Vibro-Acoustic Analysis And Active Control Of Sound Transmission Through Double-Panel Structures

Due to their superior sound insulation capability over single-panel structure,double-panel structures have found increasingly wide applications in noise control engineering.They are widely used for automobile,high-speed rail,submarine,shipping,aerospace and aeronautic structures,and so on.A thorough study of the physical nature of vibro-acoustic coupling performance of double-panel structures is critical to design the low-noise design of structures.However,for the low-frequency noise,the insulation capability of double-panel structures is not superior to their combined upper-lower panels.In fact,it is always difficult to control the low-frequency noise in the field of noise control.Here,in order to improve the sound insulation performance of double-panel structure in the low-frequency range,it is necessary to research on active control of sound transmission through double-panel structure.Considering the structural characteristics of aircraft fuselage,double-panel structure model and double-panel structure coupling with an enclosure model are built to study the vibro-acoustic coupling analysis methods of double-panel structure.Vibro-acoustics coupling performance of double-panel structure filled with porous-materials and an enclosure in mean flow is also investigated.Whilst,based on these models above,active noise control technique is applied to actively control the sound transmission through double-panel structure and to improve the sound insulation of double-panel structure in the low-frequency range.(1).The vibro-acoustic coupling performance of double-panel structures is studied.From different physical perspectives,the transverse acoustical modal expansion method(TAMEM)and acoustical waveguide modal expansion method(AWMEM)are presented to express the sound pressure distributions of the cavity sound field between the upper and lower panels of double-panel structure.The vibro-acoustic coupling relationships of double-panel structure and of double-panel structure with an acoustic enclosure are built to investigate the effects of sound pressure distribution functions of cavity sound field on calculated results of vibro-acoustic coupling performance,for example,sound transmission loss of double-panel structure,noise reduction of double-panel structure with an acoustic enclosure and sound pressure distribution of cavity.The results show that four methods could reflect the panel-air-panel resonance and standing-wave resonance phenomena and other phenomena induced by structural resonance,but the relevant frequencies may be different.Although they may result in the same calculated results of sound transmission loss,the same subsystem may have different dynamic responses for the four methods,for example,sound pressure distribution of cavity.Especially,AWMEM could provide much more sound field information,which includes not only propagating wave,but also positive-and negative-going evanescent wave.In fact,the existence of positive-and negative-going evanescent wave may influence the performance in the actual application,for example,near-field sensing strategies for active noise control and near-field acoustical holography.(2).Considering both the actual aircraft fuselage structure and flying environment,a theoretical model is proposed for the sound transmission through double-panel structures filled with air or porous material into an enclosure in external mean flow.An equivalent fluid model is employed to characterize the absorption of sound in the porous material.The sound pressure distributions of cavity and enclosure sound fields are expressed by using the structural modal function method and vibro-acoustic modal coupling method,respectively.The coupling relationships of the whole system are established by mean of structure-acoustic coupling theory.And the influences of several key system parameters on vibro-acoustic coupling performance of the whole coupling system are then systematically explored.The results are shown as follows.The porous material layer can effectively improve the sound insulation performance of double-panel structure,and the existence of porous material layer weakens the panel-air(EF)-panel resonance and standing-wave resonance phenomena and the NR is increased with the increase of thickness of cavity sound field.The enclosure couldn't alter the sound insulation performance of double-panel structure,but could significantly influence the vibro-acoustic coupling performance of the whole system.The large(infinite)dimension double-panel structure provides an upper bound NR for finite configurations for frequencies above the panel-air(EF)-panel resonance frequency and a lower bound NR for finite configurations for frequencies below the panel-air(EF)-panel resonance frequency.As the panel thickness is increased,the NR values drastically increase.The NR curves have few change for the case of porous material layer as the incidence angle increases,and the azimuth angle has almost no influence on the NR behavior of the overall system for the cases of both air layer and porous material layer.Lastly,the downstream flow(positive Mach number)has negligible influence on the NR behavior of the overall system.Nevertheless,the upstream flow(negative Mach number)has remarkable influence on NR curves and even makes the total reflection phenomenon appear.Because of the existence of porous material layer,the effect of the Mach numbers of upstream flow on NR of the overall system is tiny,and the occurrence of total reflection phenomenon is put off.(3).The problem of active noise control system based on double-panel structure is studied.In view of the control objective selection and implementation approach of control system,some new control objectives(total of twenty-five kinds)are proposed to optimize the secondary control sources and to reveal the relationship between the location of secondary control sources and control objectives.In order to reduce the sound energy in the sound radiating field,which includes infinite sound radiating field and enclosure sound field,the secondary forces are applied at the upper and lower panels,or the secondary sound sources are positioned at the cavity sound field between the upper and lower panels.By mean of directly reducing the sound energy of sound field of target or cutting off the paths of sound transmission through double-panel structure into the sound radiating field,some new control objectives are defined to optimize the secondary control sources.Results show that three kinds of secondary control sources could achieve the purposes of improving the sound insulation performance of double-panel structure,and the control effects of secondary sources is closely related to the selection of control objectives.When the locations of secondary sources are given,the physical quantities of sound field or structures in the downstream position of the locations of secondary sources should be selected as control objectives,and the ideal control effects should be gotten.Otherwise,the control effect couldn't be gotten and even the sound energies of sound field of target are increased.(4).Active insulation of double-panel structure is built by embedding the active acoustical boundaries into the boundaries of cavity between the upper and lower panels,and the active insulation control method based on the active acoustical boundary is proposed.Whilst,in combination with acoustic radiation mode,active insulation control method of double-panel structure using acoustic radiation mode error sensing strategy is also proposed.Firstly,in this study,the active acoustical boundary is replaced by the simply supported panel bounded into the boundary of the cavity sound field,which is named AABP.Three kinds of control objectives,i.e.,minimization of averaged acoustical potential energy of cavity(MAAPEC),minimization of the kinetic energy of the radiating panel(MKERP),minimization of the radiated sound field(MRSP),minimization of averaged acoustical potential energy of enclosure(MAAPEE),are adopted to optimize the control forces.The existence of AABPs changes the vibro-acoustic coupling performance of double-panel structure.the control strategy based on active acoustical boundary could achieve ideal control effect for three kinds of control objectives,MAAPEE,MAAPEC and MKERP.Among three kinds of control objectives,the control effect of MAAPEE as objective is the best.It is also found that the control performance is increased with increasing number of AABP,the location of control force applied to the AABP and the size of AABP could significantly influence the control effect.in practice,to get the optimal control performance,it is necessary to optimize the size of AABP and the location of control forces according to the actual working scene and the controlled frequency range.Numerical results indicate with the first order acoustic radiation mode as control objective,the optimal control effect could be gotten in the low frequency range(<200 Hz).And in the high frequency range,much more order acoustic radiation modes are required to achieve the active control.Lastly,through calculation and analysis,there are mainly two control mechanisms: modal suppression and modal rearrangement.