Mittu Pannala

Characterization of the time-variant behavior of a biomimetic beamforming baffle

Horseshoe bats can actively change the shapes of their noseleaves and outer ears on time scales that are comparable to the duration of the biosonar pulses and echoes. When the shape deformations and the emission or reception of the ultrasonic signals overlap in time, the result is a time-variant diffraction process. Such a dynamic process provides additional flexibility that could potentially be used to enhance the encoding of sensory information. However, such a function remains hypothetical at present. To investigate the time-variant properties of deforming baffles such as the outer ears of horseshoe bats, the acoustic behavior of a biomimetic microphone baffle modeled on these biological structures has been investigated. The methods employed to characterize this device included representations in the time-delay domain as well as in the time-frequency domain. It was found that characterization methods which do not employ Fourier transforms revealed even more substantial time-variant effects than were apparent from time-frequency domain characterizations such as beampatterns obtained for different times in the deformation cycle. Furthermore, conspicuous correlates of asymmetries in the time-variant physical shapes were found in some characterizations that could be used to link dynamic baffle geometry with acoustic behavior.

Interplay of static and dynamic features in biomimetic smart ears

Horseshoe bats (family Rhinolophidae) have sophisticated biosonar systems with outer ears (pinnae) that are characterized by static local shape features as well as dynamic non-rigid changes to their overall shapes. Here, biomimetic prototypes fabricated from elastic rubber sheets have been used to study the impact of these static and dynamic features on the acoustic device characteristics. The basic shape of the prototypes was an obliquely truncated horn augmented with three static local shape features: vertical ridge, pinna-rim incision and frontal flap (antitragus). The prototype shape was deformed dynamically using a one-point actuation mechanism to produce a biomimetic bending of the prototypes tip. In isolation, the local shape features had little impact on the device beampattern. However, strong interactions were observed between these features and the overall deformation. The further the prototype tip was bent down, the stronger the beampatterns associated with combinations of multiple features differed from the upright configuration in the prominence of sidelobes. This behavior was qualitatively similar to numerical predictions for horseshoe bats. Hence, the interplay between static and dynamic features could be a bioinspired principle for affecting large changes through the dynamic manipulations of interactions that are sensitive to small geometrical changes.

Design of a dynamic sensor inspired by bat ears

In bats, the outer ear shapes act as beamforming baffles that create a spatial sensitivity pattern for the reception of the biosonar signals. Whereas technical receivers for wave-based signals usually have rigid geometries, the outer ears of some bat species, such as horseshoe bats, can undergo non-rigid deformations as a result of muscular actuation. It is hypothesized that these deformations provide the animals with a mechanism to adapt their spatial hearing sensitivity on short, sub-second time scales. This biological approach could be of interest to engineering as an inspiration for the design of beamforming devices that combine flexibility with parsimonious implementation. To explore this possibility, a biomimetic dynamic baffle was designed based on a simple shape overall geometry based on an average bat ear. This shape was augmented with three different biomimetic local shape features, a ridge on its exposed surface as well as a flap and an incision along its rim. Dynamic non-rigid deformations of the shape were accomplished through a simple actuation mechanism based on linear actuation inserted at a single point. Despite its simplicity, the prototype device was able to reproduce the dynamic functional characteristics that have been predicted for its biological paragon in a qualitative fashion.

Dynamic Substrate for the Physical Encoding of Sensory Information in Bat Biosonar

Horseshoe bats have dynamic biosonar systems with interfaces for ultrasonic emission (reception) that change shape while diffracting the outgoing (incoming) sound waves. An information-theoretic analysis based on numerical and physical prototypes shows that these shape changes add sensory information (mutual information between distant shape conformations <20%), increase the number of resolvable directions of sound incidence, and improve the accuracy of direction finding. These results demonstrate that horseshoe bats have a highly effective substrate for dynamic encoding of sensory information.

Bat biosonar as an inspiration for dynamic sensing

Sensory systems found in biology continue to outperform their man-made peers in many respects. In particular, their ability to extract salient information from complex, unstructured environments is often superior to engineering solutions. Bat biosonar is an example for an exceptionally powerful yet highly parsimonious sensing system that is capable of operating in a wide variety of natural habitats and achieve a likewise diverse set of sensing goals. One aspect in which bat biosonar appears to differ from man-made system is its heavy reliance on diffraction-based beamforming with intricate baffle shapes. In-vivo observations of these shapes have provided evidence for non-rigid baffle deformations that coincide with the diffraction of the outgoing or incoming waves. In horseshoe bats, a bat group with one of the most elaborate biosonar systems, the emission baffles have been found to twitch in synchrony with each pulse emission. On the reception side, the animals’ outer ears can likewise be deformed while the incoming pulses impinge on them. The acoustic effects of the outer ear motions can be characterized using frequency-domain characterizations (beampatterns) revealing significant acoustic effects. However, a time-domain characterization of a biomimetic prototype has shown even stronger effects. Hence, it may be hypothesized that these mobile baffle provide a substrate for a time-variant strategy for the encoding of sensory information - a hypothesis that is well suited for further exploration with bioinspired sensing technology.

