Patent No. 4349898 Sonic Weapon system

 

Patent No. 4349898  Sonic Weapon system (Drewes, et al., Sep 14, 1982)

Abstract

A system for transmitting a parametrically pumped sonic signal through a transmission medium to a remote location is disclosed. The preferred system, which is particularly intended for use as a sonic weapon, comprises a sound source; means for separating the sound into a plurality of discrete frequency components including a fundamental component and at least one additional component, each additional component having a frequency twice that of the next lowest frequency component; means for adjusting the phase difference between each frequency component and the next lowest frequency component to substantially 90.degree.; means for colinearly focusing the components on the remote location; and means for rendering the transmission medium nonlinear between the focusing means and the remote location.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to sonic transmission systems and more particularly to systems for transmitting low frequency sonic signals with long range characteristics. Most particularly, this invention pertains to a parametrically pumped sonic weapon system.

2. Statement of the Prior Art

In order to destroy a target or kill or injure enemy personnel using a sonic signal, it is necessary to vibrate the target at or near its resonant frequency. Since the resonant frequency of commonly encountered structural targets is relatively low, typically 5-20 hertz, a low frequency sonic beam is required. In addition, the sonic beam must have sufficient range to be effective as a weapon. While these two criteria are simple to state, they are not easy to implement. The reason is Rayleigh's Law which may be written in equation form as follows:

where

.delta. equals the radius of the central disc of energy,

.lambda. equals the wavelength of the focused beam and

.phi. equals the angle subtended by the lens at the focal distance. For example, Rayleigh's Law predicts that the lens diameter required to focus a 10 hertz wave to within a 50 foot diameter zone of focus at a range of one mile is about 26,484 feet or about five miles.

Until recently, Rayleigh's Law was thought to be an absolute bar to the long-range propogation of a low frequency wave. Recently, however, it has been recognized that if two colinear sound beams are introduced into a nonlinear transmission medium, the interaction between them results in the production of the difference frequency. Thus, if the frequency of one of the beams is f and the frequency of the other beam is 2f, then the difference frequency component will also be f and may be used to augment the lower frequency sound beam. Moreover, the difference frequency component will have the range characteristics of the higher frequency component. However, this augmentation, commonly referred to as parametric pumping, will only take place if the phase difference between the two signals at the starting point of the interaction is approximately 90.degree.. Otherwise, the component produced by the non-linear interaction will tend to oppose the lower frequency signal. The theoretical basis for these conclusions is set forth in an article by O. V. Rudenko and S. I. Soluyan entitled Theoretical Foundations of Nonlinear Acoustics (English translation) Consultant's Bureau, New York 1977, pp. 145-157. Applicant is not aware of any system which utilizes these principles to focus low frequency sonic signals over relatively large distances much less one that does so with sufficient power to destroy remote targets or kill or injure enemy personnel.

SUMMARY OF THE INVENTION

According to the present invention, I have developed a sonic weapon system which takes advantage of the interaction between colinear beams in a nonlinear medium to transmit low frequency sonic signals to remote targets, structural or human, with sufficient power to destroy them. The basic system includes a high level sonic source, means for separating the raw output from the sonic source into discrete frequency components, means for adjusting the phasing between these components and means for focusing them on the target.

The sonic source may comprise any sound source having a sufficient power output capacity to maintain destructive levels at the target and render the transmission medium, typically air, nonlinear. Commercially available jet and nuclear engines are sufficient for this purpose and their use is presently preferred.

Although nonlinear augmentation as between a fundamental signal frequency f and a pump signal frequency 2 f is known, it is desirable for weapon systems applications to provide still further augmentation. This may be effected by adding a second pump signal having a frequency (4 f) twice that of the first pump signal (2 f), a third pump signal (8 f) having a frequency twice that of the second pump signal (4 f), and so on. For the same reason and with the same effect that the 2 f pump signal augments the fundamental signal, the 4 f pump signal will augment the 2 f pump signal which, in turn, augments the fundamental signal. Similarly, the 8 f pump signal will augment the 4 f pump signal which augments the 2 f pump signal, and so on. As is the case between the 2 f pump signal and the fundamental signal, each successive pump signal must be 90.degree. out of phase with the signal it augments.

The result is a low frequency sonic wave having the range characteristics of the highest frequency pump signal. The frequency of the highest frequency pump signal is, in turn, only limited by the expected losses due to divergence, sound absorption, transfer of energy to higher harmonics, etc. Based on these considerations, a maximum pump frequency of about 5,000 Hertz is presently preferred.

The frequency selector and phase controller serves to separate the raw output from the sonic source into the required fundamental and pump signal frequencies. The frequency selector and phase controller preferably comprises a combination of tubes, fans and masks. The input ends of the tubes are positioned to receive the raw output from the source such that the source output is distributed substantially equally among the tubes. A fan and a mask are disposed in the output end of each of the tubes, the mask being fixed relative to the tube.

