Sound waves generated by a moving piston

The displacement of the piston is ε sin ωt.

Notes

The fluid particles immediately adjacent to the piston remain there for all time.

As the piston moves into the fluid, the fluid is compressed and the local pressure and density increase.

As the piston moves moves back the local pressure and density decrease.

Sound waves are longitudinal waves in which the fluid particles move in the direction of wave propagation.