Small but Mighty: Insect-Inspired Microphones #ASA184

Small but Mighty: Insect-Inspired Microphones #ASA184

3D printing technology facilitates bio-inspired microphones that operate autonomously and efficiently.

Media Contact:
Ashley Piccone
AIP Media

CHICAGO, May 10, 2023 – What can an insect hear? Surprisingly, quite a lot. Though small and simple, their hearing systems are highly efficient. For example, with a membrane only 2 millimeters across, the desert locust can decompose frequencies comparable to human capability. By understanding how insects perceive sound and using 3D-printing technology to create custom materials, it is possible to develop miniature, bio-inspired microphones.

The displacement of the wax moth Acroia grisella membrane, which is one of the key sources of inspiration for designing miniature, bio-inspired microphones. Credit: Andrew Reid

Andrew Reid of the University of Strathclyde in the U.K. will present his work creating such microphones, which can autonomously collect acoustic data with little power consumption. His presentation, “Unnatural hearing — 3D printing functional polymers as a path to bio-inspired microphone design,” will take place Wednesday, May 10, at 10:05 a.m. Eastern U.S. in the Northwestern/Ohio State room, as part of the 184th Meeting of the Acoustical Society of America running May 8-12 at the Chicago Marriott Downtown Magnificent Mile Hotel.

“Insect ears are ideal templates for lowering energy and data transmission costs, reducing the size of the sensors, and removing data processing,” said Reid.

Reid’s team takes inspiration from insect ears in multiple ways. On the chemical and structural level, the researchers use 3D-printing technology to fabricate custom materials that mimic insect membranes. These synthetic membranes are highly sensitive and efficient acoustic sensors. Without 3D printing, traditional, silicon-based attempts at bio-inspired microphones lack the flexibility and customization required.

“In images, our microphone looks like any other microphone. The mechanical element is a simple diaphragm, perhaps in a slightly unusual ellipsoid or rectangular shape,” Reid said. “The interesting bits are happening on the microscale, with small variations in thickness and porosity, and on the nanoscale, with variations in material properties such as the compliance and density of the material.”

More than just the material, the entire data collection process is inspired by biological systems. Unlike traditional microphones that collect a range of information, these microphones are designed to detect a specific signal. This streamlined process is similar to how nerve endings detect and transmit signals. The specialization of the sensor enables it to quickly discern triggers without consuming a lot of energy or requiring supervision.

The bio-inspired sensors, with their small size, autonomous function, and low energy consumption, are ideal for applications that are hazardous or hard to reach, including locations embedded in a structure or within the human body.

Bio-inspired 3D-printing techniques can be applied to solve many other challenges, including working on blood-brain barrier organoids or ultrasound structural monitoring.

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A moth’s ear inspires directional passive acoustic structures

Lara Díaz-García –
Twitter: @laradigar23
Instagram: @laradigar

Centre for Ultrasonic Engineering, University of Strathclyde, Glasgow, Lanarkshire, G1 1RD, United Kingdom

Popular version of 2aSA1-Directional passive acoustic structures inspired by the ear of Achroia grisella, presented at the 183rd ASA Meeting.

Read the article in Proceedings of Meetings on Acoustics

When most people think of microphones, they think of the ones singers use or you would find in a karaoke machine, but they might not realize that much smaller microphones are all around us. Current smartphones have about three or four microphones that are small. The miniaturization of microphones is therefore a desire in technological development. These microphones are strategically placed to achieve directionality. Directionality means that the microphone’s goal is to discard undesirable noise coming from directions other than the speaker’s as well as to detect and transmit the sound signal. For hearing implant users this functionality is also desirable. Ideally, you want to be able to tell what direction a sound is coming from, as people with unimpaired hearing do.

But dealing with small size and directionality presents problems. People with unimpaired hearing can tell where sound is coming from by comparing the input received by each of our ears, conveniently sitting on opposite sides of our heads and therefore receiving sounds at slightly different times and with different intensities. The brain can do the math and compute what direction sound must be coming from. The problem is that, to use this trick, you need two microphones that are separated so the time of arrival and difference in intensity are not negligible, and that goes against microphone miniaturization. What to do if you want a small but directional microphone, then?

When looking for inspiration for novel solutions, scientists often look to nature, where energy efficiency and simple designs are prioritized in evolution. Insects are one such example that faces the challenge of directional hearing at small scales. The researchers have chosen to look at the lesser wax moth (fig 1), observed to have directional hearing in the 1980s. The males produce a mating call that the females can track even when one of their ears is pierced. This implies that, instead of using both ears as humans do, these moths’ directional hearing is achieved with just one ear.

Lesser wax moth specimen with scale bar. Image courtesy of Birgit E. Rhode (CC BY 4.0).

