As Halloween approaches, it’s the perfect time to dive into the mysterious world of bat robots in this Acoustics Today article, “The Evolution of Bat Robots.” The ability of bats to navigate their environment using ultrasound has fascinated scientists for decades, and the mystery of how they process this information has drawn researchers from various fields. It’s no wonder that engineers have been lured into this world, attempting to replicate the biosonar capabilities of bats through a variety of “bat robots.”
Despite decades of research, the intricacies of bat biosonar remain mostly uncharted. Continuous advancements in recording and data analytics technologies promise to unlock more insights into the world of bat robots. These insights will likely drive further evolution in the field. Researchers are on the cusp of developing more integrated systems that combine encoding and extraction of sensory information. These mechanical marvels, inspired by the eerie elegance of bats, may hold the key to autonomous drones capable of navigating the dark forests, just like their natural counterparts.
The Acoustics Today articleweaves together a captivating story of technological evolution, highlighting the challenges, breakthroughs, and intriguing possibilities that lie ahead. If you’re curious about how bats’ extraordinary biosonar abilities are inspiring cutting-edge drones and robotic systems, read the full article for free at AcousticsToday.org. It’s a journey that promises to leave you in awe of both nature and human ingenuity. Happy Halloween!
Presented Tuesday afternoon, November 6, 2018 176th ASA Meeting, Victoria, British Columbia
Bats use echolocation, or biological sonar, to make an auditory picture of their environment when foraging and avoiding obstacles in flight (1). To echolocate, bats emit a loud, high-pitched sound using their mouth or nose. The sound bounces off an object and returns to the bat as an echo, providing each individual with information about the object characteristics and location. While echolocation allows for the detection and discrimination of targets, the high-pitched frequency sounds that bats emit when echolocating provide a limited range of information (2). Despite being known for flying at night, some bats spend only a part of their time flying in complete darkness, suggesting that they may also rely on vision to supplement their echolocation in environments that have more light (2, 3). Previous studies have demonstrated that vision in bats influences flight behavior, which suggests bats may combine vision and echolocation to sense their environment (2). It is, therefore, accepted that bats are not blind, as the common phrase suggests, but little is known about how vision influences the way bats use echolocation.
Figure 1. Swarm of Brazilian free-tailed bats flying during daylight hours after emergence. Photo Credit – Dr. Laura Kloepper, 2018
The Brazilian free-tailed bat migrates annually from Mexico to form large maternal colonies in caves in the Southwestern United States (2). These bats forage for insects in flight and emerge from the cave in groups of thousands for nightly foraging. The bats return to the cave in the early hours of the morning, requiring them to navigate back to their complex cave environment across a vast, open landscape. This reentry occurs across periods of complete darkness as well as early morning hours when ambient light is present. This suggests that bats have the option of using both echolocation and visual cues to navigate their environment in hours of daylight. Our research addresses how bats change their echolocation calls from an open environment to the more complex cave edge environment, and how the presence of daylight may influence their level of echolocation when accomplishing this feat.
Figure 2. Spectrogram image of a sequence of bat echolocation calls recorded at the cave environment.
Compared to the calls used over a vast landscape, bats at the cave edge used more complex calls that gathered more precise information about that environment. During hours of daylight, however, these calls collected less precise information than hours of darkness. As less information was gathered acoustically by bats during daylight hours, it is likely that bats are getting information from visual cues once daybreak occurs. This supplementing of vision for echolocation indicates that despite what the phrases say, bats are not blind.
Video 1. Bats emerging for foraging during early dusk.
Moss, C. F., & Surlykke, A. 2010. Probing the natural scene by echolocation in bats. Frontiers in Behavioral Neuroscience 4: 33.
Mistry, S. 1990. Characteristics of the visually guided escape response of the Mexican free-tailed bat Tadarida BrasiliensisAnimal Behavior 39: 314-320.
Davis, W.H., Barbour, R.W. 1965. The use of vision in flight by the bat Myotis sodalis. The American Midland Naturalist 74: 497–499.
Popular version of paper 1pAB4, “Beamwidth in bat biosonar and man-made sonar” Presented Monday, May 7, 2018, 1:30-3:50 PM, LAKESHORE B, 175th ASA Meeting, Minneapolis.
Bats and Navy engineers both use sonar systems. But do they worry about the same design features?
