The Doppler Effect

                   As an ambulance speeds towards you, sirens blazing, the sound you hear is rather high in pitch. This is  because the sound waves in front of the vehicle are being squashed together by the moving ambulance. This causes more vibrations to reach your ear per second. As you know, more vibrations per second results in a higher pitched sound. When the ambulance passes you, the sound becomes lower in pitch. Behind the ambulance there are fewer vibrations per second, and a lower sound is heard. This change in pitch is known as the Doppler Effect.

                   When a vehicle travels faster than the speed of sound, about 330 meters per second, a sonic boom can be heard. As the vehicle overtakes its own sound, the sound waves spread out behind in a shockwave, or sonic boom.
 
 

    An obstacle is no match for a sound wave; the wave simply bends around it.  An example:  if a stereo is playing in a room with the door open, the sound produced by the stereo will bend around the walls surrounding the opening. This bending of a wave is called diffraction.

    All waves exhibit diffraction, not just sound waves. Without diffraction, the sound from the stereo could only be heard directly in front of the door. Instead, the air in the doorway is set into longitudinal vibration by the sound waves from the stereo. This means that each air molecule is a source of a sound wave itself. This results in each molecule producing a sound wave and emitting it outward in a spherical fashion. The final result is the diffraction of the sound wave around the doorway.