Listening Part 4: Highlight correct summary

PTE Listening Part 4- Sample 3: Highlight Correct Summary

Propagation of sound can be through any medium like water, air and even solids because sound travels both in the form of longitudinal waves as well as transverse waves. Every sound source always generates sound in the form of vibrations in the surrounding medium, and these vibrations create sound waves at the speed of sound. The velocity, pressure, and the displacement of medium vary at a fixed distance from the source at a time which in turn at an instant vary in space. An important point is that it is just the sound wave that travels, and not the particles of medium whether it is gas, liquid or even a solid material acting as a medium. However, reflection, refraction or even attenuation can take place through the mediums during the propagation of sound waves. But a medium (gas, liquid, solid, plasmas) is always required because sound cannot travel through vacuum.