Waves in the water
If you want to give the students an initial impression of sound waves, the propagation of waves in water is very useful.
Information and ideas:
The well-known photo may be a help for students to picture the propagation of sound waves in their minds. (Strictly speaking a water wave and a sound wave differ with respect to the direction of the vibration – transversal and longitudinal respectively!)
Relevant for teaching:
Vibrations and waves