Abstract:
A system and method are provided to reduce a structure&#39;s far field acousticadiation 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&#39;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.

Description:
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without payment of any royalties thereon or therefor. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to active noise control, and more particularly to a method and system for reducing a structure&#39;s far field acoustic radiation signature. 
     BACKGROUND OF THE INVENTION 
     A structure of unspecified size and shape, either stationary or moving, has some type of internal excitation. This internal excitation energy is passed through the structure until it reaches the shell of the structure. The energy on the shell then dissipates into the surrounding field and can be regarded as emanating acoustic waves. The acoustic waves on the surface of the structure are either radiating waves or non-radiating (i.e., evanescent) waves and exist in a region known as the evanescent acoustic near field. The radiating acoustic waves cause observable responses in the acoustic far field. For many underwater structures, it is desirable to reduce the observable acoustic radiation responses in the acoustic far field of a radiating structure. 
     In attempting to reduce the far field acoustic radiation from a structure using an active control system, a measurement of the amount of acoustic radiation in the acoustic far field is desirable as a means of ascertaining the performance of the active control system. In a typical application, this is not measured and is often times unmeasurable. For instance, if the structure is a moving vehicle, it is difficult and impractical to place acoustic measurement devices in the acoustic far field to measure the acoustic radiation from the vehicle. 
     Current active control methods circumvent this problem of actually measuring the far field radiation by focusing on vibrations within the structure. The active control system essentially attempts to minimize the vibrations of the structure, and the system&#39;s performance criterion is based on measurements of the structure&#39;s vibrations. However, by attempting to reduce the vibrations of a structure, the main objective of reducing the far field acoustic radiation levels can be achieved with certainty only if all the vibrations of the structure are reduced. 
     More specifically, in structural acoustic active control, the active control system reduces the outputs of the structure&#39;s modes, which are analyzed from a structural acoustics point of view. Modes are a means of classifying the frequency dependent vibrations of a structure. When a structure is excited, the modes of the structure are said to be excited and some of these modes cause acoustic radiation to the far field while some do not. With structural acoustic active control, energy to control the structure&#39;s vibrations is placed directly into the structure with the purpose of reducing the modal contributions to the acoustic radiation in the far field. Thus, intimate knowledge of the structure&#39;s modes is required to apply structural acoustic active control. However, as the complexity of the structure increases, this knowledge is more difficult and expensive to obtain. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a system and method to reduce a structure&#39;s far field acoustic radiation signature. 
     Another object of the present invention is to provide a system and method that reduces the far field acoustic radiation signature of a structure that is simpler to implement than structural acoustic active control systems. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, a system is provided to reduce a structure&#39;s far field acoustic radiation 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 connected to the sensors generates a prediction of the structure&#39;s far field acoustic radiation signature using the near field acoustic radiation. The same or different programmable controller then generates noise control signal(s) using the prediction. A plurality of acoustic radiators are suspended within the structure convert substantially all of the noise control signal(s) to airborne acoustic energy within the structure. In this way, the far field acoustic radiation is reduced without having to solve the complex problem of controlling a structure&#39;s vibration characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The sole figure is a schematic view of the far field acoustic radiation reduction system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the sole figure, a schematic view is shown of a structure 100 equipped with a system 10 for reducing the far field acoustic radiation signature in accordance with the present invention. While not shown, it is to be understood that structure 100 typically includes a variety of equipment that excites the shell of structure 100 into vibration. It is this structural vibration that is the source of both near and far field acoustic radiation signatures of structure 100. 
     System 10 includes an acoustic sensing system, represented for purposes of illustration by sensor 12, for measuring near field acoustic radiation of structure 100. It is to be understood that sensor 12 is representative of any one of a variety of well known stationary acoustic near field sensing systems. Typically, sensor 12 is representative of a plurality of sensors (e.g., hydrophones when structure 100 is submerged in a liquid) located at specific points in the acoustic near field of structure 100. For example, sensor 12 could represent line array(s), array(s) that conform to the shape of the structure, multiple tiers of sensors or arrays of sensors, sensors placed sparsely or densely in the entirety of the acoustic near field of the structure, etc. 
