By Katie Scott, Wired UK

The extraordinary auditory abilities of the orca have inspired researchers at Stanford University to create an undersea microphone that has a sensitivity range of 160 decibels but can also function at any depth.

Onur Kilic, a postdoctoral researcher in electrical engineering, set out to create a device that had a large range of sensitivity. He began by looking at the ears of marine mammals, particularly orcas. “Orcas had millions of years to optimise their sonar and it shows,” he said in a press release. “They can sense sounds over a tremendous range of frequencies and that was what we wanted to do.”

The team created a silicon chip with a thin membrane about 500 nanometers thick — about 25 times thinner than common plastic wrap. However, at depth, the air around the diaphragm would compress causing it to stop vibrating. Kilic countered this by surrounding the diaphragm with water and drilling a grid of “nano-holes” in it so that it could flow through.

This, however, brought new issues as the water impeded the diaphragm’s movements. “The kind of displacements you get of the diaphragm for the quietest sounds in the ocean is on the order of a hundred-thousandth of a nanometer,” Kilic said in a release. “That is ten thousand times smaller than the diameter of an atom.”

The team opted to employ lasers and mirrors to detect these minute movements. Kilic ran a fibreoptic cable into the microphone, with the end of the cable positioned near the inside surface of the diaphragm. He then shot light from a laser out the end of the cable onto the diaphragm.

In a statement, the team explained: “Normally a diaphragm so thin would be transparent, allowing the laser’s light to escape. But we knew that if the diameters of the holes that allowed water to pass through the diaphragm were close to the wavelength of the light from the laser, the holes would interfere with light trying to pass through the membrane. Instead of letting it pass, the holes would reflect the light back toward the tip of the fibreoptic cable, effectively turning the diaphragm into a mirror even as it still allowed water to pass.”

To capture the range of volumes that the team wanted — they had to use three of these diaphragms — each with a different diameter — to capture different sections of the range. The largest is just three-tenths of a millimeter in diameter. “Since they all measure the exact same signal – just with different degrees of responsiveness – they work like a single sensor,” Kilic said in the statement. “It is a very high dynamic range microphone, able to sense everything from the weakest sounds to those 100 million times stronger.”

Image: RayS/Flickr

Source: Wired.co.uk

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