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. An object vibrating only a few times a second has a low frequency.