The working hypothesis is that directionality must be achieved by the asymmetrical shape and characteristics of the moth’s ear itself. To test this hypothesis, the researchers designed a model that resembles the moth’s ear and checked how it behaved when exposed to sound. The model consists of a thin elliptical membrane with two halves of different thicknesses. For it, they used a readily available commercial 3D printer that allows customization of the design and fabrication of samples in just a few hours. The samples were then placed on a turning surface and the behavior of the membrane in response to sound coming from different directions was investigated (fig 2). It was found that the membrane moves more when sound comes from one direction rather than all the others (fig 3), meaning the structure is therefore passively directional. This means it could inspire a single small directional microphone in the future.

Laboratory setup to turn the sample (in orange, center of the picture) and expose it to sound from the speaker (left of the picture). Researcher’s own picture.
Image adapted from Lara Díaz-García’s original paper. Sounds coming from 0º direction elicit a stronger movement in the membrane than other directions.

2aEA3 – Insect Ears Inspire Miniature Microphones

James Windmill –
University of Strathclyde
204 George Street
Glasgow, G1 1XW
United Kingdom

Popular version of paper 2aEA3
Presented Tuesday morning, May 8, 2018
175th ASA Meeting, Minneapolis, MN

Miniature microphones are a technology that everyone uses everyday without thinking about it. They are used in smartphones, laptops, tablets, and more recently in smart home equipment. However, working with sound technology always means there are issues, like how to deal with background noise. Engineers have always looked for ways to make technology better, and in miniature microphones one of the paths for improvement has been to look at how insects hear. If you want to design a really small microphone, then why not look at how the ear of a really small animal works?

In the 1990’s researchers discovered that a small fly (Ormia ochracea) had a very directional ear. That is, it can tell the direction that sound was coming from with a lot higher accuracy than predicted. Since that discovery many engineers have made attempts to make microphones copying the mechanism in the Ormia ear. Much of the effort has spent been trying to get round the problem that the Ormia is only interested in hearing one specific frequency. Humans want microphones that cover all the frequencies we can hear. Why bother copying this insect ear? If you could make a tiny directional microphone then a lot of background noise drops simply because the microphone points towards the person speaking.

At Strathclyde we have developed a variety of microphones based on the Ormia ear mechanism. The main push in this work has been to try and get more sensitive microphones working across more frequencies. To do this we have put four microphones into one Ormia type design, as in Figure 1. So instead of a single frequency, the microphone works as a miniature directional microphone across four main frequencies [1].

insect ear

Figure 1. Four frequency Ormia inspired miniature microphone.

Work on the Ormia system at Strathclyde encouraged us to think of other things that insect ears do, and their structure, to see if there are other advantages to find. This work has taken two main themes. Firstly, many hearing systems in nature are not just simple mechanical systems; they are active sensors. That is they change how they function depending on what sound they’re listening to. So for a quiet sound they increase the amplification of the signal in the ear, or for a loud sound they turn it down. Some ears also change their frequency response, changing the frequencies they are tuned to. Strathclyde researchers have taken these ideas and produced miniature microphone systems that can do the same thing [2]. Why do this, when you can just do it in signal processing? By making the microphone “smart” you can free up processor power to do other things, or reduce the delay between a sound arriving and the electronic signal being used.

Figure 2. Graphs showing the results of a miniature microphone actively changing its frequency (A) and gain response (B).

Secondly, we thought about how you make miniature microphones. The ones we use in phones, computers etc today are made using computer chip technology, so are made very flat out of very hard silicon. Insect ears are made of a relatively soft material, and come in a huge variety of three dimensional shapes. The obvious thing it seemed to us was to try making insect inspired microphones using 3D printing techniques. This is very early work, its not easy to do. But we have had some success making microphone sensors using 3D printers [3]. Figure 3 shows an “acoustic sensor” that was inspired by how the locust hears sound.

Figure 3. 3D printed acoustic sensor inspired by the ear of a locust.

There is still a lot of work to do, both on developing these techniques and technologies, and on working out how best to use them in everyday technologies like the smartphone. Then again, a huge number of different insects have ears, each working in slightly different ways to hear different things for different reasons, so there are a lot of ears out there we can take inspiration from.

[1] Bauer R et al. (2017), Housing influence on multi-band directional MEMS microphones inspired by Ormia ochracea, IEEE Sensors Journal, 17: 5529-5536.

[2] Guerreiro J et al. (2017), Simple Ears Inspire Frequency Agility in an Engineered Acoustic Sensor System, IEEE Sensors Journal, 17: 7298-7305.

[3] Domingo-Roca R et al. (2018), Bio-inspired 3D-printed piezoelectric device for acoustic frequency selection, Sensors & Actuators: A. Physical, 271: 1-8.

