Abstract: A countermeasure device which is supplied with air in order to actively compensate for external water pressure at any depth, is disclosed. The countermeasure includes an air bag hover system and one or more acoustic projectors, both of which are connected by an air line to a high pressure air source. A sensor is provided which measures the external water pressure and transmits the measurements to a microprocessor. Upon sensing an increased external water pressure, a pressure regulator valve is opened and air is supplied to both the acoustic projectors and the air bag in order to compensate for the increased water pressure. Similarly, if the external water pressure decreases, then air is released from both the air bag and the acoustic projectors through a pressure vent valve connected to the air bag. The microprocessor can also be programmed to vary the deployment depth of the device by varying the internal pressure within the device relative to the external water pressure.

Abstract: An underwater noise generator has a submerged housing containing a receiver section responsive to remotely originating acoustic signals or magneto-inductive signals in the ELF to VLF range. The submerged housing contains a composition that reacts with water to produce gas. The signals initiate an explosive squib that blows a lid off the housing and penetrates a wall that covers the composition. Water floods into the housing and onto the composition which produces gas that creates bubbles. The bubbles are buoyed from the noise generator to the surface and, in so doing, they produce noise. Underwater noise generators can be used singularly, in multiples, or in various patterns as needed to conceal activities or otherwise deceive remote listeners. Appropriately coded magneto-inductive control signals in the ELF to VLF range are transmitted from a variety of remote sources through the sea, air, vegetation, and sediment or any combination of these conditions to activate the underwater noise generators.

Abstract: An acoustic transmission in an underwater environment having a spectrum (amplitude/frequency signature) of interest is generated by cyclically releasing energy which forms impulses which individually have spectral amplitudes principally near one end of the desired spectrum, but which impulses are repeated at a rate near the opposite end of the desired spectrum, which rate may be varied to smooth the spectral response. A multiplicity of impulses may be generated synchronously, the number depending upon the desired amplitude of the spectrum. A multi-port air gun having a stator with a series of ports, which are valved by a rotor coaxial with the stator provides the energy in the form of the release of compressed air bubbles. The rate at which the impulses are produced is controlled by controlling the speed of rotation of the rotor.

Abstract: A plurality of Helmholtz resonators with varying physical characteristics are mechanically coupled within a structure to drastically reduced acoustic transmission through the structure or along a pipe. These mechanically coupled resonator structures produce much flatter broadband acoustic attenuation than uncoupled resonators.

Abstract: A countermeasure buoy for use in an underwater environment includes a main ody portion, a propulsion portion, a hull section, and an acoustical array. The main body portion includes a first end and a second end longitudinally opposed from the first end. The propulsion portion includes a main body end connected to the first end of the main body portion and a propelling end opposite the main body end. The hull section includes a rearward end connected to the second end of the main body portion and an open end opposite to the rearward end. The acoustical array is housed within said hull section and includes a base end connected to an interior of the hull section and a free terminal end opposite to the base end. A cap member is removably attached to the open end of said hull section and a vertical alignment of the propulsion portion, the main body portion and the hull portion enables gravitational displacement of the cap member and release of the acoustical array from the hull section.

Abstract: An acoustic generating system is described incorporating a bubble generator (1) for forming pressurised gas bubbles and directing the bubbles in a plume from an outlet (7), and a low frequency sound source (2) to apply a varying pressure to the base of the plume of bubbles to modulate (8) the plume of bubbles, whereby a source of low frequency sound waves is produced. A method for generating and transmitting a low frequency acoustic wave using is also described.

Abstract: Sonar echoes of length s/c (cos .theta..sub.i +cos .theta..sub.r)+T are slated by transmitting properly timed replicas of an illuminating signal from a number of projectors spaced along a cable of target length s. Because of the length of this cable, magnetic anomaly effects can be simulated by passing a current through a conductor within it.

Abstract: A very low frequency augmentation system consists of a mobile target very w frequency projector (VLFP) adapted for operation as an augmentation device and installed within a torpedo shell housing. The housing is loaded within the torpedo tube of a submarine. A control and power interface is provided between the housing and the submarine to operate and monitor the augmentation device from the submarine. The housing is loaded in one of the submarine's torpedo tubes and the tube is flooded. The VLFP transmits low frequency vibrations into the surrounding seawater, acting as an augmentor.

