A comparison of the role of beamwidth in biological and engineered sonar

Abstract

Sonar is an important sensory modality for engineers as well as in nature. In engineering, sonar is the dominating modality for underwater sensing. In biology, it is likely to have been a central factor behind the unprecedented evolutionary success of bats, a highly diverse group that accounts for over 20% of all mammal species. However, it remains unclear to what extent engineered and biosonar follow similar design and operational principles. In the current work, the key sonar design characteristic of beamwidth is examined in technical and biosonar. To this end, beamwidth data has been obtained for 23 engineered sonar systems and from numerical beampattern predictions for 151 emission and reception elements (noseleaves and pinnae) from bat biosonar. Beamwidth data from these sources is compared to the beamwidth of a planar ellipsoidal transducer as a reference. The results show that engineered and biological both obey the basic physical limit on beamwidth as a function of the ratio of aperture size and wavelength. However, beyond that, the beamwidth data revealed very different behaviors between the engineered and the biological sonar systems. Whereas the beamwidths of the technical sonar systems were very close to the planar transducer limit, the biological samples showed a very wide scatter away from this limit. This scatter was as large -if not wider -than what was seen in a small reference data set obtained with random aluminum cones. A possible interpretation of these differences in the variability could be that whereas sonar engineers try to minimize beamwidth subject to constraints on device size, the evolutionary optimization of bat biosonar beampatterns has been directed at other factors that have left beamwidth as a byproduct. Alternatively, the biosonar systems may require beamwidth values that are larger than the physical limit and differ between species and their sensory ecological niches.This work was supported by grants from the National Science Foundation (NSF Grant No. ID 1362886) and the Naval Engineering Education Consortium (NEEC, contract number N00174-16-C-0026). Comparison of the Role of Beamwidth in Biological and Engineered Sonar Bryan D. Todd (GENERAL AUDIENCE ABSTRACT)Sonar is an important method of sensing for engineers in undersea environments, but it is also used by several species of animals for for everyday use. The most prominent species that uses sonar, or echolocation, are bats, one of the most diverse groups of mammals. The study of bat biosonar systems serves as a counterpoint to many of the concepts in technical sonar. In technical sonar, arrays are made to be larger in size, with more elements, and operate at higher frequencies in order to decrease their beamwidth which increases their resolution. Unlike technical sonars bats must rely on smaller sized systems that they can carry around and they operate in air which has worse qualities for propagating sound waves. Even with these disadvantages, bats are able to operate in complex environments, such as dense vegetation, wit...

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“…This could be due to physical constraints on changes that can be made to the shapes of a noseleaf or pinna, how much these changes can influence the beampatterns, and how different beampatterns are possible. A recent study [34] has shown that bat biosonar beampatterns are more variable than a random reference (irregular cones made from crumpled aluminum foil) in terms of beamwidth which could be seen as an indication that factors other than beamwidth have driven the evolution of these characteristics.…”

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“…This suggests that beamwidth may not be as much of a limiting factor for bats navigating in narrow foliage as it may have appeared from the conventional approach to detecting gaps. Hence, the comparatively large beamwidths of bats that are due to the small sizes of the animals relative to the employed ultrasonic wavelengths [12], are not necessarily an obstacle to navigating through narrow gaps in vegetation. As a consequence, factors that favor narrower biosonar beams, especially larger pinnae or the use of higher frequencies, may not have offered selective advantages to these species-at least not in the context of finding passageways in vegetation.…”

“…in Euderma, to 212 kHz, e.g. Cloeotis, [11]), bat biosonar has to make do with emission and reception half-power (−6 dB) beamwidths that are typically range from 10 • to over 100 • [12]. Even for bat species that are known to navigate in dense vegetation, beamwidths values of 12 • -18 • , e.g.…”

“…Plecotus auritus [15] have been reported. Man-made underwater sonar systems, by comparison, have beamwidths well below 10 • [12]. This raises the question of whether bats that rely on biosonar echoes still have sufficient sensory information available to detect gaps in the foliage that are much narrower than their beamwidths.…”

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“…On the flying route, a total of 12 sampling points were taken. At each sampling point, the direction of the sonar beam was designed to target at the trees, and the beamwidth of the sonar was randomly sampled from the interval of PLOS ONE [30,65] degrees, a range that was found to be common for bat's biosonar [53]. Fig 7 demonstrates a bird view of the scene along with the -3 dB contours of the sonar beams (highlighted in red) and the resulting impulse responses.…”