1aNS3 – Low-frequency sound control by means of bio-inspired and fractal designs

Anastasiia O. Krushynska –
Federico Bosia –
Nicola M. Pugno –
Laboratory of Bio-inspired and Graphene Nanomechanics
Department of Civil, Environmental and Mechanical Engineering
Uiversity of Trento
Via Mesiano 77
Trento, 38123, ITALY

Popular version of paper 1aNS3, “Fractal and bio-inspired labyrinthine acoustic metamaterials”
Presented Monday morning, May 7, 2018, 9:15-9:35, Nicolett 3D
175th ASA Meeting, Minneapolis

Road, rail, airports, industry, urban environments, crowds – all generate high-volume sound. When sound becomes uncomfortable or even painful to the ear, it is generally called noise. Nowadays, noise is one of the most widespread environmental problems in developed countries, negatively affecting public health and quality of life. Recent findings of the World Health Organization show that noise pollution is not only annoying for a large percentage of the population, but also causes sleep disturbance, increases the risk of cardiovascular diseases, intensifies the level of stress and hinders learning processes. Low-frequency noise is the most troublesome type and is mainly produced by road vehicles, aircraft, industrial machinery, wind turbines, compressors, air-conditioning units, etc.

The attenuation or elimination of low-frequency noise is a challenging task due to its numerous sources, its ability to bypass obstacles, and the limited efficiency of most sound barriers. The laws of acoustics tell us that if a solid wall is used to attenuate noise, sound transmission is inversely proportional to its mass per unit area and the sound frequency. This means that very heavy walls, more than ten meters thick (!), are necessary to efficiently reduce typical low-frequency noise in the frequency range between 10 and 1000 Hz.

Fortunately, modern technology can provide more innovative and efficient solutions, based on so-called acoustic metamaterials. These are engineered structures capable of effectively slowing down sound speed and reducing sound intensity thanks to enhanced internal structural losses. The latter can be induced by incorporating internal resonators, which transfer mechanical vibrational energy into heat, or by using a geometry-related mechanism, exploiting the artificial elongation of sound propagation paths by means of narrow, so-called “labyrinthine” channels. In this work, we develop labyrinthine acoustic metamaterials with long narrow channels inspired by the structure of spider webs or arranged along fractal space-filling curves. These particular designs help to extend the metamaterial functionalities as compared to simpler configurations analyzed in previous years.

What happens if a sound wave enters a straight narrow channel? Depending on the channel geometry, it can either propagate through it, or be attenuated. For narrow channels, friction effects in the vicinity of the channel walls hinder wave propagation, and can eventually lead to its total attenuation. For moderately wide channels, if the sound wavelength matches the distance between the two channel edges (i.e., it equals an integer number of half wavelengths), resonance takes place, allowing to amplify the sound transmission. Both the described effects take place at single frequencies.

But what happens if the channels are arranged in the shape of a maze or if there is a set of coiled channels? We now know that for certain configurations, other types of collective resonances can be induced – Mie resonances – that enable the achievement of total reflection at rather wide frequency ranges.

We have found out that natural spider-web designs for the channel labyrinths provide sufficient freedom for the development of metamaterials with switch on/off regimes between total transmission and total reflection that can be easily adapted for controlling low-frequency sound. In particular, we have shown that a light-weight re-configurable structure with a square cross section of 0.81 m2 is capable of totally reflecting airborne sound at frequencies of 50-100 Hz and above [1]. Moreover, by modifying the channel thickness and length, we can tune operating frequencies to desired ranges. In fact, the proposed metamaterials provide exceptional versatility for application in low-frequency sound control and noise abatement.

Incorporation of more advanced designs, e.g. coiling wave paths along space-filling curves, enables to develop more compact configurations and opens a route for creating efficient sound absorbers [2]. Space-filling curves are lines constructed by an infinite iterative process with the aim to fill in a certain area, e.g. a square or cube. Since the work of G. Peano (1890) until the 1980s, these curves were considered no more than mathematical curiosities, and only recently have they found application in fields like data science and routing systems. The use of space-filling curves for wave path labyrinths in combination with the added effect of friction in narrow channels has allowed us to achieve total reflection or to improve wave absorption of low-frequency sound. The absorption can be increased up to 100 % at selected frequencies, if a hybrid configuration with incorporated Helmholtz resonators is used [3]. This could be the next chapter to be written in the story of efficient noise abatement through innovative metamaterials.


[1] A.O. Krushynska, F. Bosia, M. Miniaci and N. M. Pugno, “Spider web-structured labyrinthine acoustic metamaterials for low-frequency sound control,” New J. Phys., vol. 19, pp. 105001, 2017.

[2] A.O. Krushynska, F. Bosia, and N. M. Pugno, “Labyrinthine acoustic metamaterials with space-coiling channels for low-frequency sounf control,” Acta Acust.united Ac., vol. 104, pp. 200–210, 2018.

[3] A.O. Krushynska, V. Romero-García, F. Bosia, N.M. Pugno, J.P. Groby, “Extra-thin metamaterials with space-coiling designs for perfect sound absorption”, (working paper), 2018.

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 –

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