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Sound waves are caused when objects vibrate, causing nearby air particles to vibrate as well. As a sound wave travels, each bit of air is vibrating parallel to the direction of wave travel, not travelling forwards. Sound is a longitudinal wave: the direction of vibration is parallel to the direction of energy transfer. Sound waves are faster in solids/liquids than gases; the particles are very close so vibrations are easily passed on. Sound waves are faster in solids than liquids; the strong forces between particles mean vibrations travel even faster.