Shruthi box is a hand-operated reed instrument used to create a drone. Having a drone is essential in Carnatic music as it establishes the key in which the music is played. The drone subtly plays in the background over which the performer renders their music.
The working of the shruthi box, similar to that of the harmonium, involves inducing vibrations in the reeds with the air pushed through the hand-operated bellows. The tone is similar to that of a single reed harmonium. By changing the position of the valves, one can set the key and its tonic, mediant and the dominant. In a typical shruthi box, three keys can be set.
Although the prominence of manual shruthi box has gone down in recent years due to the prevalence of its electronic counterpart, some still prefer the timbre of the original tone.