The Measurement of Sound

Sound is an aural sensation caused by pressure variations in the air.  The variation is always produced by some source of vibration for example a solid object or from turbulence in a liquid or gas.  These pressure fluctuations may take place very slowly, such as those caused by atmospheric changes, or very rapidly like the crack of lightening.  The velocity of sound is independent primarily on temperature.  

Frequency is the number of vibrations of pressure fluctuations per second.  The unit is the hertz (Hz).

Wavelength.  This is the distance travelled by the sound during the period of one complete cycle.  Low frequencies have long wavelengths and high frequencies have short wavelengths.

Velocity of Sound in air under normal conditions is around 340m per second.  

Waves in air are longitudinal, where the vibrations are in the direction of the motion.  The pressure fluctuations are like a spring that returns to its normal state once the excitation has ceased.  Air particles don’t speed thru the air like a bullet but rather vibrate causing changes in pressure.  Once the vibration has stopped, so does the movement of the air.