Index of content:
Volume 120, Issue 2, August 2006
- ACOUSTIC SIGNAL PROCESSING 
Wideband multichannel time-reversal processing for acoustic communications in highly reverberant environments120(2006); http://dx.doi.org/10.1121/1.2211588View Description Hide Description
The development of multichannel time-reversal (T/R) processing techniques continues to progress rapidly especially when the need to communicate in a reverberant environment is critical. The underlying T/R concept is based on time-reversing the Green’s function characterizing the uncertain communications channel mitigating the deleterious dispersion and multipath effects. In this paper, attention is focused on two major objectives: (1) wideband communications leading to a time-reference modulation technique; and (2) multichannel acoustic communications in two waveguides: a stairwell and building corridors with many obstructions, multipath returns, severe background noise, disturbances, and long propagation paths including disruptions (bends). It is shown that T/R receivers are easily extended to wideband designs. Acoustic information signals are transmitted with an eight-element array to two receivers with a significant loss in signal levels due to the propagation environment. The results of the new wideband T/R processor and modulation scheme demonstrate that the overall performance for both high and low bit level analog-to-digital converter designs. These results are validated by performing proof-of-principle acoustic communications experiments in air. It is shown that the resulting T/R receivers are capable of extracting the transmitted coded sequence from noisy microphone array measurements with zero-bit error.
120(2006); http://dx.doi.org/10.1121/1.2214393View Description Hide Description
This paper proposes an efficient array beam-forming method using spatial matched filtering(SMF) for ultrasonic imaging. In the proposed method, ultrasonic waves are transmitted from an array subaperture with fixed transmit focus as in conventional array imaging. At receive, radio frequency echo signals from each receive channel are passed through a spatial matched filter that is constructed based on the system transmit-receive spatial impulse response. The filtered echo signals are then summed without time delays. The filter concentrates and spatially registers the echo energy from each element so that the pulse-echo impulse response of the summed output is focused with acceptably low side lobes. Analytical beam pattern analysis and simulation results using a linear array show that this spatial filtering method can improve lateral resolution and contrast-to-noise ratio as compared with conventional dynamic receive focusing (DRF) methods. Experimental results with a linear array are consistent but point out the need to address additional practical issues. Spatial filtering is equivalent to synthetic aperture methods that dynamically focus on both transmit and receive throughout the field of view. In one common example of phase aberrations, the SMF method was degraded to a degree comparable to conventional DRF methods.
120(2006); http://dx.doi.org/10.1121/1.2215407View Description Hide Description
Acoustic microimaging (AMI) is a common nondestructive tool for failure analysis of microelectronic packages. Accurate estimation of the reflected ultrasonicechoes is essential for detection and location of defects inside the microelectronic packages. In this paper, an advanced AMI technique based on adaptive sparse representations is proposed to estimate the ultrasonicechoes and recover the reflectivity function. An adapted overcomplete dictionary capable of concise expression of ultrasonic signals is first learned by the focal underdetermined system solver-based column normalized dictionary learning algorithm. The ultrasonic A-scans generated by an AMI system are then decomposed into adaptive sparse representations over the learned dictionary using a sparse basis selection algorithm. Echo selection and echo estimation are further performed from the resulting adaptive sparse representations. The proposed technique offers a solution to the blind source separation problem for restoration of the reflectivity function and can separate closely spaced overlapping echoes beyond the resolution of the AMI system. Experimental verifications are carried out using both synthetic and measured data. The results show the proposed technique produces high resolution and accurate estimates for ultrasonicechoes.
120(2006); http://dx.doi.org/10.1121/1.2213523View Description Hide Description
The possibility of determining the location of an acoustic source in the presence of gross sediment uncertainties is investigated. Promising results are obtained using focalization, which involves constructing ambiguity surfaces corresponding to randomly selected realizations of the sediment parameters. Due to a parameter hierarchy in which the source location is more important than environmental parameters, it is often possible to reliably determine the source position without determining the correct sediment parameters. The examples involve multiple sediment layers, with sound speeds and range-dependent thicknesses that are unknown. An example that includes both sediment uncertainties and internal waves is also included.
Multiple scattering between two elastic cylinders and invariants of the time-reversal operator: Theory and experiment120(2006); http://dx.doi.org/10.1121/1.2217128View Description Hide Description
The decomposition-of-the-time-reversal-operator method is an ultrasonic method based on the analysis of the array response matrix used for detection and characterization. The eigenvalues and the eigenvectors of the time-reversal operator (equivalent to the singular values and the singular vectors of the array response matrix) provide information on the localization and nature of scatterers in the insonified medium. Here, the eigenmodes of the time-reversal operator are studied for two elastic cylinders: The effects of multiple scattering and anisotropicscattering are considered. Analytical expressions for the singular values are established within the isotropic scattering approximation. Then, the comparison with a complete model is presented, putting in evidence the importance of the anisotropy of the scattering. Experiments, carried out at central frequency on diameter nylon and copper cylinders embedded in water, confirm the theory. In particular, the small cylinder limit and the effect of the dominant quadrupolar normal mode of nylon are discussed.