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Abstract
How hummingbirds hum is not fully understood, but its biophysical origin is encoded in the acoustic nearfield. Hence, we studied six freely hovering Anna's hummingbirds, performing acoustic nearfield holography using a 2176 microphone array in vivo, while also directly measuring the 3D aerodynamic forces using a new aerodynamic force platform. We corroborate the acoustic measurements by developing an idealized acoustic model that integrates the aerodynamic forces with wing kinematics, which shows how the timbre of the hummingbird's hum arises from the oscillating lift and drag forces on each wing. Comparing birds and insects, we find that the characteristic humming timbre and radiated power of their flapping wings originates from the higher harmonics in the aerodynamic forces that support their bodyweight. Our model analysis across insects and birds shows that allometric deviation makes larger birds quieter and elongated flies louder, while also clarifying complex bioacoustic behavior.
Highlights
Birds, bats, and insects flap their wings to generate unsteady aerodynamic forces that lift their body into the air, which enables them to fly
To resolve how the oscillating aerodynamic force generated by flapping wings may contribute to wing hum, we developed a new aerodynamic force platform (Ingersoll and Lentink, 2018; Lentink et al, 2015; Hightower et al, 2017) to directly measure the net 3D aerodynamic force generated by freely hovering hummingbirds
The aerodynamic force platform integrates both the steady and unsteady components of the pressure field around the bird up to three times the wingbeat frequency, which are associated with its net 3D aerodynamic forces
Summary
Bats, and insects flap their wings to generate unsteady aerodynamic forces that lift their body into the air, which enables them to fly. On the other hand, suppress the aerodynamic sound generated by their wings to mitigate interference with their hearing and escape prey detection (Geyer et al, 2013; Jaworski and Peake, 2020; Kroeger et al, 1972; Sarradj et al, 2011; Clark et al, 2020) Their flapping wings generate less structural noise (Clark et al, 2020) because their feathers lack the noisy directional fastening mechanism that locks adjacent flight feathers during wing extension in other bird species (Matloff et al, 2020).
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