To find out, we have done an exhaustive review of both kinds of sonar systems, poring over the spec sheets of about two dozen engineered sonars for a variety of applications and using computer models to predict 151 functional characteristics of bat biosonar systems spanning eight different biological families. Crunching the numbers revealed profound differences between the way engineers approach sonar and the way bats do.
The most important finding from this analysis is related to a parameter called beamwidth. Beamwidth is a measure of the angle over which the emitted sonic power or receiver sensitivity is distributed. A small beamwidth implies a focused emission, where the sound energy is – ideally – concentrated with laser-like precision. But the ability to generate such a narrow beam is limited by the sonar system’s size: the larger the emitter is relative to the wavelength it uses, the finer the beam it can produce. Reviewing the design of man-made sonars indicates that beamwidth has clearly been the holy grail of sonar engineering — and in fact, the beamwidth of these systems hews closely to their theoretical minima.
Some of the random emission baffles made from crumpled aluminum foil that served as a reference for the scatter seen in the bat beam width data.
But when it comes to beamwidth, tiny bats are at a significant disadvantage: even the largest bat ears are barely ten times the size of the animals’ ultrasonic wavelength, while engineered systems can exceed their wavelengths by 100 or 1000 times. Remarkably, our analysis showed that bats seem to disregard beamwidth entirely. In our data set, the bats’ beamwidth scattered widely towards larger values; the scatter was even larger than that for random cone shapes we created from crumpled aluminum foil. Clearly, the bats’ sonar systems are not optimized for beamwidth. But we know that they are incredible capable when it comes to navigating complex environments — which begs the question: what criteria are influencing their design?
We don’t know yet. But the bats’ superior performance demonstrates every night that giant sonar arrays with narrow beamwidths aren’t the only and certainly not the most efficient path to success: smaller, leaner solutions exist. And those solutions will be necessary for compact modern systems like autonomous underwater or aerial vehicles. To make sonar-based autonomy in natural environments a reality, engineers should let go of their fixation on size and look at the bats.
Taito Banda – email@example.com Miwa Sumiya – firstname.lastname@example.org Yuya Yamamoto – email@example.com Yasufumi Yamada – firstname.lastname@example.org Faculty of Life and Medical Sciences, Doshisha UniversityKyotanabe, Kyoto, Japan
Yoshiki Nagatani – email@example.com Department of Electronics, Kobe City College of Technology, Kobe, Japan.
Hiroshi Araki – Araki.Hiroshi@ak.MitsubishiElectric.co.jp Advanced Technology R&D Center, Mitsubishi Electric Corporation, Amagaski, Japan
Kohta I. Kobayasi – firstname.lastname@example.org Shizuko Hiryu – email@example.com Faculty of Life and Medical Sciences, Doshisha University, Kyotanabe, Kyoto, Japan
Popular version of paper 4aAB4 “Analysis of bats’ gaze and flight control based on the estimation of their echolocated points with time-domain acoustic simulation.” Presented Friday morning, December 7, 2017, 8:45-9:00 AM, Salon F/G/H 174th ASA in New Orleans
Bats broadcast ultrasound and listen to the echoes to understand surrounding information. It is called echolocation. By analyzing those echoes, i.e., arrival time of echoes, bats can detect the position of objects, shape or texture [1-3]. Contrary to the way people use visual information, bats use the sound for sensing the world. How is the world different between the two by sensing? Because both senses are completely different, we cannot imagine how bats see the world.
To address this question, we simulated the echoes arriving at the bats during obstacle-avoiding flight based on the behavioral data so that we could investigate how the surrounding objects were described acoustically.
First, we arranged microphone arrays (24 microphones) and two high-speed cameras in an experimental flight chamber (Figure 1) . The timing, positions and directions of emitted ultrasound as well as the flight paths were measured. A small telemetry-microphone was attached on the back of the bat so that the intensity of emitted ultrasound could be recorded accurately . The bat was forced to follow a S-shaped flight pattern to avoid the obstacle acrylic boards.
Based on those behavioral data, we simulated propagation of sounds with the measured strength and direction emitted at the position of the bat in the experiment, and we could obtain echoes reaching both left and right ears from the obstacles. By using interaural time difference of echoes, we could acoustically identify the echolocated points in the space for all emissions (square plots in Fig.2). We also investigated how the bats show spatial and temporal changes in the echolocated points in the space as they became familiar with the space (top and bottom panels).
