When you hear a person talking, a dog barking, or 
  a police siren, you hear sound. Most sounds are 
 very loud.  Others are soft.  Sound sounds are high, 
  like a whistle.  Others are low, like a rumble of 
          thunder.
  Vibrations Make Sound
 • Remember that energy is the ability to 
  make something move, happen, or 
  change. 
 • Sound is the energy that you can hear.  
 • Sound is made by something vibrating.
 • When something vibrates, it moves 
  back and forth very fast
   Think about what happens when someone plucks a guitar string. 
 • The string vibrates.  Each time 
  the string moves, it bumps 
  against air that is nearby.  
 • The string makes the air 
  vibrate, too.  The air bumps 
  against more air, and so on.  
 • The energy of the vibrations 
  move through the air in 
  waves. 
                Click Here for sound energy
                 
                      The waves are like ripples in a pond when you toss a pebble into it.  
                            Sound Wave Ripple Effect Animation 
 Sound waves travel from the guitar in all directions. 
 When the sound waves reach your ears, the air in your 
    ear vibrates. 
 The air makes your eardrums inside your ears vibrate 
    too. 
  Signals travel to your brain, and you hear the sound of 
    the guitar.  
 http://www.harcourtschool.com/activity/science_up_clo
 se/521/deploy/interface.html