Abstract:
A sound mitigation system and method reduce the transmission of acoustic output from explosions underwater. The system is submerged to bottom terrain of a body of water and has at a series of gas-generating devices providing volumes of gas that are fed to refractory heat resistant tubing assemblies and flexible general tubing assemblies. The heat resistant and general tubing assemblies extend between adjacent gas generating devices and are provided with at least one row of holes to vent bubbles of gas and form a virtually continuous curtain of bubbles rising to the surface of the water between detonating explosives and areas of interest. The curtain of bubbles mitigates the effects of the explosions on marine mammals and endangered, threatened, or protected species within an area adjacent to the explosions.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a system and method to reduce sound underwater. More particularly, this invention reduces the transmission of acoustic output from underwater explosions to mitigate the effects on marine mammals and endangered, threatened, or protected species within the vicinity of the explosions. 
     Exploding live ordnance during training exercises and testing explosives generally tend to attract unfavorable attention. The intense shock waves that have been created by unrestricted underwater detonations have taken their toll of marine life, and in some cases, steps are being taken to restrict this activity. Particularly when explosives are detonated underwater near the shore and in some other regions in the ocean, environmentalists are alarmed by the effects of explosions on all aspects of the marine habitat. 
     However, an important part of an effective research and development program for national defense purposes is thorough testing of new explosive compounds and ordnance items in their intended underwater applications. Extensive underwater testing often must take place to see if innovative designs can survive in the harsh marine environment and function sell enough to assure successful completion of a mission. 
     Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for a system and method to reduce the acoustic output created by detonations of explosives underwater from reaching areas of interest to mitigate impact on marine mammals, fish, and the marine ecosystem as a whole. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a system and method to reduce the transmission of acoustic outputs caused by detonations of explosives underwater. 
     Another object of the invention is to provide a system and method to mitigate the effect of underwater explosions on marine mammals and endangered, threatened, or protected species within an area adjacent to the explosions. 
     Another object of the invention is to provide a system and method to form a curtain, or wall of bubbles between underwater explosions and areas of concern. 
     Another object of the invention is to provide a system and method to form a curtain, or wall of bubbles from gas generating devices that have gas producing means activated by a control unit. 
     Another object of the invention is to provide a system and method to form a curtain, or wall of bubbles vented from holes in tubing assemblies to extend from the bottom to the surface of a body of water between detonating explosives and areas of concern. 
     Another object of the invention is to provide a system on the bottom of a body of water to percolate a curtain of bubbles upward through the water to mitigate shock waves from detonating explosives. 
     Another object of the invention is to provide a system at the surface of a body of water to drop a wall, or curtain of particulate-like matter, such as sand, to mitigate the shock waves travelling from detonating explosives. 
     Another object of the invention is to provide a system and method to mitigate shock waves from detonating explosives by venting bubbles through a series of holes in elongate tubing assemblies located between the explosives and areas of concern. 
     Another object of the invention is to provide a system and method to mitigate shock waves from detonating explosives on areas of concern by venting bubbles through a series of holes that may be differently sized in elongate tubing assemblies located between the explosives and areas of concern. 
     Another object of the invention is to provide a system and method to mitigate shock waves from detonating explosives on areas of concern by using gas generating devices to create volumes of an environmentally friendly gas and force the gas volumes through tubing assemblies and into ambient water as a curtain of bubbles. 
     These and other objects of the invention will become more readily apparent from the ensuing specification when taken in conjunction with the appended claims. 
     Accordingly, the sound mitigation system and method of the invention reduce transmission of acoustic outputs from explosions underwater. The system is submerged to bottom terrain of a body of water and has at a series of gas-generating devices providing volumes of gas that are fed to elongate tubing assemblies. The tubing assemblies extend between adjacent gas generating devices and are provided with at least one row of holes to vent bubbles of gas and form a virtually continuous curtain of bubbles rising to the surface of the water between detonating explosives and areas of interest. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of the system of the apparatus of the invention for reducing the transmission of acoustic shock waves created by underwater detonations of explosives to areas of interest. 
     FIG. 2 is an isometric schematic view of the invention emitting at least one curtain of gas bubbles through ambient water between exploding explosives and an area of interest. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, sound mitigation system  10  of this invention has a plurality of gas generating devices  20  connected to a plurality of heat resistant tubing assemblies  30  and general tubing assemblies  40 . Sound mitigation system  10  creates a curtain, or wall  12  of bubbles  14  extending along its length to reduce the transmission through ambient water  5  of the acoustic outputs, or shock waves  6  that are created during underwater detonation of explosives  7 . Unmitigated pressures of such shock waves  6  can have an adverse impact on the marine environment in the vicinity, or area adjacent to the detonations. 