We analyzed changes in the echolocated points by using this acoustic simulation, corresponding to which part of objects the bats intended to gaze at. In a comparison between before and after the habituation in the same obstacle layout, there are differences in the wideness of echolocated points on the objects. By flying the same layout repeatedly, false detection of objects was reduced, and their echolocating fields became narrower.
It is natural for animals to pay their attention toward objects adequately and adapt both flight and sensing controls cooperatively as they became familiar with the space. These finding suggests that our approach in this paper, i.e., acoustic simulation based on behavioral experiment is one of effective ways to visualize how the object groups are acoustically structured and represented in the space for bats by echolocation during flight. We believe that it might serve a tip to the question; “What is it like to see as a bat?”
Figure 1 Diagram of bat flight experiment. Blue and red circles indicate microphones on the wall and the acrylic boards, respectively. Two high-speed video cameras are attached at the two corners of the room. Three acrylic boards are arranged to make bats follow S-shaped flight pattern to avoid the obstacles.
Figure 2Comparison of echolocated points between before and after space habituation. The measured positions where the bat emitted the sound are shown with circle plots meanwhile the calculated echolocated points are shown with square plots. Color variation from blue to red for both circle and square plots corresponds to temporal sequence of the flight. Sizes of circle and square plots correspond to the strength of emissions and their echoes from the obstacles at the bat, respectively.
References:  Griffim D. R., Listning in the dark, Yle University, New Haven, CT, 1958
 Simmons J.A., Echolocation in bats: signal processing of echoes for target range, Science, vol. 171, pp.925-928., 1971
 Kick S. A., Target-Detection by the Echolocating Bat, Eptesicus fuscus, J Comp Physiol, A., vol. 145, pp.431-435, 1982
 Matsuta N, Hiryu S, Fujioka E, Yamada Y, Riquimaroux H, Watanabe Y., Adaptive beam-width control of echolocation sounds by CF-FM bats, Rhinolophus ferrumequinum nippon, during prey-capture flight, J Exp Biol., vol. 206, pp.1210-1218, 2013
 Hiryu S, Shiori Y, Hosokawa T, Riquimaroux H, Watanabe Y., On-board telemetry of emitted sounds from free-flying bats: compensation for velocity and distance stabilizes echo frequency and amplitude, J Comp Physiol A., vol. 194, pp.841-851, 2008
Team at Virginia Tech hopes to create small, efficient sonar systems that use less power than current arrays
WASHINGTON, D.C., May 20, 2015 — Engineers at Virginia Tech have taken the first steps toward building a novel dynamic sonar system inspired by horseshoe bats that could be more efficient and take up less space than current man-made sonar arrays. They are presenting a prototype of their “dynamic biomimetic sonar” at the 169th Meeting of the Acoustical Society of America in Pittsburg, Penn.
Bats use biological sonar, called echolocation, to navigate and hunt, and horseshoe bats are especially skilled at using sounds to sense their environment. “Not all bats are equal when it comes to biosonar,” said Rolf Müller, a mechanical engineer at Virginia Tech. “Horseshoe bats hunt in very dense forests, and they are able to navigate and capture prey without bumping into anything. In general, they are able to cope with difficult sonar sensing environments much better than we currently can.”
To uncover the secrets behind the animal’s abilities, Müller and his team studied the ears and noses of bats in the laboratory. Using the same motion-capture technology used in Hollywood films, the team revealed that the bats rapidly deform their outer ear shapes to filter sounds according to frequency and direction and to suit different sensing tasks.
“They can switch between different ear configurations in only a tenth of a second – three times faster than a person can blink their eyes,” said Philip Caspers, a graduate student in Müller’s lab.
Unlike bat species that employ a less sophisticated sonar system, horseshoe bats emit ultrasound squeaks through their noses rather than their mouths. Using laser-Doppler measurements that detect velocity, the team showed that the noses of horseshoe bats also deform during echolocation–much like a megaphone whose walls are moving as the sound comes out.
The team has now applied the insights they’ve gathered about horseshoe bat echolocation to develop a robotic sonar system. The team’s sonar system incorporates two receiving channels and one emitting channel that are able to replicate some of the key motions in the bat’s ears and nose. For comparison, modern naval sonar arrays can have receivers that measure several meters across and many hundreds of separate receiving elements for detecting incoming signals.
By reducing the number of elements in their prototype, the team hopes to create small, efficient sonar systems that use less power and computing resources than current arrays. “Instead of getting one huge signal and letting a supercomputer churn away at it, we want to focus on getting the right signal,” Müller said.
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