The fan and mask in each tube are dimensioned such that as the fan rotates, it will alternately pass and block the flow of air through its corresponding tube. For example, a semicircular mask having a diameter equal to the internal diameter of the tube and a fan having a single matching semicircular blade are preferably used to generate the lower frequency components. Thus, as the blade rotates it will alternately pass through a position in which it is fully overlapped by the mask thus leaving half the output opening exposed, and a second position in which no portion of the blade is overlapped by the mask and the tube opening is fully blocked. This results in the generation of air pulses, the frequency of the pulses being dependent on the frequency of rotation of the fan blade. Since practical design limitations limit the maximum fan rotation speed, semicircular blades and masks will preferably not be used to generate the higher frequency components. According to the invention, this is overcome by adding additional mask sections and fan blades whereby each rotation of the fan blade results in the production of two or more pulses.

All the fans are preferably driven from a single rotating member through suitable gearing arrangements. The advantage of using a single rotating member, referred to herein as the fan speed controller, is that the fundamental and pump signal frequencies may be varied in step by simply varying the rotation rate of the controller.

Phasing is preferably accomplished by introducing a differential into the gearing for each fan. The differential permits each fan to be advanced or retarded relative to the others and thus may be used to control the relative timing and hence phase of the air pulses emanating from individual tubes.

The preferred sonic lens comprises a concave honeycomb array, each unit of the array comprising a hexagonal tubular structure closed at one end. By preselecting the cross-sections and lengths of the individual units, each unit may be tuned to enhance the reflection of a particular sonic frequency. Preferably, the array will have a number of honeycomb units tuned to each of the frequency components emanating from the frequency selector. Means are also preferably included for adjusting the pan and tilt of the individual honeycomb units whereby individual focusing of the honeycomb units may be effected. Also preferably included are means for adjusting the depth of each honeycomb unit relative to the others for fine tuning the phasing of the reflected signals.

The system is operated by focusing the output from the frequency selector and phase controller on the sonic lens and then focusing the sonic lens on the target. Upon activation of the source, the frequency selector will separate the raw output into discrete frequency components comprising a fundamental frequency at or near the resonant frequency of the target and successively higher pump frequencies. These components in turn strike the lens which then focuses them on the target. By selecting a sonic source with sufficient output to render the transmission medium nonlinear, the pump signals will augment the fundamental signal during transmission with the result being a high energy, low frequency sonic signal at the point of impact.

The destructive capability of the system during firing is maximized by adjusting the fan speed controller to match the fundamental frequency of the frequency selector and phase controller with the resonant frequency of the target. For this purpose, the preferred system also includes a laser interferometer for measuring the vibration frequency of the target. After appropriate signal conditioning, the information from the interferometer may be used to regulate the fan speed controller as required. Since accurate phasing is essential to effective operation, phasing is also preferably monitored during firing and adjusted as required. For example, signals from pressure sensors positioned to sense the amplitude peaks of each frequency component may be compared and this information used to implement phase control.

In one embodiment of the invention, level control of the individual frequency components is introduced by regulating the size of the opening to the individual tubes of the frequency selector. Portability may also be introduced as by using two flat bed trucks, one for the sound source and frequency selector and the other for the sonic lens and laser feedback system. The preferred embodiment also includes a computer for adjusting the various system parameters as required.

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As noted above, it is not necessary to rely solely on the intensity of the sound generated by the sonic source 12 to render the transmission medium, typically air, nonlinear. Thus, for example, the transmission medium may be rendered nonlinear or its nonlinearity enhanced by periodically introducing shock waves along the transmission path. Such shock waves, which are familiar when a body passes through air at supersonic speeds or highly compressed air is suddenly released, result in a rapid rise in pressure that is propagated through the medium. It is presently contemplated that such shock waves will be used to enhance the nonlinearity of the air medium by introducing them into the transmission path in synchronization with the fundamental frequency of the system 10. Since, as is discussed above, the conversion efficiency obtainable by nonlinear parametric pumping is dependent on medium nonlinearity, enhancing the nonlinearity of the medium will increase the overall efficiency of the system 10.

Still further changes and modifications may be made. For example, while the tubes 60 which make up the lens 16 have been described as having preferably hexagonal cross-sections, this is not necessary and other cross-sectional configurations, such as circular cross-sections, may be substituted. In any event, because of the high sonic levels and numerous moving parts, aerospace design techniques will preferably be employed throughout.

Since these as well as further changes and modifications may be made within the scope of the present invention, the above description should be construed as illustrative and not in a limiting sense, the scope of the invention being defined by the following claims.