Originally, a shruti is the unit of pitch in Indian music - the octave is divided into 22 (non-equal) intervals, "shrutis". A raga uses a selection of seven of these pitches.

A "shruti box" is a substitute for a tanpura as the drone in an Indian classical music group - it plays a couple of notes with very long sustain and no discernible rhythm. They are usually electronic, but there is a reed one like a cut-down Indian harmonium (an English concertina would do more or less the same job).