Design of a Bio-Inspired Smart Ear Prototype

The outer ears (pinnae) of many bat species are smart structures that undergo non-rigid deformations controlled through an intricate muscular actuation system. It is hypothesized that such non-rigid changes in the physical shape of the pinnae provide a substrate for adaption of the spatial sensitivity (reception beampattern) of the animals’ biosonar system on a short time scale. In the research presented here, a simplified biomimetic baffle shape was developed to investigate the functional properties of non-rigidly deforming baffles. This prototype had the shape of an obliquely truncated cone that was augmented with local shape features that aided in achieving a biomimetic deformation pattern and may also have direct acoustic effects on the device beampattern. The prototype was manufactured from a thin sheet of rubber and actuated parsimoniously through a single linear actuator. Despite its comparative simplicity, the prototype device was able to reproduce the deformation pattern seen in the ears of horseshoe bats qualitatively. Biomimetic baffle deformations resulted in profound, qualitative, and quantitative changes to the beampattern. Future research will investigate how the time-variant beampatterns relate to the specifics of the deformation patterns and how they could be controlled and used in an engineering context.Copyright

Biosonar dynamics in horseshoe and old-world leaf-nosed bats

Horseshoe bats (family Rhinolophidae) have evolved a capable biosonar system to allow the pursuit of prey amid dense vegetation that produces large amounts of clutter echoes. Horseshoe bats have long been known to employ dynamic effects such as Doppler effect compensation and large-scale rotations of their pinnae to realize these capabilities. Recent research has produced evidence of even more pervasive dynamical biosonar properties in horseshoe bat biosonar as well as in the related Old World leaf-nosed bats (family Hipposideridae). On the emission side, these animals employ elaborate baffle shapes that surround the exit points of their ultrasonic pulses (nostrils). During echolocation, multiple parts of these noseleaves such as the anterior leaf and the lancet in horseshoe bats are set in motion, typically in synchrony with pulse emission, hence creating a time-variant channel for the exiting wave packets. Similarly, the pinnae which diffract the incoming echoes on the reception side are frequently in m...

Dynamic encoding of sensory information in biomimetic sonar baffle

The biosonar system of horseshoe bats stands through several dynamic features that could be related to an outstanding sensory performance. The outer ears (pinnae) of the animals, for example, can change their shapes in a non-rigid fashion and thereby produce qualitative beampattern alterations. Such changes can be reproduced qualitatively with a highly simplified biomimetic baffle prototype. Such a biomimetic prototype is a crucial platform for measuring large amounts of data on the time-variant device behavior that would be difficult to obtain from an animal in vivo. Here, such time-variant data were used to investigate the question whether a dynamic deforming baffle can enhance the encoding of sensory information. This was done based on estimates of the mutual information between beampatterns belonging to different deformation stages of the biomimetic baffle. To make this estimation problem tractable, the transfer functions associated with each direction of sound incidence were sorted into “alphabets” o...

Effects of local shape features on the beampatterns of a dynamic biomimetic baffle

Horseshoe bats have mobile pinnae that can change their shapes as a result of active actuation. A common pattern is an alteration between an upright and a bent-tip geometrical configuration. Numerical predictions of reception beampatterns associated with these different shape geometries have indicated that the upright configurations are associated with beampatterns dominated by a single mainlobe whereas the bent-tip configurations have prominent sidelobes. Using a biomimetic baffle prototype, we have found that this effect can be reproduced qualitatively with just a plain obliquely truncated cone that is fabricated from flexible material (rubber) and bent at the tip. However, local shape features can have a strong impact on the quantitative expression of this effect. The three features studied here were a vertical ridge, an equivalent to the bats antitragus, and a lateral incision into the baffle rim. The effects of these features on the beampatterns where found to interact with each other and also depen...

Characterization of dynamic baffles in biosonar and biomimetic devices

The pinna shapes of horseshoe bats are actively actuated to undergo non-rigid deformations on a time scale of one tenth of a second. These cycle times are similar to the durations of the echoes that the animals receive. Hence, it is possible that the bat pinnae are undergoing a change in shape while an individual echo is impinging on them. Such a dynamic reception paradigm based on a continuously deforming baffle are an interesting concept for explaining the biosonar as well as the design of bioinspired devices. Beampatterns are the standard characterization of time-invariant reception devices. In a beampattern, device gain is given as a function of direction and frequency. The Fourier transform that underlies this representation does not represent time variant behavior. Hence, new ways to characterize the time-variant behavior of a deforming baffle have to be found. This is further complicated because a complete description of system behavior requires four independent variables: azimuth, elevation, and e...