1pAB4 – Size Matters To Engineers, But Not To Bats

Rolf Müller – rolf.mueller@vt.edu
Bryan D. Todd

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.

1aSC31 – Shape changing artificial ear inspired by bats enriches speech signals

Anupam K Gupta1,2, Jin-Ping Han ,2, Philip Caspers1, Xiaodong Cui2, Rolf Müller1

  1. Dept. of Mechanical Engineering, Virginia Tech, Blacksburg, VA, USA
  2. IBM T. J. Watson Research Center, Yorktown, NY, USA

Contact: Jin-Ping Han – hanjp@us.ibm.com

Popular version of paper 1aSC31, “Horseshoe bat inspired reception dynamics embed dynamic features into speech signals.”
Presented Monday morning, Novemeber 28, 2016
172nd ASA Meeting, Honolulu

Have you ever had difficulty understanding what someone was saying to you while walking down a busy big city street, or in a crowded restaurant? Even if that person was right next to you? Words can become difficult to make out when they get jumbled with the ambient noise – cars honking, other voices – making it hard for our ears to pick up what we want to hear. But this is not so for bats. Their ears can move and change shape to precisely pick out specific sounds in their environment.

This biosonar capability inspired our artificial ear research and improving the accuracy of automatic speech recognition (ASR) systems and speaker localization. We asked if could we enrich a speech signal with direction-dependent, dynamic features by using bat-inspired reception dynamics?

Horseshoe bats, for example, are found throughout Africa, Europe and Asia, and so-named for the shape of their noses, can change the shape of their outer ears to help extract additional information about the environment from incoming ultrasonic echoes. Their sophisticated biosonar systems emit ultrasonic pulses and listen to the incoming echoes that reflect back after hitting surrounding objects by changing their ear shape (something other mammals cannot do). This allows them to learn about the environment, helping them navigate and hunt in their home of dense forests.

While probing the environment, horseshoe bats change their ear shape to modulate the incoming echoes, increasing the information content embedded in the echoes. We believe that this shape change is one of the reasons bats’ sonar exhibit such high performance compared to technical sonar systems of similar size.

To test this, we first built a robotic bat head that mimics the ear shape changes we observed in horseshoe bats.

han1 - bats

Figure 1: Horseshoe bat inspired robotic set-up used to record speech signal

We then recorded speech signals to explore if using shape change, inspired by the bats, could embed direction-dependent dynamic features into speech signals. The potential applications of this could range from improving hearing aid accuracy to helping a machine more-accurately hear – and learn from – sounds in real-world environments.

We compiled a digital dataset of 11 US English speakers from open source speech collections provided by Carnegie Mellon University. The human acoustic utterances were shifted to the ultrasonic domain so our robot could understand and play back the sounds into microphones, while the biomimetic bat head actively moved its ears. The signals at the base of the ears were then translated back to the speech domain to extract the original signal.
This pilot study, performed at IBM Research in collaboration with Virginia Tech, showed that the ear shape change was, in fact, able to significantly modulate the signal and concluded that these changes, like in horseshoe bats, embed dynamic patterns into speech signals.

The dynamically enriched data we explored improved the accuracy of speech recognition. Compared to a traditional system for hearing and recognizing speech in noisy environments, adding structural movement to a complex outer shape surrounding a microphone, mimicking an ear, significantly improved its performance and access to directional information. In the future, this might improve performance in devices operating in difficult hearing scenarios like a busy street in a metropolitan center.


Figure 2: Example of speech signal recorded without and with the dynamic ear. Top row: speech signal without the dynamic ear, Bottom row: speech signal with the dynamic ear

Robotic Sonar System Inspired by Bats

Robotic Sonar System Inspired by Bats

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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The Acoustical Society of America (ASA) is the premier international scientific society in acoustics devoted to the science and technology of sound. Its 7,000 members worldwide represent a broad spectrum of the study of acoustics. ASA publications include The Journal of the Acoustical Society of America (the world’s leading journal on acoustics), JASA Express Letters, Proceedings of Meetings on Acoustics, Acoustics Today magazine, books, and standards on acoustics. The society also holds two major scientific meetings each year. See https://acousticalsociety.org/.