2.  SOUND WAVE

"Sound is a wave characterised by over-pressure ... at one point and a lack of particles at another point. It is a longitudinal vibration of the air that is propagated step by step" (The violin homepage).

from Team 27178 homepage

• The period = the distance between two crests of a wave thus determining the pitch of the sound perceived.  
• The frequency = the number of oscillations per second in herts [Hz].  
• The amplitude = the power of the sound

       People can hear because sound wave exist.  If the frequency become a higher, the sound become a sharper.  But, we cannot hear every sound even though they do have sound wave because the amplitude that people actually can hear is limited.  

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