Abstract: A method is presented to reduce the quantity of microbubbles having diameters of approximately 1000 microns or less in seawater as a means of ship wake signature suppression. Ultrasonic acoustic energy is projected into a volume of seawater in which the microbubbles reside, e.g., a ship's wake. The ultrasonic acoustic energy has a constant frequency selected from the range of approximately 0.5-2.5 MHz.

Abstract: A sound absorption device in accordance with this invention includes an enclosure loosely containing hollow beads. Preferably, the hollow beads have openings, and differ with respect to the volume of the hollow space defined by the walls of the beads, the wall thicknesses of the beads, the size of the openings of the beads and/or the number of openings on the beads. Through Helmholtz resonance, vorticity and tortuous path scattering, the beads damp sound waves over a broad range of frequencies. Preferably, the beads are formed from a material able to withstand relatively high temperatures, such as ceramic or nickel. The beads can thus be used to damp sound in applications which generate significant heat in addition to sound, such as aircraft engines, rocket motors or other loud, heat-generating sources, for example. Also, by forming the beads from a material with a relatively low thermal conductivity, the beads can also be used for thermal insulation in addition to sound absorption.

Abstract: A multi-technology acoustic energy barrier and absorber is disclosed that employs the teaching of U.S. Pat. No. 5,400,296 for a matrix material with at least two species of particles with differing characteristic acoustic impedances, in combination with the teaching of U.S. patent Pending Ser. No., 08/780,271, for an "Acoustic Absorption or Damping Material with Integral Viscous Damping," and with the principles underlying constrained-layer dampers.

Abstract: Acoustic absorption or vibration damping materials are produced by mixing at least two species of particles into a restricted amount of matrix material in order to produce an acoustic absorption or vibration damping material with tortuous passageways throughout the material. The tortuous passageways of the instant invention serve to: a) reduce acoustic reflectivity at the surface, b) provide channels within which the interfacing medium such as air can interact viscously, c) increase the surface area between interfacing media and, d) improve structural stiffness by adding thickness without adding weight. Particle species within the matrix material are differentiated by their acoustic impedances. A particle species of particular interest is crumb tire rubber from used tires. This material is inexpensive and its use in this application has the societal advantage of making productive use of a material that is currently polluting the environment.

Abstract: An apparatus and method for producing inverse bubbles is disclosed. One embodiment of such apparatus includes a first solution in a container and a second solution in a reservoir above the container. The reservoir has a dispensing aperture and while dispensing drops of the second solution into the container, the aperture and the surface of the first solution are maintained at a substantially constant distance from one another. Another embodiment of the apparatus involves a gun-like device dispensing drops using projectile force. The method involves producing inverse bubbles using an aqueous first solution and an aqueous second solution. Each solution includes a substance for reducing water surface tension. A drop of the second solution is dispensed into the first solution at a velocity such that the drop impacts the surface with a force slightly in excess of the force component of the surface tension of substantially pure water. A liquid containing an inverse bubble is thereby produced.

Abstract: A method for reconstructing the acoustic field on the surface of a vibrating object based on measurements of the radiated acoustic pressure includes solving the Helmholtz equation directly using the expansion of a set of independent functions that are generated by the Gram-Schmidt orthonormalization with respect to the particular solutions to the Helmholtz equation. The coefficients associated with these independent functions are determined by requiring the assumed form of solution to satisfy the pressure boundary condition at the measurement points. The errors involved in these coefficients are minimized by the least squares method. Once these coefficients are specified, the acoustic pressure at any point, including the source surface, is completely determined.

Abstract: A panel spacer is disclosed that enhances the natural damping characteristics of the material used for construction. The spacer is shaped to redirect acoustic and vibrational energy into a direction parallel to the panel surfaces and to take advantage of the faster energy propagating through the material of the spacer relative to surrounding gasses. Energy propagating through the spacer will arrive at a mid-point of the spacer well before sound originating from the same source can reach the same point through air or another gas. The phase discrepancy between the induced displacements of the spacer and the surrounding gas is used to dissipate energy by causing the spacer to interacting viscously with the gas.

