Threads locate noises in air and water

Written by: Tom Shelley | Published:

Tom Shelley reports on a clever way of locating sound sources using micro engineered arrays that would fit in a match head.

A Dutch company has a sensor technology based on pairs of very thin platinum wires on a single device, that can accurately locate sources of sound both in the air, and recently, underwater. The devices have for some time been used by automotive engineers to locate noise sources on cars under development, but they also have applications in defence for locating and identifying enemy weapons fire and incoming aircraft and a new family of devices can locate noise sources under water. These have potential applications in the defence and offshore oil and gas industries.

The basic technology was originally invented by Hans-Elias de Bree at the University of Twente in the Netherlands in 1994. He earned his doctorate in 1997 and started Microflown Technologies in 1998. John Bremner, the company's business development manager, says that the wires, which are thinner than human hairs, are heated to 200ºC. Particles of air of water crossing them are heated by the first wire, taking heat from it and thus reducing its electrical resistance. When they cross the second wire, they are heated again and also cool it, but by a smaller amount.

This allows calculation of the flow rate past the wires. If there are three pairs of wires arranged orthogonally, it is possible to determine the speed and direction of the movement of air or water. Sound waves cause air or water to move in a reciprocating manner whose frequency can be detected. Hence, by processing the changes in resistance of the wires in real time, and combining this information with the output of a microphone, it possible to determine the direction of a sound source, and identify what sort of sound it is and how far it is away.

The combination of microphone and three flow sensors is called by the company an 'Acoustic Vector Sensor' or AVS. If the flow sensors are working in air they are referred to as 'Microflown' sensors, and if they are working in water, 'Hydroflown' sensors, in which case the microphone is replaced by a hydrophone. It is possible to make a single AVS only 5mm x 5mm x 5mm. Angular accuracy is within +3°. Sensitivity is from -10dB to +170dB and frequency response is from 0.1Hz to 120kHz, although for civilian commercial applications, the bandwidth is limited to 1Hz to 20kHz.

In air, if distance is to be established for military purposes, it is possible to achieve +50m over a range of a "Few km." The first and main users of the technology are automotive engineers set with the task of locating the sources of noise in new cars, with a view to eliminating them before they go into production. There has been much research and development into systems to detect, identify and locate enemy small arms, mortars, artillery, fixed wing aircraft and helicopters.

Because the system is passive, there is no need to use radar, whose signals may be detected by the enemy. Underwater, where the AVS units are thumb sized, it is possible to locate ship and submarine propeller noises, so enemy vessels can be located passively without having to use sonar, which can also be detected by the enemy. They also have the potential to replace towed sonar arrays with items that are orders of magnitude smaller and lest expensive. In the oil and gas industries, they have the potential to be able to quickly locate leaks and developing problems.

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