Articles
Dr Rob Malkin, senior research associate in acoustics and non-destructive testing, University of Bristol
As part of my research into bio-acoustic-inspired hearing aids at the University of Bristol, I work with an interdisciplinary team of biologists, physicists and materials engineers to unravel some of the amazing hearing systems that insects possess.
Hearing in insects has evolved 16 separate times, meaning that nature has come up with a wide range of subtly different mechanisms by which sound waves can be transduced into nerve signals.
We tend to study crickets, locusts and fruit flies, as they are a good representation of the other hearing-capable insects. They have shown us how to create small, sensitive, mechanically elegant and robust hearing systems.
Current hearing aids work generally by using a microphone to receive incoming sound waves, which are then amplified and digitally filtered using quite advanced electronics. A significant portion of electrical power is dedicated to this signal processing. Finally, the desired sound signal is passed to a speaker and directed to the ear canal via a plastic tube.
Hearing aids can be quite bulky – small, in-ear ones are only really suitable for people who suffer minor hearing loss. They also tend to suffer from poor battery life and poor high-frequency (>7kHz) reproduction. This means that, as yet, we still cannot restore rich and engaging sounds to the hard of hearing.
However, by studying insects, which are adept at making loud sounds from small “speakers”, we can begin to rethink the ways we design hearing aid speakers.
We need to study the picometre-scale displacements that insect hearing organs undergo. We rely extensively on non-contact laser vibration systems, which have allowed us to probe mechanical response in stunning detail. This approach is coupled with more traditional techniques such as neurophysiology, where we study the response of nerves to acoustic stimuli. For structural details, we use micro-computed tomography.
The most inspiring thing we have found is that nature is good at designing a multi-functional device. The locust ear is capable of greatly amplifying incoming sound radiation and performing frequency analysis in a single, 2mm-diameter, ear-drum membrane. In humans, the task of amplification is performed by the middle ear bones, while frequency analysis is done separately by the cochlear. This discovery is making us rethink how much signal processing we can do by redesigning the microphones used in hearing aids.
To mimic some of the hearing systems that insects exhibit, we rely on nano-3D printing. However, there are some restrictions. 3D printing using techniques such as stereolithography is fantastic for micrometre-sized structures, while normal 3D printing is good for 5mm-plus structures. But as yet there is no suitable technique to replicate the kinds of sizes in insect ears.
Our hope is to come up with simple solutions, from making hearing aids smaller to making the batteries last longer, by shifting some of the amplification and analysis from a powered system to a passive one. Hearing aids will become more comfortable and less obtrusive.
Our next step is working with hearing aid manufacturers and audiologists to address their requirements and put some of our knowledge into a real product.