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Near-field acoustic holography (NAH) is an effective tool for visualizing acoustic sources from pressure measurements made in the near-field of sources using a microphone array. The method involving the Fourier transform and some processing in the frequency-wavenumber domain is suitable for the study of stationary acoustic sources, providing an image of the spatial acoustic field for one frequency. When the behavior of acoustic sources fluctuates in time, NAH may not be used. Unlike time domain holography or transient method, the method proposed in the paper needs no transformation in the frequency domain or any assumption about local stationary properties. It is based on a time formulation of forward sound prediction or backward sound radiation in the time-wavenumber domain. The propagation is described by an analytic impulse response used to define a digital filter. The implementation of one filter in forward propagation and its inverse to recover the acoustic field on the source plane implies by simulations that real-time NAH is viable. Since a numerical filter is used rather than a Fourier transform of the time-signal, the emission on a point of the source may be rebuilt continuously and used for other post-processing applications. [ABSTRACT FROM AUTHOR]

An investigation of structural intensity control associated with flexural vibration in beams using strain sensing is presented in this paper. The instantaneous intensity is completely taken into account in the control algorithm, i.e., all the terms are considered in the real-time control process and, in particular, the evanescent waves are considered in this approach. Previous work has shown the validity of the approach using an array of accelerometers to measure structural intensity. It is the purpose of this paper to present results on the use of strain sensing in the control of structural intensity. A finite difference approach using discrete PVDF strain sensors provides the structural intensity sensing scheme. A feed forward filtered-X LMS algorithm is adapted to this energy-based control problem, involving a nonpositive definite quadratic form in general. In this respect, the approach is limited to cases where the geometry is such that the intensity component will have the same sign for the control source and the primary disturbance. Experimental validation of the approach is conducted on a free-free beam covered with viscoelastic material and excited harmonically by a shaker. A comparison is made between classical acceleration control and structural intensity control and the performance of both approaches is presented. These results tend to indicate that intensity control allows the error sensors to be placed closer to the control source and the primary disturbance, while preserving a good control performance. [ABSTRACT FROM AUTHOR]