     Regardless of the number of sensors and system of locating same around structure 100, the output of sensor 12, i.e., the acoustic near field radiation produced by structure 100 as measured by sensor 12, is fed to a far field predictor 14. Far field predictor 14 is typically a programmable microprocessor that processes the acoustic near field measurements to obtain a prediction of the structure&#39;s far field acoustic radiation signature. Programming of far field predictor 14 may be based on a variety of prediction techniques. For example, two well known acoustic far field prediction techniques include: 1) propagation via the exterior Helmholtz integral as disclosed by Clark et al. in &#34;Numerical Propagation of Spatially Distributed Acoustic Sources Using the Exterior Helmholtz Integral Equation&#34;, Statistical Signal and Array Processing Workshop, Victoria, British Colombia, Canada, Oct. 7-9, 1992, or 2) wave vector filtering disclosed by Clark et al. in &#34;Acoustical Holography Measurements on Circular-Ribbed and Helical-Ribbed Steel Cylindrical Models&#34;, Research and Development Report, Signatures Directorate, Carderock Division, Naval Surface Warfare Center, September 1993. 
     In the Helmholtz integral approach, a discrete approximation of a closed measurement surface is used. For example, if the structure were cylindrical, discrete surface points are selected along the length of the cylinder and radially around the cylinder&#39;s endcaps. The exterior Helmholtz integral is then solved for each of the discrete surface points as a prediction of the far field acoustic radiation signature. By way of example, a MATLAB code implementation of this approach for a cylindrical structure is attached hereto as an Appendix. 
     In the wave vector filtering approach, measured data of the structure&#39;s acoustic near field is transformed from the position-time domain into the wave vector-frequency domain using the well known Fast Fourier Transform. For each frequency bin, the values of the wave vector bins around a small angle about the zero wave vector bin are summed to produce an estimate of the acoustic far field. 
     In either case, the prediction of the far field acoustic radiation signature generated by far field predictor 14 is output to a controller 16 where the predicted far field acoustic radiation signature is used as a control parameter to reduce the actual far field acoustic radiation signature. Controller 16 may be a microprocessor (e.g., the same microprocessor used for far field predictor 14 or a separate microprocessor) programmed with any well known acoustic noise control algorithms. For example, in its simplest implementation, controller 16 could be a phase shifter that processes the predicted far field acoustic radiation signature to output a phase shifted version of same (e.g., 180° out-of-phase with the predicted far field acoustic radiation signature). Other well known but more complex acoustic noise control algorithms may also be employed and include, but are not limited to, classic control, modern control and adaptive control. 
     The output of controller 16 is one or more time varying noise control signal(s) fed to one or more acoustic radiators (e.g., loudspeakers). For purpose of illustration, four radiators 18a, 18b, 18c, and 18d are shown. However, it is to be understood that more or less radiators may be used depending on the structure&#39;s size, shape and vibration radiation characteristics. The actual number of acoustic radiators and their placement within structure 100 may be determined by well known acoustic calibration techniques. 
     Each acoustic radiator receives its signal and converts same to airborne acoustic energy represented by arrows 200a-200d. The airborne acoustic energy is converted to structural excitation at the shell of structure 100. The airborne acoustic energy serves to reduce the predicted acoustic far field radiation signature without attempting to cancel the vibration of structure 100 due to internal equipment operation. Accordingly, each acoustic radiator 18a, 18b, 18c, and 18d must be mounted within structure 100 to insure that substantially all of the noise control signal is converted to airborne acoustic energy. Such vibration isolation mounting may be employed according to any one of a variety of techniques well known in the art. 
     The advantages of the present invention are numerous. When structure-borne actuators are used to reduce the structure&#39;s vibrations, the actuators must transmit their energy directly into the structure via single attachment points. The problem of determining the location of the attachment point(s) is a difficult and time-consuming task since knowledge of the structure&#39;s modes of vibration are required. Rather than attempting to solve the complex problem of controlling a structure&#39;s vibrations, the present invention allows the structure to vibrate and simply reduces the observable acoustic radiation in the far field in a novel fashion. By using airborne acoustic energy originating from within the structure, the energy used to reduce or negate the acoustic far field effects of the structure&#39;s vibrations may be transmitted to a larger portion of the structure as opposed to a single point. Thus, when the control energy is applied to the airborne path instead of the structure-borne path, the need to input sufficient energy into the structure to overcome internal structural damping is not required. Further, the complex problems of determining where to locate the structural excitation devices is no longer as difficult to solve. Finally, the present invention makes use of a prediction of the far field acoustic radiation signature developed from the easily measured near field in a far field acoustic radiation reduction system. 
     Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in the light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described. 
