Quantitative hygromorphic analysis of the Bundengan musical instrument

While most musicians prefer to keep their instruments dry, the Bundengan musicians at Wonosobo in Indonesia have long discovered that their instrument sounds better when it is played under the rain. The key to this phenomenon is the bamboo culm sheaths, a hygromorphic material used to layer the resonator of the Bundengan: when the sheaths become wet, they change their shapes from planar to tubular, such that the mechanical coupling between neighbouring sheaths become stronger, leading to a pleasing sound from the vibrating resonator. Conversely, dry sheaths have a weaker mechanical coupling, resulting in an unpleasant sound. To better understand this phenomenon, we perform a quantitative analysis of a drying bamboo culm sheath. First, a time-lapse series of the sheath images was recorded to capture the hygromorphing dynamics. Next, pairs of subsequent images are quantitatively compared to detect and analyze the motion of the drying sheath. Our data shows that the middle part of the sheath is generally stationary, while the sides of the sheaths move almost continuously. Our results indicate that the bamboo culm sheaths may be very useful, not only to construct the Bundengan, but also to develop other water-enhanced musical instruments.