     Sound mitigation system  10  can be made in modularized form that each has selected numbers of gas generating devices  20  interposed among elongate heat resistant tubing assemblies  30  and elongate general tubing assemblies  40 . This feature permits serial connection of more that one such systems  10  along mating junctures  10   a  so that a curtain of bubbles  14  can extend for as far as needed with respect to detonating explosives  7  to mitigate acoustic shock waves  6 . Gas generating devices  20  and tubing assemblies  30  and  40  of sound mitigating system  10  can be made negatively buoyant to sink to and stay on bottom terrain, or floor  8  in ambient water  5 , and/or gas generating devices  20  and tubing assemblies  30  and  40  of sound mitigating system  10  have mounting structure, such as spikes  10   b  or weights  10   c  to secure them to bottom  8  during deployment and operation. These features assure that curtain  12  of bubbles  14  being generated by and emitted from system  10  rises and/or percolates upward in a virtually continuous curtain  12  throughout its length. FIG. 2 of the drawings shows bubbles  14  of curtains  12  emitted from system  10  and rising to surface  9  as being uniformly sized and spaced apart to avoid cluttering and confusion in the drawings. In fact, the emitted bubbles  14  form virtually continuous curtains  12  of such bubbles  14 , all of bubbles  14  do expand as they rise toward surface  9 , and the expanded, or expanding bubbles  14  tend to break-up into further multitudes of bubbles  14  as they continue to rise to assure mitigation of acoustic pressure waves  6 . 
     Gas generating device  20  may have different designs in system  10 . Whichever design is selected, it must be capable of generating and/or emitting sufficient volumes of gas to feed the interconnected tubing assemblies  30  and  40  to percolate a sufficiently dense curtain  12  of bubbles  14  upward through water  5  to mitigate shock waves  6 . In shallow water, or a near shore application, gas-generating device  20  can be a manifold structure receiving pressurized gas through hoses (not shown) extending to a commercial air compressor or bank of air tanks (not shown) located at surface  9  of water  5 . Optionally, each gas-generating device  20  can be compressed air tanks and valve mechanisms submerged at bottom terrain  8  and connected in-line with tubing assemblies  30  and  40 . 
     Gas generating devices  20  are more likely to be in the category of pyrotechnic devices; however, some other chemically reacting compounds that produce gasses could be used. Each gas generating device  20  can have a can-shaped shell  21  of a ferrous or composite material that has sufficient strength and refractory properties to contain a burning solid propellant charge  22  in shell  21 . Each propellant charge  22  is initiated, or activated by activation signals over lines  23  connected to it from remote control unit  25  to burn and produce volumes of gas for bubbles  14 . The activation of propellant charges  22  can be simultaneous or selective depending on the desired effect. Simultaneous actuation of propellant charges  22  in gas generating devices  20  produces a continuous curtain  12  of bubbles  14  along the length of sound mitigating system  10 . 
     The activation signals can be electromagnetic signals on electrically conducting lines  23  to initiate explosive or pyrotechnic squibs  22   a  at propellant charges  22 . The activation signals can be mechanical vibrations or displacements (tugs, or jerks) on cord-like lines  23  from remote control unit  25  to trip a trigger mechanism  22   a ′ at propellant charges  22  to strike a percussion cap and fire propellant charge  22 . Remote control unit  25  can be at a command terminal (not shown), or control unit  25  can be on bottom  8  and a control lead  28  can extend to command terminal that is located at a safer distance from explosives  7  to activate control unit  25 . 
     Each gas generating device  20  contains an amount of propellant charge  22  to generate sufficient volumes of environmentally friendly gases to create a portion of curtain  12  of bubbles  14  via heat resistant tubing assemblies  30  and general tubing assemblies  40 . Pressures of the generated gases from gas generating devices  20  that force, or vent the gases through holes  31  and  41  of assemblies  30  and  40  create bubbles  14  of curtain  12 . The density of bubbles  14  of curtain  12  produces a sound-mitigating barrier, or attenuator for acoustic outputs caused by detonating explosives  7 . 
