Abstract:
A high intensity, low frequency underwater transducer for non-lethal deterrence of terrorist swimmers or divers in a body of water. The invention consists of a motor driven flextensional underwater transducer. In one embodiment, the phase of a transducer is sensed, enabling multiple projectors to achieve high acoustic sound pressure levels by beamforming and/or modal constructive interference (e.g. taking advantage of harbor bottom topography and boundaries.).

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims rights under 35 USC§ 119(e) from U.S. patent application Ser. No. 60/786,413 filed Mar. 27, 2006, the contents of which are incorporated herein by reference. 

   this invention was made with United States Government support under Contract No. N00014-06-C-0101 awarded ty the Office of Naval Research. The united States Government has certain rights in the invention. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to underwater sound and more particularly to high power acoustic transducers (projectors) for applications such as non-lethal deterrence of terrorist swimmers and divers. 
   2. Brief Description of Prior Developments 
   Non-lethal swimmer and diver engagement is of increasing importance in today&#39;s threat environment because many potential terrorist targets are in areas accessible to recreational boaters or swimmers who may have no malevolent intent. The potential proximity of marine mammals also necessitates non-lethal methods. 
   Modern detection sonar systems are able to differentiate between marine mammals, large fish, swimmers and divers through their signature and track. They cannot, however, discern the intentions of a human in the water. Thus there is a need for a graduated system of engagement, beginning with audible warnings, sirens, etc. that should cause the casual intruder or marine life to turn away. 
   The later stages of engagement require a method that effectively incapacitates the intruder without lethal force, since there remains the possibility that they could be demonstrators, not terrorists. The ideal method would cause divers to surface where they could be dealt with by more conventional means. 
   The parameters of an ideal deterrent may be summarized to include effectiveness, high reliability, not being easily countered, using a graduated force level; non-lethality, affordability, and having size, weight and power source requirements appropriate to the application. 
   Short of developing the equivalent of a rubber bullet for underwater use, the candidates for non-lethal underwater deterrence are light and sound. Both can create psychophysical and/or physiological effects. Light, however, suffers from short propagation distances in the turbid water typical of many harbors and rivers. It is easily countered and does not work at all in the most turbid water. 
   High-intensity, low frequency sound is useful as a non-lethal means for deterring swimmers and divers who may be terrorists. The psychophysical acoustic interactions proposed to be exploited include annoyance/aversion (avoidance of a loud sound) and/or cognitive/functional task impairment (physical symptoms). 
   The physiological (based on frequency and sound pressure level (SPL) dependent thresholds) effects of low frequency sound are hearing (up to 160 dB SPL=minor effects) including auditory pain threshold ˜220 dB SPL, vestibular function (dizziness, rotation of visual field), and bronchopulmonary resonance (coughing, gagging, choking, pain). 
   It is difficult to defend against low frequency sound unless one is inside a rigid body such as a vehicle. Thus, resonance of the lungs is an ideal candidate for the deterrent method. Experimental evidence suggests that the nominal resonance frequency of the human lung is about 20 to 70 Hz and is depth dependent. The in situ damage threshold to mice and guinea pig lungs is reported to be about 180 dB SPL. 
   What is needed for an effective deterrent for underwater terrorists, therefore, is a relatively inexpensive, high power, low frequency source of underwater sound. 
