Abstract
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.
Highlights
Castanets are percussive instruments comprising two shells that produce light and high-pitched sounds on being clapped together.1 The early evidence of the use of castanets in Spanish music can be found in the Cantigos de Santa Maria, which was written in the 13th century.2 They are often used in Spanish music, while performing regional folk dances, and occasionally in orchestral and popular music
By comparing the experimental results with theoretical models, it was found that the first mode could be explained on the basis of the Helmholtz resonance with a time-varying resonator shape due to the movement of the shells after impact, while the second mode could be considered as the standing wave mode caused by an interior volume of the shells
Because of the contactless and high-spatial-resolution imaging provided by parallel phase-shifting interferometry (PPSI) and the high-speed camera, the sound fields between the shells of the castanets were successively visualized
Summary
Castanets are percussive instruments comprising two shells that produce light and high-pitched sounds on being clapped together. The early evidence of the use of castanets in Spanish music can be found in the Cantigos de Santa Maria, which was written in the 13th century. They are often used in Spanish music, while performing regional folk dances, and occasionally in orchestral and popular music. Such sound-field measurement methods provide useful information regarding the spatial characteristics of various types of musical instruments, the spatial resolution of microphone arrays is limited because the distance between microphones cannot be reduced to less than the diameter of the microphone This essential limitation makes it difficult to spatially resolve the fine structures of sound fields; notably, spatial resolution is critical to investigating high-frequency components or small spaces, such as the gaps between the shells of castanets. The gas flow and sound waves emitted from a whistle were simultaneously visualized via parallel phase-shifting interferometry (PPSI).17–19 By applying these optical methods to measure the sound of a castanet, the sound field within the. By comparing the experimental results with theoretical models, it was found that the first mode could be explained on the basis of the Helmholtz resonance with a time-varying resonator shape due to the movement of the shells after impact, while the second mode could be considered as the standing wave mode caused by an interior volume of the shells
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