Error sensing strategy for active acoustic structure based on distributed displacement sensors

The nearfield error sensing approach, which is one of the key problems encountered in the implementation of active acoustic structure, is investigated theoretically. The basic idea involves a limited number of PVDF film pairs be boned to the surface of the primary panel and secondary panels for measuring the total radiated sound power. In this paper, first, a theoretical model associated with the active acoustic structure is established. Second, the formulae for the shape coefficient of the PVDF pairs are derived based on the acoustic radiation modes and a PVDF sensing model. Finally, a number of computer simulations are performed and the shape of the PVDF pairs and corresponding reduction in the radiated sound power are calculated. The results demonstrate the feasibility of the proposed nearfield error sensing strategy.     

Research on modal analysis of structural acoustic radiation using structural vibration modes and acoustic radiation modes

Modal analysis of structural acoustic radiation from a vibrating structure is discussed using structural vibration modes and acoustic radiation modes based on the quadratic form of acoustic power. The finite element method is employed for discretisizing the structure. The boundary element method and Rayleigh integral are used for modeling the acoustic fluid. It is shown that the power radiated by a single vibration mode is to increase the radiated power and the effect of modal interaction can lead to an increase or a decrease or no change in the radiated power, moreover, control of vibration modes is a good way to reduce both vibration and radiated sound as long as the influence of interaction of vibration modes on sound radiation is insignificant. Stiffeners may change mode shapes of a plate and thus change radiation efficiency of the plate‘s modes. The CHIEF method is adopted to obtain an acoustic radiation mode formulation without the nonuniqueness difficulty at critical frequencies for three-dimensional structures by using Moore-Penrose inverse. A pulsating cube is involved to verify the formulation. Good agreement is obtained between the numerical and analytical solutions. The shapes and radiation efficiencies of acoustic radiation modes of the cube are discussed. The structural acoustic control using structural vibration modes and acoustic radiation modes are compared and studied.     

Characterization of electro-acoustics impedance and its application to active noise control

Characteristics of radiation impedance and its inducing variation of electrical impedance for a controllable source have been investigated. An impedance-based error criterion has been proposed and its application to Active Noise Control is demonstrated through a coil driven loudspeaker. A general formula of radiation impedance is derived for two control strategies, according to the criterion of total acoustic power output. The radiation impedances of some commonly used sound sources are calculated. We discuss in detail the relation between variation of the input electrical impedance and radiation impedance for the two control strategies. The measured data of the input electrical impedance from a loudspeaker agree fairly well with theoretical analysis. An AC- bridge circuit is designed in order to measure the weak variation of electrical impedance resulted from radiation impedance. The bridge relative output is unique for a certain control strategy, from which an impedance-based error criterion is then proposed and the implementation of its application to an active control system is analyzed. Numerical results of such criterion are presented. An analogue control system is set up and experiments are carried out in a semi-anechoic chamber to verify the new control approach.     

The relationship between acoustic radiation modes and structural modes and its applications

Both acoustic radiation modes and structural modes play an important role in the field of structure-borne sound, however, little work has been done for inherent relations between these two kinds of modes. This paper is focused on the relationship between the radiation modes and structural modes and its physical mechanisms. First, a governing equation for relating the radiation mode and structural mode is given based on the characteristics of the modes. Then, using the symmetric or anti-symmetric properties of two kinds of modes, the corresponding relations are presented. And then, numerical examples are given to verify the theoretical investigations, and it has been shown that, for a simply supported rectangular panel vibrating at low frequencies, the first radiation mode is dominant corresponding to (odd, odd) structural modes; the following radiation modes are respectively dominant corresponding to (even, odd), (odd, even), and (even, even) structural modes. Finally, such relations are applied to active acoustic structural control and provide a direct help for the design of active control strategy and arrangement of the secondary forces.     

Theoretical study on active control of sound radiation based on planar sound sources

Active control of low frequency sound radiation using planar secondary sources is theoretically investigated. The primary sound field originates from a vibrating panei and the planar secondary sources are modeled as simply supported rectangular panels in an infinite baffle. The sound power of the primary and secondary panels is calculated, and then a series of formulae are derived to obtain the optimum reduction in sound power. Finally, active reduction for a number of secondary panei arrangements is examined, it is concluded that when the modal distribution of the secondary panei does coincide with that of the primary panei, one secondary panei is sufficient. Otherwise four secondary panels can guarantee considerable reduction in sound power over entire frequency range of interest.     

Theoretical study on active control of sound radiation based on planar sound source

Active control of low frequency sound radiation using planar secondary sources is theoretically investigated. The primary sound field originates from a vibrating panel and the planar secondary sources are modeled as simply supported rectangular panels in an infinite baffle. The sound power of the primary and secondary panels is calculated, and then a series of formulae are derived to obtain the optimum reduction in sound power. Finally, active reduction for a number of secondary panel arrangements is examined, it is concluded that when the modal distribution of the secondary panel does coincide with that of the primary panel, one secondary panel is sufficient. Otherwise four secondary panels can guarantee considerable reduction in sound power over entire frequency range of interest.     

The decoupling of the active structural acoustic control based on the theory of radiation modes

With the theory of radiation modes,piezoceramics are used as control actuators in the active control of sound radiation.According to the characteristics of the first four radiation mode shapes,the arrangement of four group of actuators is presented.Theoretical analysis and numerical calculation demonstrate that the four group of actuators can not only reduce the sound power effectively,but also decouple the control equations.While controlling of the sound powers of the first four radiation modes,these actuators can convert the complicated control system with multiple input/output into several simple and independent systems with single input/output.     

Theory of sound field in a room

In the normal-mode theory of Morse, it gives a series of normal modes as the solution of forced vibration in a room. But actually there is always the direct radiation besides the normal modes which represent the reverbrant sound field only. The reason is that the normal modes were assumed only in the source, and naturally normal modes only are obtained in the solution. A theory of double source is proposed, that the sound source is both the source of the direct radiation as if in free space before the boundary surfaces were reached by the direct radiation, and after the first reflection from the boundary surfaces, the source of the reflected wavelets, randomly distributed both in space an in time on the boundary surfaces that build up the normal modes after further reflections. The wave equation is formed accordingly, and the solution of the wave equation, the sound field in a room, contains explicitly both the direct radiation and the reverberant sound formed of normal modes. The approximate mean square sound pressure is found to be the dircet sound determined by the sound power of the source,and reverberant sound determined by the sound power reduced by a factor of π/2, different slightly from the result obtained from energy consideration, if the source is pure tone. There is essentially no difference for a source of band noise.     

The method eliminating nearfield effect for measurement sound absorption coefficient

The numerical simulation is used in this work to study the nearfieldradiation pressure of a piston projector,the smooth effect on the pressure fieldby using a“big”receiver and the influence of the nearfield and in the transitionregion on measurement of sound absorption coefficient.The result indicatesthat the big error would appear when using usual absorption coefficientmeasurement method.According to the reason causing the error,the methodeliminating nearfield effect (MENE) is proposed and some digital results aregiven for a/λ=3,9 and b=0,0.5a,a(a and b are the transducer and re-ceiver radii respectively).The calculation results show that the method not onlywidens measurement range in nearfield,but also gives accurate results.