Trapped Dogma

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The human ear is a miracle of mechanical development. It allows us to hear a surprising range of sounds and to communicate and navigate the world. Damage is easy to reduce and difficult to repair. The hearing aids are still large, uncomfortable and yet we are unable to deliver the rich and wonderful sounds we carry. Yet there may be a new way for us to replace damaged hearing from an unexpected source – the insect world.

Spend a summer in the countryside in a hot climate and expect that the male of the “singing” species will be heard chirping, in an attempt to attract the female. Surprisingly, how small sound levels are given to organisms. Can the study of cricket teach us something about preparing a small speaker, which is necessary to help you listen?

My colleagues are currently researching exactly this. Rubbing their wings together to create a creaky sound. The secret of their loud call is that their feathers are corrugated in specific patterns that make them very stiff, which in turn makes them very loud when rubbed together. Using laser vibration method and advanced computer modeling simulations we can replicate this idea by stitching the rigidity of a speaker surface. This makes it a simple and efficient way to make small speakers really very loud.

However insect motivation when stopped with small speakers. Hearing aids have traditionally been designed to operate in various stages. Sound signals are picked up by a microphone and then amplified electrically.

The best studied insects in bio-acoustics are the weeds, which have two large “tympanal” membranes used for hearing on either side of the chest. These membranes vibrate with sound and consequently transfer signals to the nervous system, such as the human ear drum. Recently we have seen this membrane only vibrating up and down. Upon careful dissection, we found that there was a regular variation in its thickness. While this may not seem particularly interesting at first, we were surprised when playing it.

This caused a tsunami-like vibration with a crest of a wave in place of nerve cells. In fact, this simple variation in thickness allows vast amplification of sound energy. In mammals the process of amplification is achieved with delicate middle ear bones, with some locusts receiving varying thicknesses of their ear drums. So we may be able to design microphones with inbuilt passive amplification based on this idea.

Interestingly, some bugs are also making us question what exactly a microphone can be. For example, mosquitoes and fruit fly, have small antennae on their heads that are subtle in shape yet are very sensitive to sound. Although research into these characteristics is temporary, it may direct us in the unexplained directions of microphone design.

The process of filtering the sounds that come with Hearing Aid requires fairly sophisticated electronics, which directly affects the size and battery life of the device. Here again the grasshopper can help. Along with increasing the sound waves, the tangent membrane also filters a range of frequencies. This is most likely due to the material the membrane is made of.

My colleague, Professor Daniel Robert, recently found a South American species of catch or bush cricket, which may perform similar functions. Each foreleg of the katid is less than one millimeter in size, capable of separating different frequencies into location specific vibrations, similar to the function of the human cochlea. If we can somehow incorporate this mechanical frequency separation into the microphone itself, we may be able to use its automatic filtering properties.

Biology, medicine and engineering have traditionally been quite different subjects. But by adding them, as we have in these projects, we can develop new engineering solutions based on discoveries made many years ago. So while bio-inspired hearing aids may not be forthcoming on the shelves, this new field of study may find more and more ways of addressing the needs of people with hearing loss. And our little mechanical experts can get a lot of inspiration from insects.

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