Abstract: There is disclosed a digital frequency synthesizer which converts an ultrnic burst from an echo-ranging sonar system into a continuous signal having a frequency exactly equal to the frequency of the cycles making up the burst. This burst is detected and a number representing the period of this burst generated and transmitted to a first counter. A clock pulse train is generated and transmitted to a second counter. When the numbers in the first and second counters coincide, a single output pulse is generated and the second counter reset. When this cycle is repeated, the frequency of the resultant pulse train reflects any change in frequency of the cycles making up the bursts.

Abstract: A method and apparatus for use by a personnel in combat unit to assess in bstantially real-time the threat to a combat unit posed by one or more threat projectiles. A command and control system receives a variety of input data and information signals and generates corresponding signals directed to a processing module. The processing module receives and processes the informational signals indicative of the status and characteristics of the combat unit and threat projectile. The processing module determines whether sample threat projectiles with an uncertainty region associated with each threat projectile detects the combat unit and determines therefrom a probability that the actual threat projectile detects the combat unit. The processing module also generates a probability of detection signals, to generate a report in user perceivable form, indicative of the probability of the threat projectile detecting the combat unit on a substantially real-time basis.

Abstract: Acoustic absorption and vibration damping materials are produced by mixing electrically conductive particles or strands into a piezoelectric matrix material. The electrically conductive particles or strands act as small localized electrical short-circuits within the matrix material and effectively dissipate the electric charges produced by piezoelectric effect from the pressure of acoustic or vibrational energy as heat. All energy thus converted into heat is subtracted from the original acoustic or vibrational energy, resulting in acoustic absorption and/or vibration damping.

Abstract: Underwater sound from a marine vessel driven by one or more propellers is damped by introducing air and/or other gas into the vessel's propeller flow(s) so that the turbulence of the propeller flow(s) causes mixing of the air/gas and water and disintegration of gas bubbles. The majority of the gas bubbles formed in the water are from 1 to 20 mm in diameter.

Abstract: An anechoic and decoupling coating for use on the surface of an underwater tructure for decoupling structural vibrations from the water and for absorbing waterborne acoustic waves directed toward the structure from an external source such as a sonar. The coating is an elastomeric matrix containing sealed air-filled cavities as well as random labyrinths of small water-filled passages running throughout and in open communication with a surface facing the water. Acoustic waves incident upon the water-facing surface cause time varying shear and bulk deformations within the matrix. As a result of these deformations, acoustic energy is dissipated by hysteretic damping of the elastomeric matrix as well as by viscosity due to water movement to and fro within the passages and into and out of the matrix.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.

Abstract: A composite for use on submarines and surface craft for controlling self-erated noise when listening with sonar. The composite includes two layers of PVF.sub.2 transducers separated by a layer of phase shifting or absorbing material. The inner transducer senses noise from the ship and subtracts this from the signal from the outer transducer representing noise plus the desired signal. In a second mode the sensed noise is regenerated through the outer transducer 180.degree. out of phase to cancel the noise and allow more accurate detection.

Abstract: A method and apparatus for acoustic energy cancellation in waveguide tube assemblies which enables monolithic construction thereof.

Abstract: A means of enhancing the acoustic attenuation and vibration damping of a material by embedding high characteristic acoustic impedance particles, low characteristic acoustic impedance particles, or both high and low characteristic acoustic impedance particles within the matrix of the material is disclosed. The mass of the resultant material may be very low while retaining excellent acoustic attenuation, vibration damping, and structural characteristics. When particles with mismatched characteristic acoustic impedances are embedded within the matrix of a material that can support shearing loads, propagating acoustic energy that encounters the particles of the instant invention is partially reflected in random directions. That is, the propagating energy is diffused. As the propagation vectors and modes of acoustic energy are effectively randomized, the probability of localized energy absorption and damping is increased.

