Question

Why are the stringed instruments like guitar provided with a hollow sound box ?

Answer 1

The sound box is constructed such that the column of the air inside it, has a natural
frequency which is the same as that of the strings stretched on it. Now when the strings
are made to vibrate, the air column inside the box is set into forced vibrations. Since the
sound box has a large area, it sets a large volume of air into vibration, the frequency of
which is same as that of the string producing resonance. So, due to this resonance produced a loud sound is heard.