     
         __________________________________________________________________________APPENDIX__________________________________________________________________________function [pfp]=helmholtz.sub.-- 3s(fx,fy,fz,sx,sy,sz,ex,ey,ez,pc1,pc2,pc3,pc4,pc5,pc6, pc7,pc8,pc9,dr,de,a1,a2,q)% Computes far field pressure using helmholtz integral% Uses tri-surface cylindrical conformal array% Inputs:% fx,fy,fz: far field points for pressure to be computed% sx,sy,sz: surface points for cylinder length% ex,ey,ez: endcap points for cylinder ends% pc1,pc2: pressure over cylinder length% pc3,pc4: pressure over cylinder end caps% dr: delta.sub.-- r (if pc1-pc2, then dr=radius1-radius2)% de: delta.sub.-- e (if pc3-pc4, then de=(neg. number))% a1: elarea1% a2: elarea2% q: desire endcaps in calculation? 1=y 2=n%% Outputs:% pfp: far field pressure%% function [pfp]=helmholt.sub.-- 3s(fx,fy,fz,sx,sy,sz,ex,ey,ez,pc1,pc2,%           pc3,pc4,pc5,pc6,pc7,pc8,pc9,dr,de,al,a2,q);%% Michael A. Sartori, Ph.D., June 7, 1993% UPDATES:%% helmholtz contribution from cylinder lengthsxx=reshape(sx,1,ncp*nlp);syy=reshape(sy,1,ncp*nlp);szz=reshape(sz,1,ncp*nlp);pts=reshape(pc1,1,ncp*nlp);ptd=reshape(-3*pc1+4*pc2-pc7,1,ncp*n1p)/(dr*2);  % tri-surfacecnt=0;ffmax=max(size(fx));disp([`  field point calculations: `,num2str(ffmax)])for ff=1:ffmaxcnt=cnt+1;if cnt==40;disp(]`   field point number `,num2str(ff)]),cnt=0;endfxx=fx(ff)*ones(1,ncp*nlp);fyy=fy(ff)*ones(1,ncp*nlp);fzz=fz(ff)*ones(1,ncp*nlp);rsf=radial.sub.-- dist(fxx,fyy,fzz,sxx,syy,szz);  %size: 1x(ncp*nlp)csb=((fxx-sxx).*sxx+(fyy-syy).*syy)./(rsf.*(sqrt(sxx. 2 + syy. 2)));mon=-exp(j*kay*rsf).*a1./(4*pi*rsf);pfp(ff)=sum(mon.*(-csb.*(-1../rsf+j*kay).*pts + ptd));end% helmholtz contribution from end capsif q==1disp(`  end cap calculations `);exx=reshape(ex,1,ncp*nrp);eyy=reshape(ey,1,ncp*nrp);ezz=reshape(ez,1,ncp*nrp);pts.sub.-- posz=reshape(pc3,1,ncp*nrp);ptd.sub.-- posz=reshape(-3*pc3+4*pc4-pc8,1,ncp*nrp)/(2*de);  %tri-surfacepts.sub.-- negz=reshape(pc5,1,ncp*nrp);ptd.sub.-- negz=reshape(-3*pc5+4*pc6-pc9,1,ncp*nrp)/(2*de);  %tri-surfacecnt=0;for ff=1:ffmaxcnt=cnt+1;if cnt==40;disp([`  field point number `,num2str(ff)]),cnt=0;endfxx=fx(ff)*ones(1,ncp*nrp);fyy=fy(ff)*ones(1,ncp*nrp);fzz=fz(ff)*ones(1,ncp*nrp);rsf=radial.sub.-- dist(fxx,fyy,fzz,exx,eyy,ezz);  %end cap in +zdirectioncsb=(fzz-ezz)./rsf;mon=-exp(j*kay*rsf).*a2./(4*pi*rsf);pfp(ff)=pfp(ff)+sum(mon.*(-csb.*(-1../rsf+j*kay).*pts.sub.-- posz +ptd.sub.-- posz));rsf=radial.sub.-- dist(fxx,fyy,fzz,exx,eyy,-ezz);  %end cap in -zdirectionc3b=(-fzz-ezz)./rsf;mon=-exp(j*kay*rsf).*a2./(4*pi*rsf);pfp(ff)=pfp(ff)+sum(mon.*(-csb.*(-1../rsf+j*kay).*pts.sub.-- negz +ptd.sub.-- negz));endend__________________________________________________________________________