Sound of locusts

The complex and highly sophisticated hearing of the locust is the inspiration for new research which could lead to the development of extremely sensitive microphones.

A multi-disciplinary team from Bristol University is applying a number of different technologies to investigate the workings of the extremely thin membranes in locusts’ ears that process sound.

Research leader Prof Daniel Robert works in the university’s bio nano-science department, which looks specifically at nano-scale events in biology. He said the research will allow scientists to not only better understand hearing but also to understand how the sensory cells in the ear operate in extremely fine detail.

According to Robert the main difference between the way insects process sound and the way man-made microphones pick it up is that insects convert the sound instantly into information.

‘They do not process the soundwaves like an engineer would by using electronic processing,’ he said. ‘The very instant the sound reaches the membrane it is processed mechanically. The neurons themselves get frequency-specific information without having to analyse it at all. They uses the material properties of the eardrum to immediately convert the acoustic properties of the sound into mechanical properties.’

It is these material properties that the team is investigating using a variety of nano-scientific tools. Highly sensitive equipment is required because although the eardrum is 1.5mm across, the membrane itself is only a micron thick and the vibrations that researchers hope to measure across that membrane are only one nm wide.

One of the technologies being used is micro-scanning laser Doppler vibrometry — a system which sends a coherent band of pure red light at a single frequency towards the membrane of the eardrum. The movement of the eardrum bounces the laser back and the differences between the outgoing and the returning beam are analysed and the movement is recorded. This use of the Doppler effect created by the moving eardrum allows researchers to model the vibration velocity of the organ, which in turn provides them with valuable data.

According to Robert the system was originally developed for analysing moving parts in Mercedes-Benz engines and so measure their efficiency, and has been adapted to be used for nanobiology.

The team is also producing a 3D map of the membrane using an atomic force microscope. Working in much the same way as a stylus moves across a record, a tiny cantilevered point travels across the surface of an object and can measure the unevenness of the material in nanometre detail. The result is a 3D map of the surface.

In Robert’s opinion the research could result in a number of future applications that require highly sensitive, miniaturised acoustic sensors.

‘We are trying to make microphones cleverer and more adapted to the task,’ he said. ‘Animals can measure sounds that our microphones cannot detect because of the way they process sound instantly. So the idea is to make more sensitive microphones that can do the processing mechanically instead of involving electronics.’

He added that these microphones could perhaps be used in military applications or in highly sensitive hearing aids. Looking about 10 years ahead, Robert suggested that the research could one day lead to the development of tiny, sensitive pressure probes and sensors that could used for extremely detailed scientific research.