See the sound of castanets
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K. Ishikawa et al.``Seeing the sound of castanets: Acoustic resonances between shells captured by high-speed optical visualization with 1-mm resolution,'' J. Acoust. Soc. Am., vol.148, no.5, pp.3171－3180, 2020. [Paper]
Sound fields radiated from the castanet, a Spanish percussive instrument comprising two shells, were optically visualized. A measurement system, which used parallel phase-shifting interferometry and a high-speed polarization camera, enabled the capture of instantaneous sound fields around the castanets, while the castanets were played, with the spatial resolution of 1.1 mm and frame rate of 100 000 fps. By carefully aligning the tilt of the castanets, the sound fields within the 1-mm gaps between both the shells were captured. From the visualization results, two acoustic resonances between the shells were identified. The first mode appeared between 1000 and 2000 Hz and exhibited a frequency chirp of several hundred hertz for several milliseconds after the impact. This can be explained by the Helmholtz resonance with a time-varying resonator shape, which is caused by the movement of the shells after impact. The second mode showed a resonance pattern with a single nodal diameter at the center of the shells, i.e., the standing wave mode caused by the interior volume. These physical phenomena involved in the sound radiation were identified owing to the unique features of the optical imaging method, such as contactless nature and millimeter-resolution imaging of instantaneous pressure fields.
Castanets – a Spanish percussion instrument
Castanets are percussive instruments comprising two shells that produce light and high-pitched sounds on being clapped together. The shells are often made from hardwood or glass fiber, and their shapes are circular or slightly oval. They are tied using a rope through holes in their bridge, as depicted in (a) and (b). The scalogram (time-frequency visualization) in (c) revealed that there are two modes: the first mode between 1 to 2 kHz and the second mode at 5 kHz. We visualized the sound field around and inside the castanets' shells, as describled in (d).
Imaging results
Visualization of the measured sound field with 10 µs intervals. The top shows the initial pulse wavefront generated at the moment of the contact of the two shells. The bottom shows the more complicated sound fields after 2 ms from the contact, where sound waves between the shells are captured.
Acoustic modes extraction inside castanets