   SUMMARY OF INVENTION 
   The invention consists of a motor driven flextensional underwater transducer. In one embodiment, the phase of a transducer is sensed, enabling multiple projectors to achieve high acoustic sound pressure levels by beamforming and/or modal constructive interference (e.g. taking advantage of harbor bottom topography and boundaries.) 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is further described with reference to the accompanying drawings wherein: 
       FIG. 1  is an illustration in a perspective view of a prior art flextensional transducer; 
       FIG. 2  is an illustration in a perspective view of a preferred embodiment of the transducer of the present invention; 
       FIGS. 3 and 4  are illustrations of the mode of operation of the present invention; 
       FIG. 5 . is a more detailed illustration of the motor controller, motor and cam of the invention; 
       FIG. 6  is an illustration of the cam and cam follower portion of the invention as viewed along the axis of the motor shaft; 
       FIG. 7  is an illustration of an alternate embodiment of the cam and cam follower; 
       FIG. 8  is an illustration of another embodiment of the invention which includes a mechanism for adjusting the sound pressure level of the invention as well as controlling the phase of the sound wave; and 
       FIG. 9  is an illustration of multiple transducers of the present invention in a harbor installation intended for protection of a moored vessel. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   As described above, sound suitable for deterring swimmers and divers must be low frequency (down to 20 Hz), high acoustical sound pressure level (SPL&gt;195 dB re 1 μP @ 1 m), and the projector must be able to produce this sound in shallow water (as little as 25 ft deep). Historically, piezoelectric or magnetostrictive projectors capable of meeting these requirements have been expensive to produce in large part due to the volume of high cost ceramic or magnetostrictive material required. In addition, large and expensive power amplifiers are required to drive such transducers. 
   Of particular concern is the need to avoid cavitation which is potentially damaging to the projector. The acoustic output level that will induce cavitation decreases with decreasing depth. This projector must operate at very shallow depths. The cavitation threshold also decreases with operating frequency. The operating frequency range of this projector is very low. In order to avoid cavitation, one can increase the acoustic radiating area i.e. make the projector bigger. This increase, of course, will increase projector weight. Thus a phase synchronized array of projectors may be required to achieve the desired output. 
   The present invention is a low-cost projector that provides the performance described in the concept description above. To summarize, the projector preferably has the following technical characteristics:
         Output Waveform: Continuous Transmission of a Single Tone   Operating Frequency Range: 20 Hz to 200 Hz   Sound Pressure Level (SPL):
           Greater than 195 dB re 1 μP @ 1 m   Variable from 150 dB re 1 μP @ 1 m to full rated SPL.   
           Output Phase Control: Accuracy less than 1 degree.   Maximum Transmit Duration: Greater than 10 minutes   Operating Depth: 25 ft to 75 ft   Dry Weight of Deployable Components: Less than 500 lbs.       

   Maximizing acoustic sound pressure level is of primary importance for an effective deterrent. 
     FIG. 1  is a schematic drawing of a conventional Inverse Flextensional (Class-VII) transducer shell—an efficient sound radiator with minimal size and weight. It is typically made of aluminum and is suitable for use in practicing the method of the present invention. Other classes of flextensional shells may also be used. 
   Shell  1  is driven by a “stack”  3  of piezoelectric or magnetostrictive elements. By applying an AC voltage to stack  3 , typically on the order of 2500 Volts, the length of the stack changes, causing the thinner sides of shell  1  to move at the AC drive frequency, but at an amplified displacement compared to the length change. 
   Because cost is very important for many deterrence applications, the cost of the power amplifier required for the transducer of  FIG. 1  (˜$30,000 for a device with the requisite output power), when added to the cost of the piezoelectric or magnetostrictive stack make this approach undesirable. 
     FIG. 2  is an illustration of an affordable, low frequency, high sound pressure level (SPL) source that meets all the criteria outlined above. Flextensional shell  2  is similar to that of  FIG. 1 , with the addition of opposed interior arms  10  and  12  extending inwardly from the exterior shell to inner terminal ends. Rotating cam  14  is in contact with the inner terminal ends With a variable speed motor (not shown in  FIG. 2 ), cam  14  provides the alternating force to drive the shell in a manner similar to the stack of a conventional transducer. By using a motor and cam in place of the power amplifier and stack, cost can be dramatically reduced. Such motors and controllers are available as relatively low cost Commercial Off-the-Shelf (COTS) products, even in low quantities. 
