A sound wave is a mechanical longitudinal wave: the particles of the medium vibrate back and forth in the same direction that the wave travels, creating compressions and rarefactions in the medium.

What that means

  • Mechanical : Sound needs a material medium (like air, water, or solids) to travel and cannot propagate through a vacuum.
  • Longitudinal : The oscillations of particles are parallel to the direction of propagation, forming alternating regions of high pressure (compressions) and low pressure (rarefactions).

In many textbooks and exam-style questions, when asked “what kind of wave is a sound wave?”, the expected short answer is “a longitudinal mechanical wave.”