Research On Near Field Focus Beam Forming Acoustic Image Measurement Technology

As a main passive positioning technology,acoustic image measurement technology is one of the core contents of the development of modern underwater acoustic science and technology.The sonogram measurement technique uses a passive acoustic imaging method to display a spatial distribution map of the radiation noise source to obtain the relative position of the target noise source.This is extremely important for guiding large-scale underwater equipment such as ships and for vibration and noise reduction,as well as underwater target detection.Due to the diversified and complex structure of the actual marine environment,the common beamforming methods for sound map measurement are based on the far-field plane wave hypothesis.Many of the conditions in practical applications cannot meet the far-field conditions.Acoustic mapping techniques suitable for near field need to be used.In this paper,the acoustic image measurement technique based on spherical wave near-field focusing beamforming is studied.The focusing methods of four arrays are introduced and simulated,and the sound resolution of each formation is compared and analyzed.The four formations include: horizontal array,vertical array,cross array and area array.When the number of elements is same,the area map has the highest resolution and the best focusing effect.Due to the complexity of the ocean channel,there is a large amount of interface reflection that affects the effect of the acoustic image measurement.To this end,it is necessary to study the acoustic image measurement technology under multi-path conditions.This paper discusses two anti-multi-path methods based on the horizontal line array: the virtual time-reversal mirror method and the virtual array element method.The effect of suppressing multi-path false peak interference is achieved.In addition,this paper combines the characteristics of the vertical array anti-multi-path and the characteristics of the array with high precision,and the vertical array has both advantages.In addition,the acoustic image measurement in the three-dimensional space is performed by the area array,and the scanning of the specific spatial area is realized to obtain the intuitive target position information in the three-dimensional space.Finally,various factors affecting the measurement of the sound image,such as array size,signal frequency,target distance,etc.,are analyzed.At present,the accuracy requirements for underwater acoustic passive positioning technology are getting higher and higher in various fields at home and abroad.Therefore,as the main passive positioning technology,the acoustic image measurement method needs to be continuously improved,and the sound image measurement technology tends to achieve high precision requirements.At low frequencies,conventional focus beamforming techniques sometimes fail to meet the high demands of the expected accuracy.In the third chapter,the high-precision acoustic image measurement technology is studied,including the minimum variance distortion-free response(MVDR)focused beamforming algorithm,the copy-related minimum variance distortion-free response(CMVDR)focused beamforming algorithm,and the minimum broadband steering covariance.Variance Beamforming Algorithm(STMV).In the processing of narrowband signals,the effects of two methods of conventional focusing beamforming and MVDR focusing beamforming are compared.In the processing of wideband signals,the comparative focusing beamforming and the above three high-precision acoustic image measurement techniques are compared and analyzed,which proves that the accuracy of the acoustic image measurement of the latter three methods is greatly improved compared with the conventional focusing beamforming.In the fourth chapter of this paper,in order to meet the common array size of acoustic map measurement under actual near-field conditions,a sparse array is used,and three types of sound maps are measured for low-frequency narrow-band signals and low-frequency broadband signals,including lines.The array,cross array and area array further verify the influence of the formation in the second chapter on the accuracy of the acoustic image measurement.And under the same signal conditions,the conventional focusing beamforming and three high-precision focusing beamforming methods are respectively performed to measure the sound image,which verifies the improvement of the precision of the high-precision sound image measurement technology.Finally,the advantages and disadvantages of each formation and their suitable application environments are discussed.In practice,it is necessary to combine the specific marine environment and accuracy requirements to select the appropriate array and acoustic map measurement methods to achieve the sound map measurement of the near-field target.