Acoustical genesis of uniqueness of tanpura-drone signal—Probing with non-statistical fluctuation pattern

Tanpura-drone is one of the most commonly used string instrument used as accompaniment with vocal of other musical forms of Indian Classical Music genre. It is an exceptional and intriguing one owing to its capability of the generation of rich pattern of fine overtones. The acoustic analysis of tanpura signal had been initiated by C.V. Raman. Raman observed that the virtual overtones are heard while one is playing tanpura, which as per the conventional acoustic principles should never be generated. After that different models of its vibration pattern had been proposed to understand the theory based on the essential features of the tanpura’s string-bridge interaction, for empirical simulation of the tanpura signal and also for various synthesis-oriented analysis. But various challenges arise while defining accurate algorithms for real-time sound synthesis, due to the inherent non-statistical dynamics of tanpura signal. In this work we have converted tanpura signal into a two-dimensional space. Along one dimension of the space there is time-displacement pattern. Along the other dimension the signal changes its pattern along its progression to generate musicality. Then, we propose a novel and robust methodology in the line of the works done for analyzing the pattern of non-statistical fluctuation of fluctuations inherent in the two-dimensional space of the tanpura signal along its progression with time. We have done similar analysis for the signals of other two commonly used string instruments used in Indian Classical Music – sitar and sarod, and compared the results with that of tanpura signal. When compared, it was strikingly revealed that the pattern of fluctuation of fluctuations inherent in tanpura sound is altogether different from that of the rest of the two types of string instrument. Further, we confirm computationally that the nature of the non-statistical fluctuation of the tanpura signal is consistently different from that of other string instruments like sitar and sarod signal.