A shruti box or surpeti is an instrument from the Indian subcontinent, with similitude to the harmonium. It is used as an accompaniment in a practice session or concert of Indian classical music, for singers and notably the flute.
The shruti box is used to help the instrumentalist or singer to maintain the tuning.
Before the arrival of the harmonium in the Indian subcontinent, musicians used either a tanpura or a specific pitch reference instrument. At the arrival of the harmonium in India, musicians modified it and made a keyless version of the harmonium, specifically to produce the drone sound. It was given the name shruti box or sruti box.
The shruti box is enjoying a renaissance in the West amongst traditional and contemporary musicians, who are using it for a range of different styles. Singers find it very useful as an accompaniment and instrumentalists enjoy the drone reference it gives to play along with.