Why are musical instruments like guitars provided with a hollow box?

Hint:We will use the principles of vibrations and the factors that influence the intensity of sound produced and the forced vibrations to answer this question.

Complete Step by Step Answer:An object's tendency to set a connected body to its surrounding body into vibration is termed as forced vibration. Sometimes, the vibrating body transmits its vibrations to another body, and that body starts vibrating because of forced vibrations.
In the case of stringed instruments like guitar, the single disturbance of a string does not produce enough loudness sufficient for a human ear. So, we must increase the loudness and sound intensity for a human to ear. Hence, they are provided with a hollow box that has a lot of air. So, whenever any string is subjected to disturbance, it starts vibrating, and that vibration propagates the adjacent hollow box in which the air undergoes forced vibrations, thereby increasing the loudness and sound intensity. Moreover, the surface area of the string is maintained less than the surface area of the box so that more surrounding air particles are set into forced vibration, thereby increasing the amplitude of the sound.

Note: Make sure to learn other similar factors which are used to increase sound intensity in musical instruments.