A numerical study of the role of the tragus in the big brown bat

The Maersk Institute, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark.
The Journal of the Acoustical Society of America (Impact Factor: 1.5). 01/2005; 116(6):3701-12. DOI: 10.1121/1.1815133
Source: PubMed


A comprehensive characterization of the spatial sensitivity of an outer ear from a big brown bat (Eptesicus fuscus) has been obtained using numerical methods and visualization techniques. Pinna shape information was acquired through x-ray microtomography. It was used to set up a finite-element model of diffraction from which directivities were predicted by virtue of forward wave-field projections based on a Kirchhoff integral formulation. Digital shape manipulation was used to study the role of the tragus in detailed numerical experiments. The relative position between tragus and pinna aperture was found to control the strength of an extensive asymmetric sidelobe which points in a frequency-dependent direction. An upright tragus position resulted in the strongest sidelobe sensitivity. Using a bootstrap validation paradigm, the results were found to be robust against small perturbations of the finite-element mesh boundaries. Furthermore, it was established that a major aspect of the tragus effect (position dependence) can be studied in a simple shape model, an obliquely truncated horn augmented by a flap representing the tragus. In the simulated wave field around the outer-ear structure, strong correlates of the tragus rotation were identified, which provide a direct link to the underlying physical mechanism.

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    • "Research on a mathematical characterization of the variety of bat ears has also been conducted [16]: bat ears are generated by a linear combination of 'eigenears' recovered from averaging of a large number of ears from different bat species . Measurements to understand the role of the tragus have been performed [17], as well as flexible receivers developed to analyse the effect of ear deformations on the associated beam pattern [18]. The right external ear of the bat Rousettus leschenaultii is considered in this paper, and the features that make this bat ear so directional are investigated over a particular range of frequencies. "
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    ABSTRACT: The directional properties of bat ears as receivers is a current area of interest in ultrasound research. This paper presents a new approach to analyse the relationship between morphological features and acoustical properties of the external ear of bat species. The beam pattern of Rousettus leschenaultiiʼs right ear is measured and compared to that of receiver structures whose design is inspired by the bat ear itself and made of appropriate geometric shapes. The regular shape of these receivers makes it possible to control the key reception parameters and thus to understand the effect on the associated beam pattern of the parameters themselves. Measurements show one receiver structure has a beam pattern very similar to that of R. leschenaultiiʼs ear, thus explaining the function of individual parts constituting its ear. As it is applicable to all bat species, this approach can provide a useful tool to investigate acoustics in bats, and possibly other mammals.
    Full-text · Article · Sep 2015 · Bioinspiration & Biomimetics
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    • "In accordance with this prediction, previous work has produced evidence for the importance of various local pinna features in other bat species. One such feature is the tragus, a protrusion of the anterior pinna rim that was found to impact the animals sound localization ability [16], presumably through the generation of sidelobes in the beampattern [17] [18] [19]. Similar "
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    ABSTRACT: 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 prototype's 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.
    Full-text · Article · May 2013 · Bioinspiration & Biomimetics
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    • "This model has the advantage that it allows an approximate analytic treatment, and our results show that it can indeed be linked to actual bat pinna shapes in a quantitative manner. The current findings also provide quantitative guidance how this model may be expanded with a bioinspired approach (Müller and Hallam 2004) to produce a wider set of beamforming properties that could include novel functional principles. "
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    ABSTRACT: A quantitative analysis of the interspecific variability between beamforming baffle shapes in the biosonar system of bats is presented. The data set analyzed consisted of 100 outer ear (pinna) shapes from at least 59 species. A vector-space representation suitable for principal component analysis (PCA) was constructed by virtue of a transform of the pinna surfaces into cylindrical coordinates. The central axis of the cylindrical transform was found by minimizing a potential function. The shapes were aligned by means of their respective axes and their center of gravity. The average pinna of the sample was a symmetrical, obliquely truncated horn. The first seven eigenvalues accounted already for two-thirds of the variability around the mean, which indicates that most of the biodiversity in the bat pinna can be understood in a more low-dimensional space. The first three principal components show that most of the variability of the bat pinna sample is in terms of opening angle, left-right asymmetry, and selective changes in width at the top or the bottom of the pinna. The beampattern effects of these individual components have been characterized. These insights could be used to design bioinspired beamforming devices from the diversity in biosonar.
    Full-text · Article · Jun 2011 · Bioinspiration & Biomimetics
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