The basso continuo principle, as embodied in Rameau's theory of functional harmony, was paralleled by the introduction of drone instruments in the classical music of India. In order to understand how these two systems are tied together in human music perception, we studied the role of tambūrā interactions with North Indian rags played on the sitār. Raman (1914-1922) had applied his theory of discontinuous wave motion to mechanical and musical properties of the strings of the violin. He noted the remarkable, powerful harmonic series that arose from the nonlinear interaction of the tambürã string and grazing contact with its curved bridge. We analyzed the waveforms of the most common drone tunings. Each of the four strings was played with and without juari ("life-giving" threads). The upward transfer and spread of energy into higher partials imparts richness to tambūrā tones and underlies the use of different drone tunings for different rags. Specific notes of rāg scales are selectively and dynamically enhanced by different drone tunings. Based on coincident features of spectral and musical scale degrees, we computed an index of spectral complexity of the interactions of tambūrā tunings with rãg scales. We speculate that the use of juari contributes to stable pitch centers, implied scale modulation, and an improvisational flexibility.