Patent Publication Number: US-9906857-B2

Title: Underwater acoustic projector

Description:
FIELD 
     This disclosure relates generally to an underwater acoustic projector. More specifically, this disclosure relates to an underwater acoustic projector having a moving coil apparatus. 
     BACKGROUND 
     Low frequency acoustic projectors for underwater acoustic applications can be expensive due to their generally complex designs and components. One type of low frequency acoustic projector is a moving coil type low frequency projector. Underwater acoustic moving coil type low frequency projectors are generally large, expensive, and include complex designs. The complex designs are at least partially a result of the moving coil apparatus being sensitive to pressure differentials between an internal cavity side of a projector face and an external side of the projector face that is in communication with the water. Moving coil type low frequency acoustic projectors can be de-tuned (e.g., frequency modified) if the forces acting on the projector face are not maintained in static equilibrium. To maintain static equilibrium, pressure is often supplied to the internal cavity, which can limit depth of operation and increase the size and complexity of the design, and accordingly, increase the expense as well. Known methods of supplying pressure to the internal cavity include using a self-contained underwater breathing apparatus (SCUBA) system, an air-backed system, or an oil-compensated system. 
     SUMMARY 
     An underwater acoustic projector for projecting acoustic energy through water and a method for designing the underwater acoustic projector are described. A method for maintaining a pressure differential in the underwater acoustic projector (“acoustic projector”) is also described. 
     In one embodiment, the acoustic projector can be a low frequency acoustic projector. In such an embodiment, the acoustic projector can have a frequency below about 1,000 Hz. It is to be appreciated that this frequency range is exemplary and that the frequency of the acoustic projector can vary beyond the stated range. 
     The underwater acoustic projector can be a moving coil type acoustic projector that includes a moving coil apparatus. 
     Acoustic projectors as described herein can be standalone devices which can be submerged underwater without being coupled to an underwater vehicle, an underwater vessel, or the like. Alternatively, the acoustic projector can be coupled to an underwater vehicle, underwater vessel, or the like. 
     The acoustic projector can be designed to be expendable. Accordingly, the acoustic projector can have a lifetime (once submerged underwater) that is based on a lifetime of an expendable device with which it is used (e.g., a sonobuoy, or the like). 
     The acoustic projector can receive water in an internal cavity of its housing to maintain the pressure differential between the internal cavity and a projector face of the acoustic projector in static equilibrium. This may increase a range of water depths for which the acoustic projector can be used. 
     An underwater acoustic projector for projecting acoustic energy through water is described. In one embodiment, the underwater acoustic projector includes a housing including a first plurality of apertures so that an internal cavity of the housing is in fluid communication with an external environment. The internal cavity includes an acoustic baffle disposed at a first end of the internal cavity that fluidly seals a first portion of the internal cavity from a second portion of the internal cavity. The underwater acoustic projector further includes a moving coil and a fixed magnet that are disposed in the second portion of the internal cavity. A projector face is disposed at a second end of the internal cavity, the projector face including a piston disposed in a space between an outer diaphragm and an inner diaphragm, and the piston is connected to the moving coil. 
     A method of designing an underwater acoustic projector is described. In one embodiment, the method includes designing a housing including a first plurality of apertures so that an internal cavity of the housing is in fluid communication with an external environment. The internal cavity includes an acoustic baffle disposed at a first end of the internal cavity that fluidly seals a first portion of the internal cavity from a second portion of the internal cavity. 
     The underwater acoustic projector further includes a moving coil and a fixed magnet that are disposed in the second portion of the internal cavity. A projector face is disposed at a second end of the internal cavity, the projector face including a piston disposed in a space between an outer diaphragm and an inner diaphragm, and the piston is connected to the moving coil. 
     A method of compensating for pressure differences in an underwater acoustic projector is also described. In one embodiment, the method includes submerging the acoustic projector in water such that water flows into an internal cavity of the acoustic projector and contacts a surface of an inner diaphragm of a projector face and the water is in contact with a surface of an outer diaphragm of the projector face. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       References are made to the accompanying drawings that form a part of this disclosure and which illustrate embodiments in which the systems and methods described in this specification can be practiced. 
         FIG. 1  illustrates an underwater acoustic projector. 
         FIG. 2  illustrates a sectional view of the underwater acoustic projector of  FIG. 1 . 
         FIG. 3  illustrates the underwater acoustic projector of  FIG. 1  installed in a pressure hull. 
         FIG. 4  illustrates a sectional view of the underwater acoustic projector of  FIG. 1  installed in the pressure hull of  FIG. 3 . 
       Like reference numbers represent like parts throughout. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an underwater acoustic projector  100  (hereinafter the acoustic projector  100 ). The acoustic projector  100  can be submerged in any body of water, including but not limited to saltwater, freshwater, brackish water, and the like. Submerging the acoustic projector  100  includes submerging a portion of the acoustic projector  100  as well as submerging the entire acoustic projector  100 . 
     The acoustic projector  100  includes a cylindrical housing  105 . The housing  105  includes a plurality of apertures  110 . The apertures  110  are spaced circumferentially from one another about a circumference of the housing  105 . The apertures  110  are designed to permit introduction of water into an internal cavity (internal cavity  200  illustrated in  FIG. 2 ) of the housing  105 . Permitting water into the internal cavity  200  equalizes the pressure between the internal cavity  200  and the water outside of the housing  105 . The housing  105  also includes an outer diaphragm  115  disposed at one end of the housing  105 . The outer diaphragm  115  includes an outer surface  115 A that is in contact with water when the acoustic projector  100  is submerged. The outer diaphragm is a part of a projector face (projector face  215  illustrated in  FIG. 2 ) of the acoustic projector  100 . 