Abstract: An apparatus for reducing acoustic radiation from an enclosure containing a fluid includes one or more vibration sensors in communication with surfaces of the enclosure. The vibration sensors feed signals corresponding to detected vibrations in the surface to a radiation filter. The radiation filter assigns weights to the signals and generates a summation signal which is then input to a control unit, with the summation signal ideally representing only those vibrations that will actually radiate from the enclosure. The control unit uses a reference signal and the summation signal to calculate a cancellation waveform to offset the cause of the detected vibrations. The cancellation signal is input to a fluid displacement unit which applies pressure oscillations to the fluid corresponding to the cancellation waveform.

Abstract: A system and method are provided to reduce a structure's far field acoustic adiation signature. A plurality of acoustic sensors are positioned in the acoustic near field of the structure for measuring the near field acoustic radiation of the structure. A programmable controller generates a prediction of the structure's far field acoustic radiation signature using the near field acoustic radiation. The prediction is then used to generate a noise control signal applied to a plurality of acoustic radiators suspended within the structure. The radiators convert substantially all of the noise control signal to airborne acoustic energy within the structure.

Abstract: A method of detecting the onset of turbulence in connection with a body mng through a fluid medium and of reducing body drag and as well reducing the amount of drag noise radiated from the body. First, the body is supplied with sensors each for generating a signal suitable for measuring amplitude of pressure fluctuations of the medium proximate a region of said sidewall of the body in at least a region of the body in which turbulence is expected to occur. During a reference stage during which the body moves through the fluid medium when it is known that turbulence is occurring around at least a portion of said body, the sensors each generate reference temporal pressure data representing fluctuations in pressure of the fluid medium around said body.

Abstract: Electromagnets are disposed in an elastomeric coating affixed to a body esed to a fluid. Controllable magnetic forces created by the electromagnets produce a wavy-wall configuration in the surface of the coating which may be static, dynamic, or static in part and dynamic in part, and which interfaces with the fluid to controllably alter noise, turbulence and drag characteristics of the body mechanically. Electrodes may be fixed on the surface of the coating to produce electric force fields for interacting with the magnetic fields and the wavy-wall, to produce magnetohydrodynamically generated Lorentz forces which controllably further alter noise, turbulence and drag characteristics of the body.

Abstract: An explosive actuated acoustic device emits sound to be used in torpedo countermeasures. Numbered devices are delivered over an extended area and sink through the water. The devices are actuated at different times as they sink, to provide sound masking over an extended period of time. The devices also include safety devices which prevent premature actuation from jarring or jolting and from impact with the water.

Abstract: The present invention relates to acoustic and vibration attenuation compoe material, particularly to damping tiles of the Type VI class 1, suitable for use in aircraft, ship and submarine applications, that is inexpensive, does not result in an unacceptable weight penalty, and is conformable-in-place to complex curvatures. The present acoustic and vibration attenuation composite comprises a constraining layer of at least one ply of graphite fabric together with curable phenolic resin bonded to a viscoelastic layer.

Abstract: A method of detect the onset of turbulence in connection with a body moving hrough a fluid medium. First, the body is supplied with sensors each for generating a signal suitable for measuring amplitude of pressure fluctuations of the medium proximate a region of said sidewall of the body in at least a region of the body in which turbulence is expected to occur. During a reference stage during which the body moves through the fluid medium when it is known that turbulence is occurring around at least a portion of said body , the sensors each generate reference temporal pressure data representing fluctuations in pressure of the fluid medium around said body. In response to reference temporal pressure data generated by sensors in a turbulence zone at which turbulence is occurring and sensors in a transition zone between the turbulence zone and a laminar flow zone, a method-of-delay phase portrait is generated for each of a progression of selected delay intervals.

Abstract: A radar system is shown to include a transmitter for transmitting a pulsed electromagnetic signal having a frequency less than 500 Hz and a receiver for receiving a (scattered) reflected signal provided from the pulsed electromagnetic signal (scattered) reflected from an anomaly below the surface of the water. The radar system further includes a switch for inhibiting the receiver from receiving a reflected signal provided from the pulsed electromagnetic signal reflected from the surface of the water and a signal processor for controlling interoperability of the transmitter, the receiver and the switch. With such an arrangement, a radar system is provided for detecting anomalies such as a wake of a moving vessel (if and when this produces a conductivity anomaly), or a plume of oil beneath the surface of water.

