Abstract:
A sonar system includes an objective having reflecting surface(s) with coincident forward radiant axes. Each of the reflecting surfaces defines sets of equivalent acoustic output/receiving locations allowing the use of a plurality of transducers with each reflecting surface. When used in a projection mode, and depending upon the frequency radiated, the sound sources may function as a distributed, functionally continuous sound source. In a passive mode use of a field reflector allows determination of bearings.

Description:
PRIORITY CLAIM 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/066,605 filed 21 Feb. 2008. 
    
    
     BACKGROUND 
     1. Technical Field 
     The field relates broadly to sonar and more particularly to transducer arrays and to sound field shaping and focusing elements for use with transducer arrays. 
     2. Description of the Problem 
     In active sonar systems arrays of acoustic transducers operate to combine the outputs from a large plurality of transducers to combine the sound waves from the individual transducers to narrow and intensify the sound field into a sound beam and, by selectively weighting or adjusting the phase of the input signal to individual transducers, to direct the beam. Typically the transducers have been deployed in a plane arranged in rows and columns, though other approaches are known, see for example WO 2006/016156. Directivity is improved and dispersion reduced with each additional row or column of transducers, however, improved directivity is largely a byproduct of increased destructive summation between the sound fields produced by individual transducers. While such arrays allow beams of high intensity to be generated, the gains obtained with each additional row or column of transducers quickly grow small in comparison to the number of transducers added at each step. While such diminishing returns are characteristic of many systems, they are pronounced in flat matrix like arrays. 
     Parabolic dishes have also been used to reduce the dispersion of a sound field to produce a beam. Meyer et al., in U.S. Pat. No. 5,821,470, describe a Broadband Acoustical Transmitting System based on a parabolic reflector incorporating two loudspeaker transducers. Meyer may be taken as representative of such systems in that he provided one transducer be spaced from the dish, forward along the intended axis of propagation of sound and located at the focal point of the dish. The focal point transducer was horn loaded and oriented to propagate sound backward along the radiant axis and into the dish for reflection in a collimated beam. Meyer added a second transducer for low frequency sound. The second transducer was located opposed to the horn loaded transducer and on the radiant axis of the dish, flush mounted in the dish and oriented for forward propagation of sound centered on the radiant axis. At this location the low frequency transducer derives relatively little benefit from the dish as a focusing element, though the dish still serves as a baffle. 
     Underwater systems, particularly sonar systems, frequently rely on piezoelectric transducers which can be readily applied both to the efficient generation of sound and for the efficient generation of electrical signals in response to impinging sound. Thus arrays of such transducers are more readily applied both for sound generation and sound reception when immersed in water than systems using transducers optimized for use in a compressible medium such as those built for atmospheric use. 
     SUMMARY 
     A sound system for the projection or reception of sound employs a reflecting structure including an “acoustic objective” which includes at least a first objective reflecting surface. The acoustic objective comprises one or more reflecting surfaces characterized by a focus which comprises a set of non-coincident acoustic transducer locations for either generation or detection of sound. The acoustic objective defines a forward radiant axis for the system when in its sound projection mode, and where the acoustic objective includes more than one reflecting surface, the surfaces may be positioned one relative to the other to have a common forward radiant axis. 
     The focus points usually lie in a plane and may be a closed loop such as a ring. Each acoustic objective may be positioned and shaped to define its own set of equivalent acoustic projection/receiver locations. The selection of the shape of each acoustic objective may be done to produce a set of equivalent transducer locations in a ring of non-zero circumference centered on the forward radiant axis. 
     An array of transducers is positioned on the dispersed focus for each acoustic objective with the transducers located at discrete locations and with the transducers aimed to radiate into, or receive sound from, its respective reflecting surface of the acoustic objective. Depending upon the selected shape for the focus, the transducers of a given array are spaced to operate in their radiating mode as a closed loop line array. The spacing between the transducers is then determined by reference to the highest intended operating frequency of the array. The transducers may be aimed to radiate inwardly toward or outwardly from the forward radiant axis, depending upon whether the acoustic objective is a cone type or a dish type. 
     An example objective includes an inner reflecting surface (cone type) and an outer reflecting surface (dish type). The inner reflecting surface is formed from a cone reflector having its axis aligned on the intended radiant axis. The outer reflecting surface may be a forward concave annular ring resembling a dish and typically is disposed around the cone reflector. The shapes of the reflecting surfaces may be made parabolic relative to the forward radiant axis to define an inner focal ring and an outer focal ring. 
     The sound system may be operated as a directionally sensitive listening device. Addition of a field reflector forward from the base system along the radiant axis allows the system to be used to determine a bearing to a sound source where the transducer elements are discrete elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevation of a sound field for a active sonar transducer array. 
         FIG. 2  is a perspective view of an embodiment of an underwater sound projector having inner and outer reflecting surfaces with coincident forward radiant axes. 
         FIG. 3  is a cross sectional diagram depicting operation of an inner reflecting surface for a sound radiator. 
         FIG. 4  is a cross sectional view of a projector having dual reflecting surfaces. 
         FIG. 5  is an exploded view of another embodiment of a projector; 
         FIG. 6  is an exploded view of the projector of  FIG. 5  modified to operate as a passive sonar element. 
         FIG. 7  is a exploded sectional view of the embodiment of  FIG. 6 . 
         FIG. 8  is a block diagram of support circuitry for the device. 
     
