How to tune the Bundengan string

Bundengan is an endangered musical instrument from Indonesia. The bundengan has a set of strings, with small bamboo clips attached to each string. To tune the sound of these strings, musicians alter the position and the mass of the bamboo clips until a satisfactory sound is obtained. Previous investigations revealed that the vibration frequency spectra of the strings exhibit strange inter-mode patterns that evolve when the position and the mass of bamboo clips are varied. To better understand this phenomenon, we have investigated the role of the bamboo clips in the tuning of the strings. We have discovered that, when bamboo clips are attached to a string, the clips divide the string into different segments, where the vibration modes in each segment are controlled by the position of the clips, while the degree of the vibration mode coupling between neighbouring segments is controlled by the mass of the small bamboo clips: heavy and light clips lead to weak and strong mode coupling, respectively. Our results show how the bundengan musicians use the simple bamboo clips to perform complex tuning of the string, resulting in non-harmonic frequency spectra that imitate the sounds of metal percussion in the Gamelan ensemble.