One form of sensor array is a sonar array. Sonar arrays are often deployed on surface or sub-surface vessels to detect objects both on and below the surface. Typically the array comprises a group of pressure sensors, such as hydrophones, being fed along communication channels to be analysed by a sonar processing system on the vessel. The sensors are deployed in various locations about the vessel, to facilitate all round cover, or they may be towed in a line behind the vessel. The data from the sensors are processed to form directional outputs, each of which is sensitive to acoustic signals coming from a particular direction, hereafter referred to as directional beams.
Sensor arrays that are mounted upon the vessel, such as its flanks or bow, are hereinafter referred to as hull-arrays. Such arrays are usually planar, or may be curved but with a radius of curvature that is large compared with an acoustic wavelength. In free-field conditions, the directional response of such arrays is ambiguous, that is they possess a back-lobe at the same orientation to the plane of the array as the main (front) lobe; they give little or no attenuation to noise/interfering targets that lie in the back-lobes of their directional beams. Interfering targets from behind the array are of little concern to arrays mounted directly on the hull because, except at very low frequencies, the vessel to which they are attached acts as a highly effective baffle. However hull-arrays are subject to unwanted noise sources arising from vibrations transmitted through the hull of the vessel, and whilst much of this noise is attenuated via the side-lobe response of the array, noise components lying in the back-lobe have the same directional characteristics as the wanted signal and cannot be rejected via conventional signal processing methods.
The traditional approach to reducing the vibration noise detected via the back-lobe of a hull array is to insert a baffle between the array and the hull in order to isolate mechanically, as far as possible, the sensors from the hull vibrations. Mechanical baffling is extremely effective at higher frequencies but at low frequencies the decoupling performance decreases and becomes ineffective unless the baffle is made very thick. However thick baffles add considerable weight (and cost) to the vessel, which can affect the vessel's performance. On submarines, the baffle's compressibility can give rise to undesirable variations in buoyancy with depth changes, particularly for large arrays.
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