Periodic Reporting for period 1 - InWingSpeak (From Insect Wings to Miniature Loudspeakers - A Bionic Modelling Approach)
Reporting period: 2019-04-01 to 2021-03-31
Crickets and bush-crickets use their wings to produce a wide range of often impressively loud courtship songs to attract distant mating partners. These songs are different for each species and highly variable in “rhythm” (the temporal structure) and pitch. Many species also produce – like bats – ultrasonic songs far beyond the human hearing range. The wings of these small insects have evolved to be miniaturised and optimised resonators, analogous to the vibrating bodies of guitars or violins. Parts of the wings are specifically tuned to the animals’ song frequencies and thus enable the production and radiation of highly amplified acoustic signals. Each cricket is therefore equipped with its own miniaturised, natural loudspeakers and studying these wings has the potential to unearth unique solutions for the design of bioinspired, efficient and light-weight acoustic transducers.
The goal of this project was to use bioacoustic and state-of-the-art imaging techniques to investigate the morphology, the biomechanical properties and sound-production capabilities of the wings of various cricket and bush-cricket species in detail. By combining computer tomography for the creation of three-dimensional, virtual wings and laser vibrometry that allows to explore the resonances of the real wings, computer models of the wings have been developed. As a result, InWingSpeak produced several computational models of song-producing cricket and bush-cricket wings (see Figure 1). These allowed simulating the song production process and, crucially, the virtual manipulation of wing structures. By studying the effects of changes in wing shape, thickness and material properties, the relationship between wing morphology and emerging resonances and songs was analysed. On the one hand, the resulting knowledge has given insights into the evolution of acoustic communication and insect wings as sound-producing structures. On the other hand, the results also have the potential to inspire and inform engineers in the design of biomimetic miniature loudspeakers for medical devices like hearing aids.
The goal of this project was to use bioacoustic and state-of-the-art imaging techniques to investigate the morphology, the biomechanical properties and sound-production capabilities of the wings of various cricket and bush-cricket species in detail. By combining computer tomography for the creation of three-dimensional, virtual wings and laser vibrometry that allows to explore the resonances of the real wings, computer models of the wings have been developed. As a result, InWingSpeak produced several computational models of song-producing cricket and bush-cricket wings (see Figure 1). These allowed simulating the song production process and, crucially, the virtual manipulation of wing structures. By studying the effects of changes in wing shape, thickness and material properties, the relationship between wing morphology and emerging resonances and songs was analysed. On the one hand, the resulting knowledge has given insights into the evolution of acoustic communication and insect wings as sound-producing structures. On the other hand, the results also have the potential to inspire and inform engineers in the design of biomimetic miniature loudspeakers for medical devices like hearing aids.
In the course of the project, various species of crickets and bush-crickets were collected and their individual songs recorded. After analysing the songs, laser vibrometry was performed to record the vibratory behaviour of the wings producing the songs. Using this method, a laser beam scans the wing surface set into motion by stimulation with broadband sound. The light reflected from the moving wing carries information about the vibration and can be used to create detailed vibration maps that show which parts of the wing respond to which sound frequency. These vibration maps therefore show the specific resonance, the “tuning” of the wings (see Figure 1, middle row). Following this characterisation of the bioacoustics and biomechanical properties, the wings were imaged using high-resolution microscopy and micro-computational tomography to produce 2D and 3D wing models. These virtual wings served as the basis for the creation of initial computational (finite-element) models which allow to simulate the wings’ vibratory behaviour. Various approaches for implementing the wing morphology and modelling technique were investigated that ultimately resulted in models that were able to satisfactorily simulate the vibrations of real wings (see Figure 1, bottom row). These models include the 2D wing shape, the thickness and distribution of veins crossing the wing and the thickness of the wing membrane itself. Additionally, material properties like density and stiffness for the type of cuticle that forms the wing (a complex compound biomaterial mostly made of chitin) have to be specified.
In a further step, the structure and material properties of the virtual models were altered to study the impact of certain morphological changes on the wing vibrations. Alterations were inspired by evolutionary or developmentally conceivable changes like decreasing or increasing membrane and vein thicknesses and changes in wing or vein stiffness (the latter could be influenced by, for example, water content or an animal’s diet). In general, increases in wing stiffness resulted in expected increases in wing resonance (which would lead to higher-pitched song) but changing the stiffness of the veins also showed that the size and location of the vibrations is influenced by this parameter. Similarly, increasing the vein thickness had a pronounced impact on the vibratory behaviour (higher resonances and more confined vibrations). Interestingly, vein morphology seemed to have a greater impact on wing tuning than membrane morphology, suggesting that small changes in wing venation have the potential to produce significant changes in song frequency and amplitude. Further manipulation of vein morphology showed that an inflation of only one central vein (as seen in certain cricket species with unusually high pitch) can change the harmonic composition of the vibrations and thus song frequency and even its “timbre”. Detailed results are currently being prepared for publication in at least two international scientific journals.
In a further step, the structure and material properties of the virtual models were altered to study the impact of certain morphological changes on the wing vibrations. Alterations were inspired by evolutionary or developmentally conceivable changes like decreasing or increasing membrane and vein thicknesses and changes in wing or vein stiffness (the latter could be influenced by, for example, water content or an animal’s diet). In general, increases in wing stiffness resulted in expected increases in wing resonance (which would lead to higher-pitched song) but changing the stiffness of the veins also showed that the size and location of the vibrations is influenced by this parameter. Similarly, increasing the vein thickness had a pronounced impact on the vibratory behaviour (higher resonances and more confined vibrations). Interestingly, vein morphology seemed to have a greater impact on wing tuning than membrane morphology, suggesting that small changes in wing venation have the potential to produce significant changes in song frequency and amplitude. Further manipulation of vein morphology showed that an inflation of only one central vein (as seen in certain cricket species with unusually high pitch) can change the harmonic composition of the vibrations and thus song frequency and even its “timbre”. Detailed results are currently being prepared for publication in at least two international scientific journals.
The computational models that have been developed as a result of the InWingSpeak project have so far proven to allow a reasonable simulation of the vibratory behaviour of real-life insect wings. These models thus pave the way for further research into areas of evolution of song production that have been hard to access so far. Current and future work will, for example, also focus on investigating the vibrations of fossilised wings to gain a better understanding of the nature of the songs produced by the ancestors of our modern-day crickets millions of years ago.
Additionally, further investigations of how the morphology of these miniature loudspeakers influences the frequency and amplitude of the ultrasonic songs will provide insights into efficient high-frequency signal generation using small sound sources.
Additionally, further investigations of how the morphology of these miniature loudspeakers influences the frequency and amplitude of the ultrasonic songs will provide insights into efficient high-frequency signal generation using small sound sources.