Sound wave is called longitudinal wave because it travels longitudinally in the direction to the propagation of the wave.It is a mechanical wave. To produce sound wave there needs a disturbance.
Like you see the waves in the river or in the sea which is due to the air flowing at its surface or due to the internal disturbance.So to say, when there is a cause there is an effect. Here Air or internal disturbance is the cause and the effect is the wave.Similarly when you strike, for example, strike a metal pie at its one end and you hear the sound at its other end very easily.The travelling mode of these waves are longitudinal.To make still more, take a piece of wood of which the mid part of it is vertically slit.Pass a long piece of thread through this slit.Hold the one end of the string and the other end also some body to hold it. Now get somebody to hold the slit- wood vertically.The string has to be reasonably taut .Now either of one of who is holding the string can move his hand up & down so that the string(Thread) starts moving up and down.Now you will see the vibration of the string with waves travelling through the slit longitudinally.Similar is the case with sound wave. I think the example makes you clear how does sound wave propagate.Thank You for your question.Note: Sound needs medium to travel.It cannot travel in vacuum.
Do the experiment as mentioned above, or similar experiment.
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