Baffle shapes are commonly used in engineered devices to interface sound sources with the free field. Examples are acoustic horns seen in megaphones and horn-loaded loudspeakers. Typical for these devices are simple, static shapes that serve primarily an impedance-matching function. Diffracting baffles linked to a sound source are also common in the biosonar system of bats. In particular in bat groups that emit their ultrasonic pulses nasally, the nostrils are always surrounded by some baffle shape. This is the case across several large and diverse bat families such as horseshoe bats (Rhinolophidae), Old World leaf-nosed bats (Hipposideridae), and New World leaf-nosed bats (Phyllostomidae). However, biosonar baffles differ from their technical counterparts in two important ways: They typically have a much greater geometrical complexity and they are capable of non-rigid shape changes over time. Although simple horn shapes can be found in the noseleaves of many bat species, they are rarely as plain and regular as in megaphones and other technical applications of acoustical horns. Instead, the baffles are broken up into several parts that are frequently augmented with intricate local shape features such as ridges, furrows, and spikes. Furthermore, we have observed that in species belonging to the horseshoe bats and the related Old World leaf-nosed bats these local shape features are often not static, but can undergo displacements as well as non-rigid deformations. At least some of these dynamic effects are not passive byproducts of e.g., sound production or exhalation, but due to specific muscular actuation that can be controlled by the animals. To study these intricate, dynamic baffles as inspirations for smart structures, we have recreated the degrees of freedoms that Old World leaf-nosed bats have in deforming their noseleaves in a digital model using computer animation techniques. In its current form, our model has 6 degrees of freedom that can be used to test interactions between different motions using actuation patterns that occur in life as well as patterns that have not been observed, but could aid understanding. Because of the high-dimensional parameter space spanned by the different degrees of freedom, a high-performance computing platform has been used to characterize the acoustic behavior across a larger number of deformed no seleaf shapes. A physical test bed is currently under construction for implementing baffle motions that have been found to result in interesting changes of the acoustic device characteristics and could hence be of use to engineering applications.
Comparative echolocation and foraging ecology of horseshoe bats (Rhinolophidae) and Old World leaf-nosed bats (Hipposideridae)1