     Many compounds are available that can be selected for propellant charges  22  that burn at controlled rates to generate controlled volumes of gases over a useful period of time and will not violently explode and tear apart system  10 . This useful period of time is at least the time is takes to create curtain  12  of bubbles  14  that extends from system  10  at bottom  8  to surface  9  of water  5  and, during which, detonation of explosives  7  occurs. Among the many that might be selected, typical compounds for propellant charge  22  have properties of burning at a slow rate, e.g. under seven thousand meters per second (but most likely under a couple of feet per second), producing high gas volumes per unit volume of compound, and burning relatively cool, e.g. in the range of eight to nine hundred degrees Fahrenheit. One such compound for propellant charge  22  is sodium azide. Other compounds that may have different rates of burning, gas volume productions, and burning temperature can be selected as well. 
     Each side of each of gas generating devices  20  is securely connected to a heat resistant tubing assembly  30 . Each heat resistant tubing assembly  30  is comprised of a tube made from heat resistant material, either metal or composite that has sufficient weight to remain on bottom  8  and not be buoyed upward even when it is filled with gas generated in adjacent gas generating devices  20 . The refractory material of each heat resistant tubing assembly  30  allows for transition of the hot gases between each of gas generating devices  20  and each general tubing assembly  40  and thereby protects each general tubing assembly  40  from the high temperature gases emitted from each of gas generating devices  20  as propellant charge  22  is being burned. In addition, each heat resistant tubing assembly  30  is perforated to have at least one row of holes  31  on its upper half to vent a first portion of gases generated by gas generating devices  20  as bubbles  14  in part of curtain  12 . 
     A general tubing assembly  40  is connected to each heat resistant tubing assembly  30  to transmit and vent other, or second portions of the gases generated by gas generating devices  20  as bubbles  14  in another part of curtain  12  of bubbles  14 . The vented bubbles  14  of curtain  12  from general tubing assemblies  40  along with vented bubbles  14  from heat resistant tubing assemblies  30  complete the length of curtain  12  of bubbles  14  in a sound-mitigating region. Each general tubing assembly  40  has at least one row of holes  41  on its upper half to vent the second portions of the gases generated by gas generating devices  20  as part of curtain  12  of bubbles  14 . Holes  41  may be aligned with holes  31 . 
     The sizes and spacing of holes  31  and  41  depend on depth of water  5  where sound mitigating systems  10  are situated, the velocity of currents in ambient water  5 , the frequencies of acoustic output from detonating explosive  7 , and the depth of explosive  7 . Under some scenarios, the distribution and sizes of holes  31  and  41  might be altered along the lengths of heat resistant tubing assemblies  30  and general tubing assemblies  40 , respectively. The different distributions and sizes of holes  31  and  41  may be provided for in each heat resistant tubing assembly  30  and each general tubing assembly  40  by adjustable shutters  51  in holes  31  and  41  to provide different densities of bubbles  14  in curtain  12  of bubbles  14  as a particular need or situation may dictate. 
     Each general tubing assembly  40  is made of flexible material, such as rubber, PVC, or combination of material (i.e. fire hose). Gas generating devices  20 , heat resistant tubing assemblies  30 , and general tubing assemblies  40  of sound mitigating system  10  have sufficient combined weight or can be sufficiently weighted to keep sound mitigating system  10  in place on bottom  8  after it has been deployed in water  5  and while gases from gas generating devices  20  fill it and are being vented through it. The flexible material of each general tubing section  40  is flexible enough to be bent into a desired shape by tools and/or divers. This feature allows shaping of general tubing assemblies  40  and modification of the overall configuration of sound mitigating system  10  along its length for different attenuation tasks. 
     The diameters of each heat resistant tubing assembly  30  and general tubing assembly  40  are determined as a function of the net weight, composition, and rate of burning of propellant charge  22 . The dimensions of each heat resistant tubing assembly and general tubing assembly  40  also are determined to take into account the depths at which sound mitigating system  10  is deployed and the velocities of currents in ambient water  5 . The frequencies to be mitigated from the acoustic outputs from detonating explosive  7  and the depth of explosive  7  are other factors to take into consideration when dimensioning heat resistant tubing assemblies  30  and general tubing assemblies  40 . 