Abstract: A remote control system protected against enemy take-over and particularly uited for the undersea control of acoustic decays and torpedoes. System includes dual electronics systems. Command signal includes plurality of frequencies which disable one of dual systems and energizes the other system.

Abstract: A torpedo countermeasure decoy which produces and broadcasts signals that e usually more attractive to a homing torpedo than those emanating from its intended target. Timely reset staircase generators combined with timely opened and closed gates generate stairstep signals which are added by a summing amplifier to produce a programmed composite signal thereof. A bank of voltage-controlled oscillators are driven by said composite signal in such a manner as to produce a predetermined plurality of discrete frequency signals within a predetermined plurality of consecutive but separated frequency bands within a given frequency spectrum, all of which are then effectively broadcast within the aqueous medium in the ambient environment of the torpedo's intended target--that is, the vessel to be protected.

Abstract: Simulation of ship's noises by means of liquid jets penetrating into the water.

Abstract: A sonar countermeasure comprising a decoy device that carries apparatus for eceiving, processing and transmitting sonar signals. Processing apparatus includes magnetic tape storage, staggered record and read heads and time varying partial erase heads which produce realistic "echo" of received sonar search signals.

Abstract: Methods for applying coverings to surfaces by casting at least a portion of the covering in a mould from a solidifiable liquid mix. The mould may be applied directly to the surface or to an already cast layer of the covering. In another embodiment an outer portion of the covering is cast in a mould and is affixed to the surface, or to an already cast layer of the covering, so as to define a void between the outer portion and the surface or already cast layer. The void is filled with solidifiable liquid mix.

Abstract: A countermeasure for acoustic homing torpedoes where a plurality of sound ansducers are transversely arrayed across the attack path of the torpedo. The transducers are energized in a time sequence to simulate a relative transducer movement in a direction across the path of the torpedo. The torpedo is thereby misdirected by the high level acoustic signal produced in the transducers slowly changing the location of the signal. The signal is interrupted to provide a silent period, such absence of a signal being interpreted by the torpedo as a miss causing the torpedo to initiate a turn and commence a search which will "exhaust" the torpedo. This sequence is repeated while the torpedo is within a close range of the target.

Abstract: An inexpensive decoy which simulates the evasive tactics of a submarine ur attack for pulse echo type search systems and which can be ejected through the flare tube of a submarine.

Abstract: A unique system of disposition-controlled, water-filled, fabric bags is sended from and towed beneath a ship to effect the modification of the inherent water pressure signature thereof and thus prevent detonation of a marine mine programmed to be exploded thereby.

Abstract: Material for providing acoustic protection against a source of noise, the material comprising a substrate and resonators, wherein said resonators formed on the substrate are constituted by thread-like and/or area-occupying composite elements whose structural characteristics (density, modulus of elasticity, shear modulus, damping factor, piezoelectric factor, etc. ...) and whose shape and/or size are selected to associate a predetermined resonant frequency with each resonator, and also to absorb the sound pressure energy from the noise source at said resonant frequency and dissipate it in the form of mechanical heat energy and/or electrical energy. The invention also relates to apparatus constituted by a wall including at least one layer of the above material.

Abstract: A split cylinder compliant tube baffle for absorbing and/or canceling noise signals generated within a submarine or surface ship is described. The split cylinder tube baffle includes an air-filled tube having a longitudinal gap extending the length of the tube. The baffle further comprises a rubber jacket for enclosing the gap and end covers for sealing end portions of the tube. A split tube baffle assembly is generally constructed of tube baffles adjacently aligned to form a panel. The panels can then be stacked for increased attenuation of acoustic signals. The split cylinder baffle offers significant size reduction and improved performance over conventional compliant tube baffle designs.

