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emit their ultrasonic pulses either through the mouth, like human speech, or through the nostrils. In species with nasal ultrasound emission, the nostrils are surrounded by elaborate emission baffles, so-called noseleaves. Like the pinnae, the noseleaves usually come with a fair amount of geometric complexity. In bats, these features have been shown to result in likewise complex distributions of the pinna’s sensitivity over direction and frequency (i.e., beam pattern). Attempts to replicate some of this complexity have been made either using highly simplified versions of the biological shapes (Figure 4; Müller and Hallam, 2004; Pannala et al. 2013) or by creating physical replicas of actual bat noseleaves or pinnae based on digitized specimens (Caspers and Müller, 2018).
These developments have not only led to systems that can be readily recognized as bat robots but have also demonstrated that shape detail in the noseleaves (Gupta et al., 2015) and pinnae (Müller et al., 2008) create useful acoustic properties. For example, a ridge on the inner pinna surface of a certain bat species was found to create a fan of frequency-dependent sidelobes of the biosonar
Figure 3. Gallery of noseleaves and pinnae in different bat species: A: Hildebrandt’s horseshoe bat (Rhinolophus hildebrandtii). B and C: Schneider’s roundleaf bat (Hipposideros speoris). D: Trident bat (Asellia tridens). All photos courtesy of M. Brock Fenton.
Figure 4. Early attempt at fitting a bat robot with a biomimetic noseleaf and pinnae. All shapes were highly simplified and placed around large transducers, which failed to yield effects because the diffracting surfaces did not receive much sound energy.
1994), which can be seen as a virtual bat robot, has shown that two such beams were sufficient for a sensorimotor system to track and capture a flying prey. In this case, one harmonic beam is used to determine the target direction in the horizontal plane and the other in the vertical plane.
Evolving Beautiful Acoustics from
Ugly Faces
When looking at portrait photos from virtually any bat species, it hard to miss that the facial features of the animals are reminiscent of gargoyles and other imaginary creatures from the Gothic period (Figure 3). To begin with, the outer ears (pinnae) of almost all bat species are, by human standards, very large compared with the size of the head. Besides their conspicuous size, bat pinnae often feature striking shape details such as combinations of different curvatures, washboard patterns, and flaps along the rim.
Additional Gothic facial features can be found in association with sound emission. Different bat species
Winter 2020 • Acoustics Today 33