     Gas generating devices  20 , heat resistant tubing assemblies  30  and general tubing assemblies  40  can be assembled and coupled together as sound mitigating system  10  at a distant depot, on a vessel on surface  9  above submerged explosives  7 , or on bottom  8  by a team of divers. The assembled sound mitigating system  10  can be arranged in a variety of configurations to mitigate acoustic outputs from detonating explosives  7 . The drawings show sound mitigating system  10  shaped in a semicircle, or crescent-shape to the right of explosives  7  on bottom  8 . This shape creates a semicircular or crescent shaped curtain  12  of bubbles  14 . If desired, sound mitigating system  10  could be formed as a complete circle spaced from and around explosives  7 , straight line, or any other practicable shape to mitigate the shock waves  6  coming from detonating explosives  7  so that volumes, or areas of interest on the other side of sound mitigating system  10  do not receive the same intensities. 
     More or less gas generating devices  20 , heat resistant tubing assemblies  30 , and general tubing assemblies  40  are coupled together until the desired length is reached for different configurations or lengths for different sound mitigating systems  10  to accommodate different explosives  7  and/or areas. Then, flexible general tubing assemblies  40  are selectably bent until the desired shape is reached. Suitable mating couplings  24 ,  34 , and  44  can be provided at the ends of gas generating devices  20 , heat resistant tubing assemblies  30 , and general tubing assemblies  40  to permit expedient tailoring of sound mitigating system  10 . These couplings  24 ,  34 , and  44  can be mating male-and-female threaded structure, standard quick-connect pipe fitting structure, etc. Gas generating devices  20  at the ends of sound mitigating system  10  have their ends  20   a  closed. 
     FIG. 2 shows semicircular, or crescent-shaped sound mitigating system  10  next to a virtually identical sound mitigating system  10 ′ to increase mitigation of acoustic pressure waves  6  coming from detonating explosives. The areas, or volumes to the right of these systems  10 ,  10 ′ will be subjected to mitigated pressures. More sound mitigation systems  10 ,  10 ′ that might be differently shaped can be layered, or spaced apart from one another depending on the extent of mitigation practicably desired and the net explosive weight of the explosives  7 . A common control unit  25  can be connected to all gas generating devices  20  of all systems  10 ,  10 ′, etc. 
     The layered sound mitigation systems  10 ,  10 ′, etc. create a plurality of layered virtual curtains  12  of bubbles  14  that extend from bottom terrain  8  of water  5  to surface  9 . The plurality of curtains  12  of bubbles  14  are layered with respect to one another between detonating explosives  7  and areas of interest located to the right of the systems. 
     Having the teachings of this invention in mind, modifications and alternate embodiments of sound mitigating system  10  may be adapted. Its uncomplicated design lends itself to numerous modifications to permit its use in the hostile marine environment. For examples, the constituents of sound mitigating system  10  can be made larger or smaller to produce effective curtains  12  of bubbles  14  at different operational depths of deployment and be fabricated from a wide variety of materials to assure sufficient strength and long term reliable operation under adverse operational conditions. Gas generating devices  20  could have different shapes, and couplings  24 ,  34 , and  44  could have different arrangements of different numbers of differently shaped structural members to engage devices  20  and tubing assemblies  30  and  40 . Control unit  25 , squibs  22   a  and trigger mechanisms  22   a ′ could be modified to be actuated by acoustic or magneto-electric signals. 
     A modification of sound mitigation system  10  could utilize sand, or other heavier-than-water particulate matter that is dropped through water  5  in a virtually continuously extending curtain to mitigate acoustic pressure waves. The constituents of the embodiment of sound mitigating system  10  shown in FIG. 1 could be modified to be buoyant and float at surface  9 . The components designated  20  could be containers of particulate-like matter, such as sand, for example. The sand could by fed from containers  20  to tubing assemblies  30  and  40  by conveyor-like mechanisms, or compressed gas feeder arrangements, for examples. Grains of sand would be dropped through holes  31  and  41  that are now oriented to be downwardly facing. The curtain of sand, or other particulate-like matter falling through water  5  will serve to mitigate the acoustic pressure waves coming from exploding explosives  7 . 
     The disclosed components and their arrangements as disclosed herein, all contribute to the novel features of this invention. Sound mitigating system  10  is an uncomplicated, cost-effective, system that can be deployed, left in place for prolonged periods of time, and be reliably used when needed at a later date. Therefore, sound mitigating system  10 , as disclosed herein is not to be construed as limiting, but rather, is intended to be demonstrative of this inventive concept. 
     It should be readily understood that many modifications and variations of the present invention are possible within the purview of the claimed invention. It is to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.