       FIG. 2  illustrates a sectional perspective view of the acoustic projector  100  of  FIG. 1  along a plane that extends along a longitudinal axis of the acoustic projector  100  from the line A-A′. 
     The internal cavity  200  of the housing  105  includes an acoustic baffle  205 , a moving coil apparatus  210 , and a projector face  215 . As illustrated, the internal cavity  200  is divided into three portions  201 - 203  by these features. The moving coil apparatus  210  is disposed between the acoustic baffle  205  and the projector face  215  in portion  202  of the housing  105 . The portion  202  of the housing  105  includes the plurality of apertures  110 . Accordingly, the portion  202  of the housing  105  can be flooded with water when the acoustic projector  100  is submerged. 
     The acoustic baffle  205  is disposed in portion  201  of the housing  105 . The illustrated acoustic baffle  205  includes an air cavity  205 A radially sealed by a gasket  205 B disposed between an outer diaphragm  205 C and an inner diaphragm  205 D. The air cavity  205 A is substantially liquid tight to prevent ingress of water into the air cavity  205 A. When submerged underwater, surface  206 C of the outer diaphragm  205 C and surface  206 D of the inner diaphragm  205 D are both in contact with water. The design of the acoustic baffle  205  is application-specific and can be selected to accommodate particular depth and pressure requirements for the acoustic projector  100 . The acoustic baffle  205  can absorb acoustic energy in order to direct the acoustic energy produced by the acoustic projector  100  in the desired direction and can, for example, be an acoustic sound absorbing rubber. The desired direction is illustrated by the arrow P, which is projecting outward from the projector face  215 . 
     The projector face  215  includes an inner diaphragm  225  and the outer diaphragm  115 , with a piston  220  disposed between the inner and outer diaphragms  225 ,  115 . A space  230  is created between the inner and outer diaphragms  225 ,  115  that corresponds to the thickness t of the piston  220 . The space  230  can be filled with a liquid in order to maintain the space  230  when the acoustic projector  100  is submerged in water. Examples of the liquid include, but are not limited to, oil, water, or the like. The liquid can be added to the space  230  through one or more apertures  232 . The one or more apertures  232  can be sealed by, for example, a setscrew, when oil is used. In such embodiments, the one or more apertures  232  can include an aperture for adding oil and an aperture for air to escape the space  230  when the space  230  is filled with the oil. When the liquid is water, the one or more apertures  232  can allow water into the space  230  as the acoustic projector  100  is submerged. In such a case, the one or more apertures  232  can be designed (e.g., size, number of apertures, or the like) to control the inflow of water. A surface  225 B of the inner diaphragm  225  is in communication with portion  202  of the housing  105 . A surface  115 A of the outer diaphragm  115  is in communication with the water when submerged. In order for the acoustic projector  100  to function properly (e.g., prevent alteration of the frequency), the pressure on the surface  115 A and the pressure on the surface  225 B are maintained in static equilibrium by flooding the portion  202  of the housing  105  with water. 
     Disposed between the acoustic baffle  205  and the projector face  215  is the moving coil apparatus  210 . The moving coil apparatus  210  operates according to principles known in the art and includes a moving coil  210 A and a fixed magnet  210 B. The moving coil  210 A is connected to the piston  220 . Pulses of electricity can be sent through the moving coil  210 A, which rapidly reverses the polarity of its magnetic field. As a result, the moving coil  210 A alternates between being attracted to the fixed magnet  210 B and being repelled by the fixed magnet  210 B. The vibration of the moving coil  210 A causes the piston  220  to vibrate back and forth, generating acoustic energy. 
       FIG. 3  illustrates the acoustic projector  100  of  FIG. 1  in a pressure hull  300 , for example of an underwater vehicle such as an unmanned underwater vehicle. The pressure hull  300  can be part of any underwater body that is fluidly sealed to prevent entry of water such as, but not limited to, an underwater vehicle, an underwater vessel, or the like.  FIG. 4  illustrates a sectional view of the acoustic projector  100  installed in the pressure hull  300  of  FIG. 3  along a plane that extends along a longitudinal axis of the acoustic projector  100  from the line B-B′. 
     The pressure hull  300  is sealed to prevent entry of water into the vehicle of which it is part. The pressure hull  300  has a larger diameter than the housing  105  such that the acoustic projector  100  can be inserted into the pressure hull  300 . The pressure hull  300  includes a cavity face  305  disposed at an end of the acoustic projector  100  including the projector face  115 . The cavity face  305  is securely connected to an internal wall of the pressure hull  300  and the housing  105  of the acoustic projector  100 . The cavity face  305  includes a plurality of apertures  310  for permitting introduction of water into an internal cavity  315  of the pressure hull  300 . Water entering the plurality of apertures  310  can also enter the plurality of apertures  110  on the housing  105  of the acoustic projector  100 . Accordingly, the acoustic projector  100  can rely on pressurization with the water as described above even when installed in the pressure hull  300 . The materials used for the pressure hull  300  can be application specific and may, for example, be selected based on depth and pressure requirements of the application. 
     The terminology used in this specification is intended to describe particular embodiments and is not intended to be limiting. The terms “a,” “an,” and “the” include the plural forms as well, unless clearly indicated otherwise. The terms “comprises” and/or “comprising,” when used in this specification, specify the presence of the stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, and/or components. 
     With regard to the preceding description, it is to be understood that changes may be made in detail, especially in matters of the construction materials employed and the shape, size, and arrangement of parts without departing from the scope of the present disclosure. This specification and the embodiments described are exemplary only, with the true scope and spirit of the disclosure being indicated by the claims that follow.