Theoretical Analysis On Transmission And Attenuation Ofstructure-borne Sound Generated By Power Machineries

Transmission of structure-borne sound and structural vibration caused by power machinery in ships and structure of ocean engineering is an issue of great interest to researchers for years. Vibration at high level not only affects operation of machinery and electronic facilities but also causes structural fatigue destruction. Transmission of structure-borne sound also causes secondary sound radiation, which is bad for habitability on board. For this reason, fundamental researches on mechanism of transmission and attenuation of structure-borne sound caused by power machinery on board are carried out in this paper by theoretical analysis associated with simplified experiment.Complex structure is divided into simpler ones. Investigations in this thesis start with the mechanism of vibration energy transmission at single-corner structure with different attenuation means applied at corner interface. Then structure-borne sound attenuation in complex multi-corner structures is carried out by using results form single-corner structure, which simplifies analyses and computations. For the particularity of ship and structure of ocean engineering, attenuation effects of passive control means such as blocking mass and dynamic vibration absorber on vibration energy transmission are investigated theoretically and experimentally. Besides, feasibility of active control with secondary control forces and moments for structure vibration is also carried out.Transmission and attenuation means of structure-borne sound is involved from chapter 2 to chapter 5 in this thesis. And the model under discussion is developed form infinite to finite scale, from sinusoidal wave to random time dependent wave incidence and from single-corner to multi-corner structure, taking bending longitudinal waves as well as near field into consideration.For analysis of single corner structure, wave approach is used to deduce equilibrium equations at corner interface. Some new non-dimensional coefficients implying corresponding ratios of characteristic impedances between plates and attenuation facilities were creatively introduced to make equilibrium equations non-dimensional so as to generalize equations governing transmission and reflection coefficients at corner interface with different kinds of attenuation facilities attached. It is shown from theoretical analysis that transmission of structure-borne sound in single corner structure depends mostly on these non-dimensional coefficients, which not only indicates that introduction of these non-dimensional coefficients help to reveal relevant physical sense by making connection of transmission and reflection coefficients with corresponding ratios of characteristic impedances and simplified deduction of formulas but also facilitates similarity experiment with small models for investigations on structural vibration in large practical engineering structure. And it also provides an approach for design of new attenuation facilities. Besides, a new model description for single-corner structure of finite scale is presented by using wave approach associated with transmission and reflection coefficients at corner interface of two semi-finite plates, which convert solution of modal and natural frequencies for a continuous structure into the problem to solve eigenvalue and eigenvector of a non-dimensional matrix.Numerical computations on some typical corner interface of two steel plates with different attenuation facilities attached are carried out to investigate effects of parameters of corner structure and attenuation facilities on vibration energy transmission. It is found that: 1) Structure-borne sound transmission at corner interface without any attenuation facility depends mostly on thickness of the plates and angles of the corner. 2) Blocking mass attached at corner interface acts as a kind of low pass filter which divides the whole frequency domain into"transmission band"and"attenuation band"with an intermediate transition band in between. The transmission loss in"attenuation band"depends mainly on mass per unit length and band width of"pass band"on mass moment of inertia per unit length of the blocking mass. 3) Vibration energy transmission at corner interface with dynamic vibration absorber attached depends greatly on whether the absorber attached acts at resonance and is relatively lower right after the nature frequency of dynamic vibration absorber. And dynamic vibration absorber attached provides less energy transmission of bending wave than blocking mass at the end of"passing band".Experiments on simplified models are carried out as well. Ratios of vibration energy density and insertion losses at corner interfaces with different attenuation means applied are tested and compared to validate some results from theoretical analysis and numerical investigations. Factors which cause discrepancies in the test are discussed in detail. It is found that results from measurement and prediction show almost the same trend with each other and the discrepancies between them increase with frequency. Furthermore a new kind of reciprocity experiment for structural vibration based on"reciprocity principle"for structure-borne sound transmission between bending and longitudinal waves, which converts the measurement of transverse velocities under longitudinal excitations into that of longitudinal velocities under transverse excitations and facilitate the test.A simplified method is introduced for analysis in multi-corner structure. Firstly, vibration velocity fields are expressed as the combination of elastic waves satisfying transverse and longitudinal vibration differential equations and propagating in both directions in plates. Then linear equilibrium equations governing boundary velocities and excitation forces at corner interfaces are deduced by using transmission and reflection equations, which avoids solving complex vibration PDE and simplifies analysis. Numerical investigations are carried out on a double-corner structure consisting of three steel plates with double blocking masses attached. It is found that transmission losses in double-corner structure fluctuate greatly with frequency, which depends mostly on whether the intermediate plate acts at resonance and anti-resonance. At the same time, double blocking mass at double corner interfaces are also effective for attenuation of structure-borne sound transmission just as they behave in single-corner structures. Experiment on a simplified model analogous to the outer hull of a ship is also carried out to compare the averaged transmission loss in eight 1/1 octave band at two double-corner structure with and without double blocking masses attached respectively. The results form measurement and prediction match well with each other for the reason that both spatial and frequency average are achieved. Besides, single-corner structure with double blocking masses attached is treated as the special case of double-corner structure and transmission losses in single-corner structure with double blocking masses are computed and compared with those with single blocking mass of the same weight attached by using equations governing multi-corner structure. It is found that reduction of bending wave energy at single corner depends greatly on whether the finite plate between double blocking masses act at resonance. Sketch of double blocking masses can achieve more transmission loss than that of single blocking mass under the same weight being used at single-corner structure. For longitudinal wave incidence, the second blocking mass attached downstream achieves better attenuation effect than the second one attached upstream and it makes no difference for bending wave incidence.Active structure-bone sound control by secondary control forces and moment is discussed in chapter 6, similar to the approach being used for investigation on structure-borne sound attenuation in multi-corner structure. Vibration velocity fields under ANC are divided into primary and secondary field. Equilibrium equations in both fields are deduced and generalized by using non-dimensional coefficients. The solution for optimized control fore and moment are also given out, which validate the feasibility of ANC in corner structures.The mechanism of structure-borne sound transmission in build-up structures as well as the effect of attenuation means is investigated in detail in this thesis. The results from theoretical analysis and simplified experiment are helpful to acoustic design on ships and structure of ocean engineering keeping structural intensity and geometry sketch unchanged. It also makes it possible to realize optimize attenuation of structure-borne sound in complex structures by combination of all attenuation means discussed above. And the researches in this paper offered a theoretical fundament for further research in this field.