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Hand clapping, a ubiquitous human behavior, serves diverse daily-life purposes. Despite prior research, a comprehensive understanding of its physical mechanisms remains elusive. To bridge this gap, we integrate human data, parametric experiments, finite-element simulations, and theoretical frameworks to investigate the acoustic properties of clapping sound and their connections with the fluid flow and soft matter collision. Motion-audio synchronization reveals the flow-excitation nature of the hand cavity resonance. The classical Helmholtz resonator model, incorporating occasional pipe standing wave contributions for finger grooves, reliably predicts clapping sound frequencies across various real and engineered hand configurations. Material elasticity, coupled with the dynamic collision process, has minor effects on the sound frequency but a major impact on the temporal evolution of the sound signals, as reflected by the quality factors of resonance. Both spatial and dynamic factors for sound intensity are examined. We establish a quadratic scaling relationship between hand cavity gauge pressure and clapping speed, elucidating the positive correlation between faster claps and louder sounds. Our work advances the knowledge of hand-clapping acoustics and offers insights into sound signal synthesis, processing, and recognition. Furthermore, these findings may facilitate low-cost acoustical diagnostics in architecture and enhance rhythmic sound patterns in music and language education.