Floating underwater sensor for vectoring low-frequency sounds

Compact, lightweight, low power, directional acoustic sensors capable of operating at low frequency

Photonics Electronics Sensors

A fine micro-fabricated web immersed in the ocean undergoes cyclic mechanical deformation (comparable to the stretching of a trampoline) in the presence of acoustic excitation. This deformation is governed by the viscous forces due to oscillatory motion of the media particles in the sound wave and can be detected by an optical probe (not shown). Directionality is provided by the fact that the out-of-plane deformation of the mesh strongly depends on the angle between the flow direction and the normal vector of the mesh. By optical monitoring of the deformation of the mesh, one can extract information regarding both sound intensity and the orientation of the wave vector. Image: Four viscous flow meters (104) attached to a floating base and a retaining thread (106) coupling the floating base to an anchor.

Scientists at the Naval Research Laboratory have recently invented a device for sensing the presence, direction, and source of acoustic signals in water. The patented technology is available via patent license agreement to companies that would make, use, or sell it commercially.

One factor in the detection and characterization of acoustic signals is determining its direction of arrival (DOA). The following conventional methods for detecting the DOA of acoustic signals have several limitations, especially for low-frequency signals

  1. Multi-sensor systems for low-frequency acoustic signals are limited by the necessary large size of the arrays required to provide the directionality.
  2. Similarly, accelerometers implemented as a neutrally buoyant body immersed in the sound waves underwater require a prohibitively large test mass for operating in 10 Hz frequency range.
  3. Micromechanical sensors show promise for applications in very low-frequency underwater acoustics but have two significant drawbacks: the amount of deformation to be measured is very small and the sensor is assumed to be somehow rigidly mounted underwater.

NRL researchers have developed a system for sensing acoustic signals that address these limitations of prior systems. The vector sensor system shown in the diagram can be used to detect and characterize low-frequency sound waves in water by detecting a periodic motion of the media particles associated with the sound waves. The orientation of the particle velocity deduced from such measurements can provide information regarding the wave vector of the sound waves, the DOA for the acoustic signal, and the location of the sound of interest. The approach exploits the acoustically-induced normal displacement of fine mesh as a measure of the collinear projection of the particle velocity in the sound wave.

The technology can be used to detect sound waves in water in the vicinity of pressure-release surfaces (a water-air boundary or hull(s) of submerged vessels at low frequencies). The mesh-type velocimeter can be used for monitoring slowly-varying viscous flow down to the direct current limit.

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