A Study On The Theory And Method Of Nearfield Acoustic Holography On Reconstructing Vibration Velocities Of Sound Sources In The Non-free Field

Noise control and low noise design of mechanical and electrical products are usually based on the noise source identification techique.Nearfield acoustic holography(NAH)is an important one of all noise source identification techniques because of its high resolution.The high resolution is achieved because the evanescent waves can be acquired on the pressure measurement surface near the source surface.The reconstructed results of NAH can be either the pressure or normal velocities of the source.The normal velocities of the source can used to identify the noise source.To perform the NAH,the free field needs to be satisfied or the incoming field can be negelected.However,there are always disturbing sources or reflection from the wall or other equipments,which results in a noisy environment.If the free field condition is not satisfied,there will be a significant reconstruction error or wrong noise source localization.Therefore,the realization of performing NAH in a non-anechoic environment is attracting more and more researchers.Recently,Sound field separation technique(SFST)has been introduced to obtain the outgoing field from the target source.The incoming filed from the other side can be separated.The outgoing field can be recognized the radiated field of the source in the free field,and the NAH can be performed.The SFST omits the scattering field from the incoming wave.When the strength of the incoming wave is large enough,the reconstruction accuracy will be decreased.In order to accurately localize noise sources in non-free fields,the inverse patch transfer functions(iPTF)method was proposed in recent years.The distrubing source and reflective sound on the measurement surface can be eliminated during the reconstruction process.However,there are still many problems to be solved in the iPTF method,which limits its practical application to some extent.In this paper,the shortcomings of the iPTF method are discussed and analyzed.The corresponding solutions are proposed,and some innovative researches based on this method are carried out.For instance,a new Green function for the planar sound source is proposed to use in the iPTF method,which improves the accuracy of the noise source localization.In order to simplify the measurement process,a new theory of the source velocity reconstruction based on the acoustically rigid boundary is proposed.A new microphone array satisfying the condition is designed and called acoustic mask,which not only realizes the accurate reconstruction,but also realizes the moveable and portable measurement.In this paper,the theoretical modeling,numerical simulation and experimental research are combined.The main contents of the dissertation are summarized as follows:(1)The significance of the research is firstly described,and the advantage of using the NAH technique to locate and locate the noise source is illustrated in the introduction.The development and research status of the NAH technique are reviewed.The reason for selecting iPTF method to identify and localize noise sources in the non-free field is given.(2)Based on the basic theory of the patch transfer functions method for solving the acoustic field in the forward direction,the normal velocity of the sound source can be inversely solved if the sound pressure and particle velocity are obtained on the measurement surface.In order to improve the cost and efficiency of acquiring the sound pressure and particle velocity on the measurement surface by p-u probes,the double pressure surfaces using the easily calibrated and low cost microphone array are proposed to be used in the iPTF method.The reconstruction formula of sound source velocity based on double pressure measurement surfaces is deduced.In this paper,the normal velocities of the baffled piston sources are reconstructed in a noisy environment by the numerical simulation.The results show that the velocities of the sound sources using double sound pressure surfaces are accurate.Some important factors which affect the reconstruction accuracy of the source velocities,e.g.,disturbing source strength,the condition number of thetransfer matrix,and the measurement noise,are considered.According to the simulation results,the suggestions of reconstruction parameters selection of the iPTF method are given.The experimental results verify that the velocities of the two loudspeakers can be reconstructed by the iPTF method based on the double pressure measurement surfaces.(3)The Green’s function used in the original iPTF method is composed of normal modes,which does not reflect the evanescent wave propagation.The evanescent wave Green’s function in the closed space under Neumann boundary condition is proposed for the iPTF method,which not only makes the reconstruction formula based on this method have more explicit physical meanings,but also can improve computational efficiency and precision.The normal velocities of two planar coherent piston sources using either the p-u probe for the pressure-velocity measurement or the microphone for the double pressure measurement are precisely reconstructed in the numerical simulation,which proves that this Green’s function is effective for both methods.In the experiment,the two loudspeakers in the baffle in a noisy environment verify the application of evanescent wave Green function to improve the reconstruction accuracy and efficiency based on either the single layer velocity measurement surface(p-u probe)or the double pressure measurement surface(microphone).It is also proved that the higher reconstruction precision can be obtained using the evanescent wave Green function under the Neumann boundary condition for the iPTF method than using the normal wave Green function.(4)The normal velocity under the acoustically rigid boundary condition should be zero.According to this theory,the modified iPTF method based on the acoustically rigid boundary condition is proposed.And a measurement device composed of the microphone array satisfying the condition is designed,which not only can accurately acquire the sound pressure in the sound field,but also ensure that the normal velocity of acquiring point is zero,thereby avoiding obtaining the normal velocities in the iPTF method.Because the sound pressure acquisition device can also weaken the influence of the incoming field to some extent,it is called acoustic mask in this dissertation.The numerical simulation is also used to reconstruct the normal velocities of the dual piston sources under the disturbing monopole.The results show the feasibility of the acoustic mask.The influence of different gaps and disturbing source intensity on the reconstruction accuracy is also considered.In the experiment,the acoustic impedance of the surface of the acoustic mask was measured in the standing wave tube.The objective is to make sure the inner surface of the acoustic mask satisfy the acoustic rigid boundary condition and the mask could be used for the experimental measurement.The normal velocities of two coherent loudspeakers on the baffle were reconstructed using the acoustic mask,which verified the modified method.In addition,the experimental results show that the iPTF method based on the acoustic mask can be used to reconstruct the normal velocities of the local surface of the sources.(5)The acoustic mask is further studied for an engineering environment.The iPTF method based on the acoustic mask is used on the vibration reconstruction of the cylindrical surface of a rotating motor.Experiments on acceleration reconstruction and noise source localization of motor housing surface were performed.Another acoustical mask is designed and applied to the cylindrical surface.At the peak frequencies of the noise,the normal acceleration of the motor on the cylindrical surface is reconstructed.Compared with the actual measurement results of the accelerameters,the reconstruction error is satisfied.The iPTF method based on the acoustic mask is promising to be used in the engineering environment.