    
    
     DETAILED DESCRIPTION 
     The present application incorporates by reference U.S. patent application Ser. No. 11/454,914 filed 16 Jun. 2006 for an Acoustic Energy Projection System, now issued as U.S. Pat. No. 7,621,369. 
     In  FIG. 1  a carrier  10 , such as a submersible watercraft, projects a sonar beam in a beam pattern  14  at a height H above the sea bottom  12 . To tighten the beam pattern  14  and deliver high energy levels into the beam, transducer arrays have been employed which have had large surface areas over which rows and columns of transducers were disposed. The degree of flatness, or narrowness, and the direction in which the flatness to be obtained, dictated the pattern of the array. 
     Referring to  FIG. 2  an underwater acoustic system  11  for producing a narrow beam pattern or receiving sound from a narrow field of view is shown featuring an objective  17  comprising two acoustically reflective surfaces  14  and  16 , the first corresponding to the outer surface of an inner cone  15  and a second reflecting surface  16  formed by a forward concave annular bowl formed by the interior of shell  20  and which is disposed circumferentially outwardly from the cone  15 . The reflecting surfaces  14  and  16  of objective  17  may have hyperbolic or parabolic section taken in planes including the radiant axis RA. This is shown for reflecting surface  14  in  FIG. 3 . A cone having a reflecting surface which has a constant hyperbolic or parabolic section in any section including the radiant axis RA will have a focal ring FR of equivalent acoustic output (or input) locations in a circle encircling the radiant axis. The focal ring FR has a non-zero circumference and encircles the cone  15  centered on the radiant axis RA. The focal ring FR defines the location of equivalent output or input points for sound radiating forward (i.e. in the direction the apex of the cone points in) or for sound received inbound along the axis. If the reflecting surfaces of the objective are scalloped the focal ring FR will not be continuous. 
     A possible location for an annular array  18  of acoustic transducers is near the base of cone  15 . Transducers are located along the focal ring FR and, when operating in projection mode, are oriented to direct sound energy against the reflecting surface  14 . Such placement of the transducers results in a highly collimated forward sound field exhibiting little dispersion. It might be observed that if the transducers are moved forward and backward along the radiant axis RA (as indicated by double headed arrow A), the field can be made more dispersive, or given a far field convergence point forward from cone  14 . A second array of transducers may be used with reflecting surface  16 . 
     A plurality of transducers may be placed at discrete, evenly spaced locations along a focal line or ring. Transducers may be directed inwardly or outwardly perpendicular to the radiant axis RA with generated sound being reflected forward along the radiant axis in a low dispersion, collimated beam. For inwardly directed systems using a cone some leakage occurs toward the tip of the cone reflector due to lack of reflective surface area there. Thus a substantial portion of a tip for a cone  15  may be dispensed with leaving a truncated cone. With transducers arranged in what is in effect an annular, closed loop line array, divergence of the sound field directed into the reflecting surface is reduced. Transducers are located discretely spaced from one another by no more than one quarter of a wavelength of the highest intended operating frequency of the device. The beam may be turned by physically turning the sound system. When used as a listening device sound system exhibits should exhibit directional sensitivity comparable to a parabolic receiving dish, but having more receiving transducers. 
     The parabolic section for a hyperbolic cone reflector follows the equation:
 
