The shruti box is a very straightforward drone instrument. It is often used to accompany Indian wind instruments, mainly the flute. There are bellows on one side of the instrument. 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. Although its origins are unclear, the shruti box probably originated from the harmonium.
