A shruti box is a small wooden instrument that traditionally works on a system of bellows. It is similar to a harmonium and is used to provide a drone in a practice session or concert of Indian classical music. It is used as an accompaniment to other instruments and notably the flute. Use of the shruti box has widened with the cross-cultural influences of world music and new age music to provide a drone for many other instruments as well as vocalists.
By pumping this with one of your hands, air flows into the reeds housed on the opposite side. Each hole where the brass reed is placed has a movable plug. By rotating these one can open and close the air flow. Hence, opening the flow will make the reed sing and closing it will mute the sound. The shruti box probably originated from the harmonium.