Index of content:
Volume 125, Issue 6, June 2009
- ACOUSTIC SIGNAL PROCESSING 
Synchronized time-reversal focusing with application to remote imaging from a distant virtual source array125(2009); http://dx.doi.org/10.1121/1.3117374View Description Hide Description
Time-reversing the transfer function between a time-reversal mirror (TRM) and a distant probe source location generates an acoustic spatio-temporal focus at the location. It is shown that a TR focus behaves as a “virtual” source (in the far-field limit) in the outbound direction with respect to the TRM. By extension, a collection of TRM-to-probe source transfer functions constitutes a virtual source array (VSA) that can serve as a remote platform for active imaging methods such as beam-steering and other coherent wavefront techniques. As a demonstration, a set of a-priori sampled TRM-to-VSA transfer functions are steered to coherently focus at a selected location beyond the VSA for which the transfer function is not known a-priori. In this case the VSA acts as a lens that refocuses the TRM field to the target location. Under proper conditions, the resolution is comparable to that of standard TR. While the specific application of active focusing is presented as a validation of the concept, the relationship between coherent focusing and the transfer function implies that the virtual array concept may find use in a range of imaging methods, both active and passive. Possible applications are discussed, and simulation and experimental results are presented.
125(2009); http://dx.doi.org/10.1121/1.3126925View Description Hide Description
A method is introduced where blind source separation of acoustical sources is combined with spatial processing to remove non-Gaussian, broadband interferers from space-time displays such as bearing track recorder displays. This differs from most standard techniques such as generalized sidelobe cancellers in that the separation of signals is not done spatially. The algorithm performance is compared to adaptive beamforming techniques such as minimum variance distortionless response beamforming. Simulations and experiments using two acoustic sources were used to verify the performance of the algorithm. Simulations were also used to determine the effectiveness of the algorithm under various signal to interference, signal to noise, and array geometry conditions. A voice activity detection algorithm was used to benchmark the performance of the source isolation.
Truncated aperture extrapolation for Fourier-based near-field acoustic holography by means of border-padding125(2009); http://dx.doi.org/10.1121/1.3126994View Description Hide Description
Although near-field acoustic holography (NAH) is recognized as a powerful and extremely fast acoustic imaging method based on the inverse solution of the wave-equation, its practical implementation has suffered from problems with the use of the discrete Fourier transformation(DFT) in combination with small aperture sizes and windowing. In this paper, a method is presented that extrapolates the finite spatial aperture before the DFT is applied, which is based on the impulse response information of the known aperture data. The developed method called linear predictive border-padding is an aperture extrapolation technique that greatly reduces leakage and spatial truncation errors in planar NAH (PNAH). Numerical simulations and actual measurements on a hard-disk drive and a cooling fan illustrate the low error, high speed, and utilization of border-padding. Border-padding is an aperture extrapolation technique that makes PNAH a practical and accurate inverse near-field acoustic imaging method.