The other person will repeat what is heard. String carries sound better than air. Put your ear against the wall and have one student tap the wall from some distance away with eraser end of a pencil. As the tapping continues, lift your ear away from wall so that sound reaches you through the air. Which is louder - through the air or the wall.
Sounds also differ in pitch.
Pitch is the highness or lowness of a sound. An object vibrating very fast makes high-pitched sounds. Lower pitched sounds are made by objects vibrating more slowly. The number of times per second an object vibrates is its frequency. An object vibrating many times a second has high frequency.