Abstract:
An acoustic radiator for underwater application is provided by opposing boundaries mutually spaced and centered on a common axis, a plurality of radial barriers located perpendicular to and connected between the top and bottom boundaries to define a plurality of adjacent radial waveguides, and a plurality of transducers disposed in each radial waveguide, and with one group of transducers being located radially outwardly from another group, the groups being defined in part by all members of the group being the same distance from the apex of the radial waveguide.

Description:
BACKGROUND 
     1. Technical Field 
     The disclosure relates to transducer arrays for producing sound, and more particularly to a high power sound source for use in liquids. 
     2. Description of the Problem 
     Sound is a disturbance in the physical properties of an elastic material/medium that propagates through the material. The disturbed physical properties can be alternation in pressure, the displacement of particles or a change in the density of the elastic material/medium. Sound in the form of an acoustic pressure wave will have alternating zones of high and low pressure, which can be referred to as the compression and rarefaction waves. An acoustic pressure wave propagating through a liquid medium can produce phase changes and otherwise affect physical properties of the liquid medium due to changing pressure. Pressure drops in a liquid medium can result in the liquid medium temporarily assuming a gaseous state, gasses dissolved in the liquid leaving solution, or both. In other words bubbles can form and collapse. Such bubbles are termed acoustic cavitation bubbles. Usually acoustic cavitation bubbles rapidly collapse, which in turn can produce intense shock waves. 
     Whether acoustic cavitation bubbles are a problem in a given situation depends upon the system. For example, in systems where the pressure variation is highest at the surfaces of the transducers acoustic cavitation bubbles occur along these surfaces and their occurrence decreases rapidly with increasing distance from the surface of the transducer. In such systems the transducer surfaces are vulnerable to damage from acoustic cavitation. 
     The acoustic cavitation phenomenon can also limit the amount of power that can be transferred from the transducer element(s) to the propagating medium and distort the resulting signal. A cavitation resistant array was proposed in U.S. Pat. No. 6,050,361 in which interstices of the sonar array between transducers was designed to match the specific acoustic impedance of water. 
     The present applicant has a pending United States Patent Application for an Omni-Directional Radiator for Multi-Transducer Array (Ser. No. 12/590,182, filed 4 Nov. 2009, which is incorporated herein by reference) which teaches use of a full or partial toroidal waveguide in sonar applications which limits cavitation for a given power input level. The radiator includes two facing interior surfaces forming boundaries. Acoustic transducers are arranged in a constellation along one of the interior surfaces of a waveguide to face the opposed surface. The facing interior surfaces extend outwardly from a central base or core of the waveguide and terminate at a mouth. Pressure waves propagating outwardly in the waveguide may be reinforced along a portion or substantially the full depth waveguide, including being summed in a cumulative or cascade manner, with operation of outer transducers being delayed and phase compensated to achieve coherent reinforcement of the pressure wave as it propagates outwardly from the core. The waveguide may be divided into channels by the use of interior radial baffles to increase output amplitude. 
     SUMMARY 
     An acoustic radiator for underwater application is provided by opposing boundaries mutually spaced, perpendicular to and centered on a common axis and a plurality of radial barriers located perpendicular to and connected between the top and bottom boundaries to define a plurality of adjacent radial waveguides. A plurality of transducers is disposed in each radial waveguide. The transducers are organized into at least first and second groups or ranks. The groups are characterized in part by the distance of the members of the group from the common axis or apex of the radial waveguide, with at least one group having members located further from the common axis than the other group. 
     Additional effects, features and advantages will be apparent in the written description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The contribution to the art believed novel is set forth in the appended claims. The preferred mode of use will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of an omni-directional acoustic radiator in accord with one embodiment of the invention. 
         FIG. 2  is a cross-sectional view of the omni-directional radiator taken along section lines  2 - 2  in  FIG. 1 . 
         FIG. 3  is a perspective view of an omni-directional acoustic radiator in accord with one embodiment of the invention. 
         FIG. 4  is a cross-sectional view of the omni-directional radiator taken along section lines  4 - 4  in  FIG. 1 . 
         FIG. 5  is a block schematic of drive circuitry for the radiator. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings and more particularly to  FIGS. 1-2 , an acoustic radiator  10  is shown. Acoustic radiator  10  may be employed to radiate sound in a liquid medium, typically fresh or sea water, and can operate through a full 360-degree arc or circle in a plane perpendicular to a vertical axis A, or in 45-degree arc segments corresponding to each of 8 radial waveguides  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26  and  28 . Radial waveguides  14 - 28  are arrayed in a plane and acoustic radiator  10  exhibits minimal vertical spread in an emission plane perpendicular to the A axis and parallel to the plane of the waveguides. 