 Y=X   2 /4 F  
 
     where F is the focus, X is width and Y is height. Non-parabolic section curves are conceivable, as is a cone reflector with straight faces. Of course such surfaces do not have the same focus characteristics as hyperbolic shapes do. 
       FIG. 4  is an exploded view of a acoustic system  40  which comprises an objective having first and second reflectors, including an inner cone reflector  44  and an outer bowl reflector  46 . The outer bowl reflector  46  is supported on the interior of an outer shell  42  which encircles the assembled projection system  40 . A plurality of transducers  54  are disposed in inner and out closed loop line arrays, nestled for support between a lower transducer cradle  50  and an upper transducer cradle  52 . An annular phase plug or summation cone  48  rests on top of the upper transducer cradle  52 . The terminology “on top of” treats “up” as being in the direction of propagation of acoustic radiation regardless of the actual orientation of the acoustic system  40  when used in its projection mode. The assembly of lower and upper cradles  50 ,  52 , arrays of transducers  54  and phase wedge  38  rest on a bottom interior surface of the shell  42 . For underwater use the inner cone reflector  44 , outer shell  42 , cradles  50 ,  52  and phase wedge  48  may be cast in aluminum. 
       FIG. 5  illustrates in cross section an objective for the acoustic system  40  of  FIG. 4 . Inner and outer reflecting surfaces  44  and  46  can be seen to a cross sectional profiles in the plane including radiant axis RA. The assembly of lower transducer cradle  50 , inner and out arrays of transducers  54  and  58 , upper transducer cradle  52  and the annular phase wedge  48  rest on a support plate  76 . A support electronics package  78  may be housed within inner cone  44 . An open tube support post  60  centered on the radiant axis of projector  40  and extending through the apex of inner cone  44  provides a foundation in which to mount a field reflector as described with reference to  FIG. 6 . The outer bowl reflector  46  is parabolic in its sections, but differs from a conventional parabolic dish in that the bases of the parabolic sections to not meet at a single point in the base of the bowl, but instead forms a circle defining an annular gap (in which the transducer cradle and inner cone are fitted). Support plate  76  can substantially cover this gap excluding ports for introduction of electrical leads. Inner and outer arrays of transducers  54 ,  58  are provided along the respective focal rings for the pair of reflecting surfaces. 
       FIG. 6  illustrates fitting of a field reflector  62  to the support post  60  using a stud  59  and retaining nut  61 . Sound projected forward from the base system  40  and impinging against the field reflector  62  is projected radially outwardly from the radiant axis in a plane transverse to the radiant axis RA up to and including the full 360 degree arc. Field reflector  62  allows use of acoustic system  40  in its receiving mode to be used to determine a bearing to a sound source. Conventionally, in an underwater acoustic device, the acoustic transducers  54 ,  58  are piezoelectric elements and efficiently generate electrical signals from impinging sound waves as well as operating to produce sound waves from electrical signals applied to the transducers. Thus, with the application of appropriate electronics, acoustic system  40  operates in a projection mode or in a listening mode. With field reflector  62  fitted to the acoustic system  40  the sound signals reaching given transducers  54 ,  58  in the inner and outer arrays will vary with the bearing to the sound source. Acoustic system  40 , with the fitted secondary radial reflector  62 , can be used to determine the relative bearing to the target from the strength of the outputs from the transducers  54 ,  58  based on knowledge of the position of the transducers in the arrays. Field reflector  62 , in order to operate for bearing determination, is formed as a cone with an inverted apex (relative to the direction of propagation of sound in the projection mode) centered on the radiant axis of the system  40 . At present other configurations for the field reflector have not been considered, although other shapes might be dictated by changes to the objective, or if system  40  were to be configured for different types of geological surveys, in a manner analogous to use of a Schmidt camera for sky surveys. Field reflector  62  is not limited to use of the system in a passive mode but can also be used to radially distribute an outgoing pulse. 
       FIG. 7 , with minor modifications to the mounting method for the field reflector  62 , illustrates sections of the sound system of  FIG. 6 . The sections of the device may be cast as sections. Lower cradle section  50 A and upper cradle section  52 A provide support two transducers, one associated with the inner array and one with the outer array. The curvature of the outer bowl reflector section  46 A and the inner cone reflector section  44 A bowing away from the cradles is illustrated. A ring bracket  67  is shown as an alternative method of attaching the field reflector section  62 A. A phase wedge section  48 A rests on top of upper transducer cradle section  52 A. 
       FIG. 8  illustrates a simplified block diagram of representative sound processing circuitry for the system of  FIGS. 5-7 . Arrays of transducers  54  and  58  are represented connected to amplification stages  110 ,  120  which represent conventional amplification and analogue to digital conversion both for the generation of sound and the processing of sound inputs and could in theory be used to apply the phase adjustment and weighting of the signals to the individual transducers as is done in planar arrays. Digital signal processing circuitry (DSP)  130  can be you used to analyze incoming signals transducer by transducer to determine a bearing based on the strength of the signal received from each transducer and knowledge of the individual transducers&#39; positions in the arrays. Generally outgoing pulses will be concurrent around the arrays. 
     The acoustic systems disclosed here allow the outputs from a potentially large plurality of sources located at acoustically equivalent locations with minimal destructive summing of the sources to produce a collimated sound field in an underwater environment. Employed in a passive or listening mode the system can be used for bearing determinations among other applications.