Humming hummingbirds, insect flight tones, and a model of animal flight sound

Abstract

Why do hummingbirds hum and insects whine when their wings flap in flight? Gutin proposed that a spinning propeller produces tonal sound because the location of the center of aerodynamic pressure on each blade oscillates relative to an external receiver. Animal wings also move, and in addition, aerodynamic force produced by animal wings fluctuates in magnitude and direction over the course of the wingbeat. Here we model animal wing tone as the equal, opposite reaction to aerodynamic forces on the wing, using Lowson's equation for the sound field produced by a moving point force. Two assumptions of Lowson's equation are met: animal flight is low (< 0.3) Mach and animals from albatrosses to mosquitoes are acoustically compact, meaning they have a small spatial extent relative to the wavelength of their wingbeat frequency. This model predicts the acoustic waveform of a hovering Costa's Hummingbird (Calypte costae), which varies in the X, Y, and Z directions around the animal. We modeled the wing forces of a hovering animal as a sinusoid with an amplitude equal to body weight. This model predicts wing sound pressure levels below a hovering hummingbird and mosquito to within 2 dB; and that far-field mosquito wing tone attenuates to 20 dB within about 0.2 m of the animal, while hummingbird humming attenuates to 20 dB at about 10 m. Wing tones play a role in communication of certain insects, such as mosquitoes, and influences predator-prey interactions, because it potentially reveals the predator's presence to its intended prey.

Auditory sensory range of male mosquitoes for the detection of female flight sound

“…Counterintuitively, considering their smaller body size, the particle velocity iso-surface for males reaches further than that of females (425.2 ± 77.9 mm, n = 15, p = 0.035; figure 2 ). Since the intensity of the Gutin sound is proportional to the body mass and wingbeat frequency in the far field [ 11 ], the higher frequency of males outweighs the effect of their smaller size and, therefore, the male sound reaches slightly further. However, because the male JO is sensitive to the male–female difference tone [ 4 ], this male sound would not be detected by other males at this distance.…”

“…Acoustic communication requires at least one sound source, and complementary sensors. For mosquitoes, sound is generated by the flight apparatus [ 11 ]. Flapping wings generate periodic three-dimensional aerodynamic forces, and the fluctuations in the magnitude and direction of aerodynamic forces result in a dipole-like Gutin sound [ 12 ].…”

“…Flapping wings generate periodic three-dimensional aerodynamic forces, and the fluctuations in the magnitude and direction of aerodynamic forces result in a dipole-like Gutin sound [ 12 ]. Among the possible range of sound-generating mechanisms, the tone produced by the flapping of mosquito wings is primarily produced by the mechanism associated with Gutin sound, just as a rotor blade produces a tone at the blade-passing frequency [ 11 – 13 ]. The wingbeat frequencies of male mosquitoes are higher than those of females; therefore, male flight-tones are higher in frequency than female flight-tones [ 4 , 9 , 14 ].…”

Flight Sound of an Insect Wing: Application of Metamaterial Concepts to Bioinspired Flapping Wings

“…Besides, it appeared that wingbeat frequency is not linked to the structural resonant frequency of the wing 14 . Yet, the existing studies pay little, if not at all, attention to the vibro-acoustics of a wing, i.e., to the sound generation process during flight 23 . This aspect is relevant in light of applications of bio-inspired concepts to the design of artificial wings for micro-robotic devices 24,25 .…”