Research On Active Acoustic Structure Based On Modal Analysis Approach

Active acoustic strt, cture (AAS) proposed in recent years has been viewed as anencouraging approach to actively control sound radiation from vibrating structures,which is an application of intelligent structures into the field of noise control. In thisdissertation, the key problems encountered in the implementation of such a structurehave been investigated theoretically and experimentally based on modal analysisapproach which links structural vibration modes to acoustic radiation modes. The mainresearch works done are listed as follows.(1) Active control of sound radiation from vibrating structure are summarizedsystematically, and the configuration and main features of AAS are analyzed and thekey technical problems needed to be solved are pointed out.(2) Sound radiation from vibrating structures is analyzed based on structuralvibration mode and acoustic radiation mode respectively. The approach used tocalculate the normal velocity of vibrating structure surface and sound pressure isprovide, and a near-field approach is used to obtain the sound power output of avibrating body. Then structural vibration modes and acoustic radiation modes arecombined to investigate sound radiation from a vibrating structure and two key issuesare solved: 1) the dominant radiation modes corresponding to different structuralvibration modes or frequencies are determined; 2) the effects of structural modalcoupling on the total radiated sound power are assessed quantitatively.(3)The structural-acoustic coupled model of AAS has been established. Physicalmechanisms of active control of sound radiation with AAS are investigated and therules for secondary sound sources arrangement are given. Based on structural vibrationmode and acoustic radiation mode, the laws of the arrangement optimization ofsecondary sound sources are derived for different kinds of conditions. The effects of thearea and modal distribution of secondary panel on active reduction are examined.(4)Two kinds of near-field error sensing strategies for AAS based on distributeddisplacement and near-field sound pressure have been studied. A limited number ofPVDF film pairs could be boned to the surface of the primary panel and secondarypanels for measuring the total the radiated sound power. The shape of the PVDF pairsand corresponding reduction in the radiated sound power are achieved. In designingPVDF sensors, it is difficult but important to choose the maximum order of shapecoefficients and the order of the structural modes, and to determine the location of the center line for placing the PVDF pairs. A new approach of designing PVDF pairs inpartitioned frequency band is presented to resolve the conflict in the choosing process.The optimized design approach and the criterion for determining the location of thecenter line for placing the PVDF pairs is given. Active control using structure surfacepressure and measuring plane pressure sensing is investigated respectively. Three kindof near-field pressure based active control cost functions for AAS are presented andapplied to active control of radiated single and broadband noise. Computer simulationson sound power reduction under three cost functions are conducted to show the validityof the control strategies. Active control effect from two different kinds of error sensingstrategies is compared.(5) Physical mechanism of noise reduction in AAS is investigated from severaldifferent points of view. Firstly, the physical mechanism is analyzed based on acousticradiation mode and the relationship between the radiation modes which could becancelled and the number of secondary sound sources is obtained. Secondly, under theminimization of the total sound power output, the physical mechanism is investigatedby analyzing the sound power output change of the primary and secondary structures.The results show that there are three mechanisms in active control, which are energyrestraint, energy absorption and energy un-absorption. The change of sound intensitydistribution of structure surface is calculated to validate the above conclusion. Finally,the change of sound intensity and pressure distribution is calculated and analyzed. Fornear-field sound intensity distribution, the effect of active control is revealed byamplitude restraint and direction adjusting for sound intensity.(6) Experiments for active control of sound radiation from a vibrating steel plateusing distributed planar secondary sources are conducted. The experimental resultsshow that: 1) using one planar secondary source, the sound power of (odd, odd)modescan be reduced. 2) Using two planar secondary sources, the sound power of not only(odd, odd) modes but also (odd, even) modes can be reduced. 3) The area andarrangement location of planar secondary sources have important influence on noisereduction. Using one planar secondary source, the larger the area is, the better thecontrol effect is. 4) The near field pressure based error sensing strategies are effectiveand feasible. The sound power calculated in terms of the sound pressures abovenear-field measuring plane can be used as an objective function, which is consistentwith the total radiated sound power. 5) After control, the far field pressure and intensitycan be reduced and partial acoustic energy is transferred into near field, the distributionof near field pressure and intensity are also changed distinctly.