Question

how will you know that the sound requires a medium of propagation

Answer 1

see the bell jar experiment on you tube it will help you

Answer 2

Take a bell jar with a hole under it through which vacuum pump is passed. Place an electric bell inside it. Close the jar with a wooden cork. Ring the bell, we can hear the sound. Now start sucking the air from the jar through the vacuum pump. The sound starts becoming fainter and fainter. Finally, the sound stops. This shows that sound needs a medium to travel