Basically a drone device used in Indian classical, folk & popular music. They come in two basic varieties - Acoustic and Electronic, both of which have their advantages. I use the electronic because it's hands free, fully tunable and has a volume contol. The acoustic ones are more versatile with shifting drones around whilst singing - we saw Maz O'Connor at the Fylde last year who did this to great effect. So - they're coming into Folk, slowly but surely, although they've been part of the world=ethnoc-free-jazz scene for decades. My first Shruti box was made out of formica and had four fully tunable tambura-pattern drones, but I left it on a train back in 1994. These days I'm never without my trusty little Swar Sudha, and it's loud enough to kick out some serious noise with the clarinet...