From ears to engineering

Modern microphones, such as those used in hearing aids, and transducers such as ultrasound scanners, are remarkably sensitive. However, they still struggle to deal with issues such as background noise, which may need to be removed by downstream digital processing, or identifying the direction from which a sound originates. Electronics engineer Prof Windmill’s highly multi-disciplinary team provide solutions to these problems, taking their inspiration from the natural world, in particular the insects.

Inspiring insects
With about a million species known to science, insects are an extremely varied group of organisms, and have evolved a diverse array of different hearing organs. Previous research into insect hearing has largely focused on the noisiest groups: the grasshoppers, crickets, locusts, and cicadas; however, Prof Windmill’s research covers a wide range of other insects including flies and moths. In the course of their work, Prof Windmill and colleagues have discovered some remarkable insects, including a moth, the greater wax moth, with the ability to hear sounds up to 300 kilohertz, higher than any other animal; the insect with the highest frequency call – a genus of katydid dubbed ‘Supersonus,’ from the South American rainforest; and the loudest (relative to its size) animal on earth – a water boatman that generates a mating call by rubbing its penis against its abdomen.

To elucidate the mechanisms of hearing in an insect, Prof Windmill’s lab use an array of techniques, including behavioural observation, microscopy and X-ray microtomography, 3D laser vibrometry, and electrical examination of the signals passing through the auditory nerve. The results are translated into a three-dimensional computer model of the ear’s structure, which can then be used to simulate how it responds to sound. Comparing their experimental data with the computer models gives the researchers a thorough understanding of how the organism’s hearing works – both in terms of its mechanics, and downstream signal processing at the neural level.

Then, the baton is passed to the engineers, physicists, mathematicians and material scientists of the team who develop new microphones (instruments for sensing acoustic and ultrasonic waves) and transducers (instruments that can both generate and sense sound – such as are used in hospital ultrasound scanners) based on these findings.

Comparing their experimental data with the computer models gives the researchers a thorough understanding of how the organism’s hearing works

Navigating by sound
While larger animals can detect the direction of a sound source by the difference in timing and amplitude as sound waves are received at each of their two ears, for smaller animals such as insects, the distance between their two hearing organs is likely too small for this to work. Thus, smaller organisms have come up with a variety of innovative techniques that are now coming to the interest of engineers.

Ormia ochracea is a tiny, nocturnal fly which lays its eggs on crickets. It therefore needs to locate its cricket hosts in the dark, which it does by the sound of the male cricket’s mating call. Since the mid-nineties, it has been known that the Ormia’s two tympanic membranes (ear drums) which are located around the base of their front legs, are directly coupled to each other by a strut. In effect, the structure forms a tiny, highly sensitive see-saw which rocks if the sound waves reaching the two tympanic membranes are in any way different in intensity or timing. This ingenious system amplifies minute differences in sound reaching the two membranes, enabling the insect to detect the direction a sound is coming from.

In Prof Windmill’s lab, their three-dimensional computer models and simulations of the Ormia system have been used to develop tiny acoustic sensors for use in hearing aids. Until recently, the sensors were built from silicon using standard Microelectromechanical Systems (MEMS, or ‘micromachine’) techniques – but now the team are moving into the realm of 3D-printing. This is enabling them to more easily design complex three-dimensional structures and to use more flexible materials, simulating more closely the mechanical properties of biological structures.

A hearing aid could focus on the frequency range of human speech and fade out the ‘noise’ from other sources

More recently, Prof Windmill, with colleagues from the Université François Rabelais de Tours, France, has identified a tiny moth, Achroia grisella, the lesser wax moth, which is able to determine the directionality of sound with just one ear, which has a maximum response to sound arising from a particular angle. These moths then use their behaviour – scanning with their head to search for the source of a sound, and then maintaining the same angle between themselves and the sound as they move – to locate their singing partners for mating.

Blocking out the noise
In fact, it is in the butterflies and moths (Lepidoptera) that the greatest number of ‘acoustic’ insects are found. Around 55% of Lepidoptera have tympanal ears, and many use ultrasound for mating. To enable them to hone in on the mating call of their own species, these moths are able to physically adapt the response of their eardrums to focus on particular frequencies of sound.

There are many situations in which this property could be useful: for instance, a hearing aid could focus on the frequency range of human speech and fade out the ‘noise’ from other sources. Like the Lepidoptera, our own ears, and those of many animals, do this automatically through a feedback system – the nature of the sounds heard changes the response of the ear. Until now, however, engineered microphones have relied on downstream digital processing of all the sound signals received, which can cause time delays as well as using energy and increasing the total size of the microphone system.

Now Prof Windmill’s team, after studying the hearing system of the large yellow underwing (Noctua pronuba), have developed a MEMS microphone which can adapt its sensitivity to different frequencies depending on the sound received. This may be particularly good news to users of hearing aids or cochlear implants who struggle with background noise. Thanks to Prof Windmill’s multidisciplinary team and the combination of fundamental biology with applied engineering, the amazing adaptations of the insect world could be coming to a microphone near you.