     Radial waveguides  14 - 28  are defined by pairs of radial barriers  42  which converge on the central core  12  from the perimeter  50  of the acoustic radiator  10 . The radial barriers  42  are located in planes including the vertical axis A, which is centered within central core  12 . Radial waveguides  14 - 28  have rectangular cross sectional profiles with sides defined by the radial barriers  42  and opposed top and bottom boundaries provided by disks  44  and  46 , which may be mounted perpendicular to and connected to the radial barriers  42  and centered on the central axis A. 
     Radial waveguides  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26  and  28  resemble horns in some respects. Horns are conventionally employed as acoustic transformers in low impedance, highly compressible transmission mediums, such as air. In a highly compressible medium a horn increases the efficiency of coupling energy from a transducer/driver to the air by constraining expansion of the air in response to transducer movement in the vicinity of the transducer. In a liquid medium impedance matching functions are not significant at moderate power input levels, however the containment functionality provided still has application in a liquid transmission medium where acoustic cavitation is possible, enabling increased power input from piezoelectric transducers installed in the radial waveguides  14 - 28 . 
     Piezoelectric transducers  36  are supported by suitable braces (not shown) in the waveguides or on the radial barriers  42 . Increased power input is achieved using two ranks  32 ,  34 , or arrays, of transducers  36 . The second rank  34  is disposed radially outwardly (or at a greater displacement) from the apex  40  of each of the waveguides  14 - 28  than the first rank  32  of transducers  36 . The first rank  32  of transducers  36  is located proximate to the apex  40  for each radial waveguide  14 - 28  at a central core  12 . By initiating a sound wave using the first rank  32  and reinforcing the pressure wave by operating the second rank in phase with the phase of the sound wave as it passes the second rank toward the mouth  38  of a radial waveguide, the second rank  34  can be operated to maintain acoustic wave amplitude. Radial barriers  42  prevent omnidirectional propagation of the acoustic wave from any given rank of transducers  36 , which could operate to cancel the signal. 
     By constraining displacement of liquid medium the phenomenon of the sound wave producing a change in phase of the medium is depressed because the transducer appears to operating at greater than its actual depth. This allows a step up in transducer operational intensity both initially and as it propagates from an apex  40  toward the mouth  38  of a given radial waveguide. The generation of acoustic cavitation bubbles during initial generation and reinforcement of the compression and rarefaction portions of an acoustic wave is retarded. 
     The first (inner) and second (outer) ranks  32 ,  34  of piezoelectric transducers  36  illustrate one way of stacking the transducers so that they are facing one another and spaced. For the first embodiment, the transducers  36  are disposed in what may be characterized as partial toroids located parallel to the plane of the acoustic radiator  10  with the center point of the full toroid located on the central axis A. The transducers  36  of the ranks are mutually spaced, facing one another and located in the toroids. A second embodiment illustrated in  FIGS. 3-4  employs an inner rank  48  of piezoelectric transducers with the transducers mounted spaced from one another in a cylinder parallel to the central axis A. The outer rank  34  is unchanged from that used in the first embodiment and the second embodiment is otherwise physically identical to the first embodiment. 
     Piezoelectric acoustic transducers  17  are conventionally provided as circular disks, though such a shape is not necessarily best. 
     The outer rank  34  of transducers  36  should add enough energy, synchronized with the wave, to at least maintain the acoustic wave&#39;s amplitude notwithstanding the expanding circumference of a wave front in a radial waveguide. 
     Referring to  FIG. 5 , a block diagram circuit  60  illustrates a mechanism for control over transducer  46  inner and outer ranks  32  and  34  or  48  and  34 . Block diagram circuit  60  is adapted for use of the system in a water environment, though its use in other liquid environments should not be discounted. A variety of factors must be taken into account in generating a high intensity underwater sound pulse, such as water depth (represented by pressure), salinity of the water and temperature of the water. All of the these factors affect water density and the speed of sound in water. In addition, other factors may be relevant to consideration of the possible onset of acoustic cavitation, such as the concentration of dissolved gasses, such as oxygen and nitrogen, in the water. Such measurements as are available (typically pressure, temperature and salinity) are provided a digital signal processor  62  which adjusts the base wave form for two channels (inner rank, outer rank) and generates a delay factor for transmission to the outer rank channel. The circuit channels correspond to the two ranks. Final amplifier stages  70 A-B provide differential levels of amplification depending upon the number of transducers in a rank. 
     The inner and outer rank channels are schematically substantially identical save that the channel for the inner rank does not provide for delay of the base signal and may not require feedback protection for the final amplifier stage. Each channel includes a bandpass filter  64 , an equalizer  66 , dynamic phase adjustment  68  and final stage amplification  70 . The outer channel adds delay elements  72  and amplification stage feedback protection  74 . 
     The acoustic radiator  10  may also be operated as a highly directional receiver.