Longitudinal wave is a wave in which particles of a medium vibrate parallel to the direction of propogation of wave. it has large pressurised area called compressions and lower pressurised are called rarefactions

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     The sound travels due to the disturbance created in air or any other medium due to vibrations of particles.  When speak the movements in our mouth create changes in pressure in air.  These pressure changes propagate in air , in the same direction as the wave is traveling.

   The particles in air or the medium vibrate (oscillate about a mean position) in the same direction as the overall wave propagation.  So it is called Longitudinal wave. 

   If the particles vibrated perpendicular to the velocity, then it is called a transverse wave.

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