Study On Improved Methods Of Passive Phase-Conjugate Method For Sound Focusing

The radiated noise of naval ship is an important factor that threats its security and affects its combat effectiveness.Noise reduction requires the accurate location and radiation characteristics of the noise source,in order to make a noise reduction plan.In the common situation,the complex ocean environment can influence the location,the focusing and imaging of the sound source.However,it is reported that the focal spot focused by phase-conjugate method can be enhanced by the complexity of medium in certain situation.When its focusing setp is realized by numerical simulation,such procedure is also called passive phase-conjugate sound focusing,which is widely used in underwater source locating and imaging.Although phase-conjugate method can achieve back propagation and adaptive focusing,its focal spot size is large.In the free space,its focal spot size is limited by the half wavelength diffraction limit,and if the distance between the source and the array becomes further,the focal spot are larger.Since a large focus is not conducive to accurately locating,in view of the above limitations of the phase-conjugate method,this thesis is devoted to reducing the focus size and proposes two improved algorithms of the passive phase-conjugate method.They are the artificial iterative phase-conjugate method and the split artificial iterative phase-conjugate method.Their abilities of reducing the focus size are proved.The thesis contents are described as follows:(1)The basic theory of phase-conjugate method is introduced,and the relationships between the three transmission and reception forms is combed.In the free space,the far-field beam function is given using the uniform line array based on phase-conjugate method to focus a point-like source.It is pointed out that reducing the focus size can be achieved by changing the linear array parameters,which is verified by numerical simulations.It is found that the monopole phase conjugation(PC/M),which measures sound pressure and sends signals by monopole source,and the dipole phase conjugation(PC/D),which measures sound pressure gradient and sends signals by dipole source,have better performance on focusing than the other transmission and reception forms.This conclusion supports the following researches on selecting transmission and reception forms.(2)Based on passive phase-conjugate method,an artificial iterative phase-conjugate method is proposed.It enhances focusing ability by iterating the procedure of phase-conjugate focusing,and the directivity of the array is also employed to obtain a small focus.Its two calculating formulas are proposed based on PC/M and PC/D.In the far field of the free space,theoretical analysis shows that the focal size of a point-like source is inversely proportional to the iteration number.The focal size decreases as iteration number increases,which in turn brings the side lobe closer and leads to the misjudgment of the sound source position.To solve this problem,the focal spot by the phase-conjugate method is employed as the searching field of the artificial iterative phase-conjugate method,in which grating lobe disappears if the element number of the uniform line array Ne and the iteration number N are satisfied by N<Ne-1.Numerical simulations and experiments verify the effectiveness of this proposed algorithm.Using a linear array,the focal size can achieve nearly 1/3 times a wavelength at a distance from the sound source equal to the wavelength,and the diffraction limitation is overcome.In the far field,the focal size achieves 5.6%of the focus by the passive phase-conjugate method,and its focus is reduced.(3)Based on the artificial iterative phase-conjugate method,a split artificial iterative phase conjugate method is proposed.This method introduces the split cross-correlation formula into the artificial iterative phase-conjugate method,which has the advantage that the focus size can be reduced by increasing the iteration number,but it also has a smaller focus and better side-lobe suppression than the artificial iterative phase-conjugate method has.Two calculating formulas of the split artificial iterative phase-conjugate method are proposed based on PC/M and PC/D.In the far field of the free space,theoretical analysis shows that the focal size focused by the split artificial iterative phase-conjugate method achieves 1/2 of the focus by the artificial iterative phase-conjugate method in case that their iteration numbers are the same.The focusing ability of this improved algorithm is verified by numerical simulations and experiments.Using a linear array,the focal size can achieve nearly 1/7 times a wavelength at a distance from the sound source equal to the wavelength,and the diffraction limitation is overcome.In the far field,its focus size achieves 2.8%of the focus size focused by the passive phase-conjugate method,and reducing focus is realized.(4)After the discussion on the linear array focusing,in the free space,the point-like source focused by the phase-conjugate method and the proposed improved algorithms using the three common planar arrays(circular array,cross-shaped array,rectangular array)are discussed.Their focal beam functions are presented,and the array parameters that may affect the focal spot size are analyzed theoretically.In the free space,focal patterns of the three planar arrays are compared,and the array parameters that can affect the focus size are confirmed refer to the conclusions of theoretical analysis.The two proposed algorithms can enhance the focusing effect of the planar array is proved by theoretical analysis,numerical simulations and experiments.In the near-and far-field,when the array characteristic size and element number are the same,the circular array using the dipole source has the best performance in focusing.When the other parameters are unchanged,increasing the size of the circular array can reduce the focus.The above researches provide some practical schemes for enhancing the acoustic focusing patterns by the passive phase-conjugate method.They may provide some new ideas on improving the focusing and imaging effect based on passive phase-conjugate method.