO NEAR RAILWAY TRACKS!!