The purpose of the sound box is to amplify the volume of the instrument, but it also gives the instrument its distinctive sound. A sound box is found in most string instruments. The most notable exceptions are some electrically amplified instruments like the solid body electric guitar or the electric violin, and the piano which uses only a sound board instead. Drumhead lutes such as the banjo or erhu have at least one open end of the sound box covered with animal skin (or a skin-like acrylic material). Open back banjos are normally used for clawhammer and frailing, while those used for bluegrass have the back covered with a resonator.
Loudspeakers also are mounted on a sound box to enhance their output, particularly bass speakers.