Abstract: A method and a submarine are disclosed influencing an acoustic source emitting an acoustig signal having a first frequency spectrum with at least one intensity maximum. In order to make detection of the frequency spectrum by means of passive acoustic locating systems more difficult, the first frequency spectrum is stochastically modulated by influencing the acoustic source.

Abstract: The invention relates to an underwater mine or decoy which should lie on the ocean bottom or which has buoyancy characteristics. The mine or decoy is made up of at least an expandable material and a foldable, preferably webbed shell which determines the external form of the mine. The disadvantages associated with the storage, transportation and stowage of heavy mines of fixed shape in submarines, small ships and aircraft are thereby avoided. It is possible to manufacture decoys or mines rapidly and economically, and they may have variable magnetic and/or sonar reflecting properties. The release of the decoys or mines from submerged submarines via forceful ejection is possible.

Abstract: A method for operating submerged submarines and a submarine are disclosed which are used to camouflage submerged submarines which, while submerged and travelling, draw a trail of heated cooling water behind themselves. In order to reduce the risk of detection of such submerged submarines by means of heat-sensitive detectors, either the density of the heated cooling water is increased through the introduction of additives, or, the heated cooling water is brought, by mechanical means, to a depth far below the submarine for preventing heated-up cooling water to rise up to the sea surface.

Abstract: An expendable moving echo radiator suitable for providing a decoy to attr a homing torpedo and divert the torpedo away from its intended target is disclosed. The present invention provides an expandable and collapsible curtain for deployment from a capsule launched from a submarine or other sea vessel. In its expanded configuration, the curtain is characterized by a physical profile sufficient to reflect acoustic waves and to generate echoes substantially similar to echo signals generated by an actual, full size submarine or other target. The present invention is further provided with propulsion means, as well as means for enhancing its capability to capture a torpedo's sensors. It can also be provided with explosives to destroy lured torpedoes. The expendable device can be used to simulate a submarine for anti-submarine warfare (ASW) training, thereby affording the advantage that a package of easily transportable size becomes a full size submarine-like target.

Abstract: A method and an apparatus are disclosed serving to minimize acoustic emission of submerged submarines. Moving mechanical elements in an inner region give off vibrations onto an outer hull via a vibration transmission path. The vibrations are damped with damping means in the vibration transmission path. The damping means is configured as an evacuated intermediate space and is inserted in the vibration transmission path.

Abstract: A method and an apparatus are disclosed serving to reduce acoustic emission from submerged submarines in which vibrations from moving mechanical elements within an inner region are transmitted to an outer hull via a vibration transmission path. For decoupling the outer hull as completely as possible from the moving mechanical elements, for example periodically moving nuclear reactor control rods, the vibrations are actively damped. Means are provided for ascertaining the motion of the mechanical elements relative to the outer hull and for superimposing upon said motion of counter-motion of opposite direction.

Abstract: An efficient yet relatively simple tactical acoustic decoy apparatus is dlosed for providing protective cover for a sea going vessel against homing and/or fire control devices operating on infrared, sonar or microwave reflected energy which comprises a plurality of decoy rounds constructed for launch by a decoy round launching mechanism, a load such as a transducer assembly within each decoy round which when deployed provides an effective countermeasure against homing or fire control devices and means for deploying the load.

Abstract: An active adaptive noise canceller that inserts delays in the weight update logic of an adaptive filter employed by the canceller to make the filter stable. It has been found that there is a great deal of flexibility regarding the selection of the delay values. This insensitivity permits designing the delays in advance, and not having to adjust them to different situations as they change, thus no longer requiring a training mode. The canceller dramatically reduces the amount of hardward needed to perform active adaptive noise cancelling, and eliminates the need for the training mode, which in some applications, including automobiles, for example, can be as objectionable as the noise sources that are to be suppressed.

Abstract: An acoustic matching member for a sonic transducer is disclosed which comprises a solid material, for example, a glass, in which a plurality of voids have been formed. A method is also included of forming an acoustic matching member for a transducer which includes the steps of forming the member from a material in which a plurality of voids have been introduced whereby the velocity of sound in the material with voids is substantially less than that of the material without voids in the direction of sound propagation of the member.