The pronounced noise of flapping-wing micro air vehicles (FWMAVs) critically limits their operational stealth. Although previous studies have investigated fundamental aeroacoustic principles of flapping wings, the oversimplified kinematic models and deformation characteristics in existing research has failed to explain the complex noise generation mechanisms in operational FWMAVs. Critical knowledge gaps persist regarding the spatial-temporal noise signatures and the acoustic impact of key design parameters, hindering noise reduction strategies. This experimental study explores the spatial and frequency acoustic signatures of a biologically inspired hummingbird-style FWMAV in its hovering state. The results indicated that both the FWMAV and its wing exhibit distinct dipole acoustic patterns, with flapping-wing, rather than transmission, mechanisms dominating the primary noise sources. Sound directivity analysis reveals maximum sound pressure levels at lateral azimuths in the horizontal plane, while spectral decomposition identifies that aerodynamic loading, transmission resonance, and membrane flutter/vortex shedding dominate the low-, mid-, and high-frequency regimes, respectively. Synchronized measurements between wing motion and FWMAV's noise identify wing pitching motion as the primary noise generation phase. Finally, parametric studies demonstrated that large-span, low-frequency wing design, and proper membrane material selection are helpful for noise reduction at equivalent lift outputs. These findings provide theoretical foundations and engineering guidelines for FWMAV noise control.

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