The more air is made to move and vibrate, the stronger the volume of the sound becomes. This is why many instruments, such as the violin, have hollow bodies. The hollow body amplifies the sound that is produced with the instrument's strings.

Pianos also have hollow, wooden bodies. Without this body, the only sound would be produced with strings. The result would be a very quiet, thin sound. 

The hollow body makes a large amount of air vibrate inside it. This causes sound waves to travel to the listener's ears. 

Tubes and horns are also useful in amplifying sounds.