     FIGS. 3 and 4 , illustrate this in more detail. In  FIG. 3 , cam  14  is shown just beginning to force arms  10  and  12  in an outward direction, with the net result that the sides of shell  2  apart. As the cam rotates farther ( FIG. 4 ), arms  10  and  12  move closer together, resulting in the sides of the shell moving centrally. When shell  2  is immersed in water, the rotating cam thus creates an alternating movement of the shell, and thereby produces a sound wave. 
     FIG. 5  illustrates further details of the invention. Arms  10  and  12  are moved by cam  14  shown in longitudinal cross-section. Cam  14  is rotationally driven by motor  16  via shaft  18 . For clarity, details of bearing, seals, etc. that are required for implementation of the invention are not shown. Such details are obvious to those skilled in the art of mechanical design. 
   Motor  16  is connected to motor controller  20  by connection means  22 . Motor  16  may be a rotary electrical motor or a rotary air motor. Motor controller  20  has means for maintaining a constant speed of shaft  18  as well as means for varying the rotation speed. The rotational speed determines the rate of flexure of shell  2  and thereby the frequency of the sound wave. 
     FIG. 6  illustrates cam  14  in transverse cross-section in additional detail. Arms  1  and  12  preferably remain in contact with cam  14  throughout the rotational cycle. 
     FIG. 7  illustrates an alternate embodiment. Cam  20  in this case is a four-lobed cam producing a vibrational frequency of shell  2  at twice rate of cam  14 . 
     FIG. 8  illustrates a preferred embodiment. In this embodiment, arms  10  and  12  have a taper to match that of tapered cam  30 . Cam  30  may have a simple cross-section similar to cam  14  or may be a multi-lobed cam in cross-section similar to cam  20 . Adjusting mechanism  32  and  34  provide means to adjust the position of cam  30  into the space between arms  10  and  12 . As the cam is moved further into this space, the amplitude of motion of the arms and thereby shell  2  is increased, thus increasing the output sound pressure level of the transducer. Note that shaft  18  is free to rotate cam  30 , independent of adjusting mechanism  32  and  34 . Again, mounting brackets, seals and other mechanisms are not shown for clarity. These details are obvious to one skilled in the art of mechanical design. 
   In  FIG. 8 , sensor  42  is attached to the distal end of shaft  18 . This sensor is used to detect the rotational position of shaft  18  and thereby the rotational position of cam  30 . Sensor  42  may be a potentiometer, digital encoder or other type of rotational sensor. The rotational position of cam  30  is communicated to controller  44  by connection  46 . Controller  44  thus not only knows the position of cam  30  at any instant, but can determine the rotational speed of the cam. This information may be sent to variable speed motor controller  20  via communication means  48 . Thus, a “closed-loop” system has been disclosed, whereby both the frequency and phase of the sound wave emanating from the transducer may be kept constant or adjusted as required. 
     FIG. 9  illustrates an example of ship protection in a harbor using two systems such as those of  FIG. 8 . The harbor perimeter  50  has an inlet  52  for access. Ship  54  is shown moored to a pier in the harbor. It is desired to create a high sound intensity in region  60  of the harbor to thwart potential waterborne swimmer or diver terrorists. By placing two transducers  2  in the correct locations, controlling their phase and amplitude jointly by master controller  56  through their individual controllers  44 , the sound waves produced may be in phase in region  60  and thereby of greater magnitude that that of a single transducer. 
   Additional transducers may be used, employing methods known as beamforming. In addition, certain properties of the topography of the harbor floor may be taken in to account to provide maximum sound pressure levels at desired locations. These methods for employing multiple transducers are well known to those versed in underwater acoustics. 
   Those skilled in the art will also appreciate that this transducer is a low-cost solution for systems that can be deployed from different platforms such as a pier facility; large ship; small boat and unmanned underwater and surface vehicles. The size, weight and power source of the method and apparatus of the present invention are applicable to piers and ships. Versions suitable for small boats are also possible. 
   While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.