D. Kent Lewis

Verification of the localized‐wave transmission effect

An acoustic array driven with a designed set of localized‐wave (LW) solutions of the scalar‐wave equation generates a robust, well‐behaved, transient pencil beam of ultrasound in water. The performance of the LW‐pulse‐driven array theoretically and experimentally exceeds a tenfold improvement over related continuous‐wave excitations of the same array.

Diffraction of ultrasonic waves by penny‐shaped cracks in metals: Theory and experiment

In this paper an analytical solution to the diffraction of elastic waves by penny‐shaped cracks in metals is compared with experimental observations. The analysis, which is based on elastodynamic ray theory, is valid for the region of ka ≳ 1. A digitized spectrum analysis system is described which measures the frequency components of the waves diffracted from a 2500‐μ‐radius crack in diffusion bonded titanium. The amplitude spectra show good agreement between experiment and theory. The theoretically predicted periodicity of the diffracted spectra provides a simple formula for the inverse problem. Application of this formula to the experimental measurements determines the crack size with excellent accuracy.

Transient waves: Reconstruction and processing

New solutions to the wave equation have been shown to exhibit enhanced localization and energy fluence characteristics. The transmission and reception of these localized waves create unique problems, since they are essentially transient wave fronts in both time and space. Nonetheless, the ability to transmit wave energy through space with these interesting properties has many potential applications in a variety of applications areas. To realize their potential, new methods must be developed to analyze and process these waves. In this paper, approaches to design receiving arrays to reconstruct these special transient waves from noisy measurement data are discussed.

Transient wave estimation: A multichannel deconvolution application

The evolution of new concepts in wave theory have led to proof‐in‐principle experiments aimed at validating the generation of a specified wave front. Not only have these concepts initiated research in transient wave theory, but they have also caused renewed effort in multichannel signal processing. In this paper, the development of a processor to deconvolve a transient acoustic wave from sensor array measurements is discussed. The design of the multichannel deconvolver coupled with model‐based signal processing techniques using acoustic pressure field measurements is discussed. Here, it is shown that an efficient solution to this problem can be obtained using a vector form of the Levinson–Wiggins–Robinson (LWR) algorithm.

GENERATION OF UNIPOLAR ULTRASONIC PULSES BY SIGNAL PROCESSING

Synthetic unipolar pulses have been generated, using signal processing, in a pulse/echo ultrasonic system. Analysis reveals that sending-receiving transduction and propagation can each be partially described by the mathematical process of differentiation. Multiple integration of the received signals yield unipolar pulses if the excitation signals applied to the transducer are unipolar.

Experimental results of localized wave experiments

The theory for several low diffraction beams has been upheld by experimental verification in several tests to date. Both the modified pulse spectrum and the superposed Gaussian beams have been verified by Livermore projects in the laboratory and in open water tests. Test methods, instrumentation, data analysis techniques, and results will be discussed in this presentation. [This work was performed under the auspices of the Department of Energy by Lawrence Livermore National Laboratory under Contract W‐7405‐ENG‐48.]

Localized wave beam forming tests

Much has been reported about work done creating the localized wave (LW) acoustic pulse using active beam creation. This presentation will show work done in passive beam forming using the LW source signals as array filter coefficients. The simple tests which were performed showed that the LW filter can detect wide bandwidth signals in an idealized medium, and that there is an apparent increase in signal resolution. These results suggest that there may be applications for other low‐diffraction beam techniques in passive signal processing. [This work was performed by the Lawrence Livermore National Laboratory under the auspices of the U.S. Department of Energy under Contract No. W‐7405‐ENG‐48.]

Broadband processing using localized waves in a shallow ocean environment

In this paper the MPS localized wave pulse is used as the basis for designing a broadband signal processor to discriminate between two point sources. The nonseparable nature of the MPS pulse requires a ‘‘best fit’’ approach to processor design. This approach illustrates the kinds of problems one would encounter when using localized waves in a processor. The processor design is described, then applied to simulated acoustic signals generated in a shallow ocean environment. The results show that the MPS‐based processor can discriminate between the two sources at least as well as a more conventional broadband beamformer. No attempt is made to estimate the positions of the sources. [This work was performed under the auspices of the Department of Energy by the Lawrence Livermore National Laboratory under Contract W‐7405‐Eng‐48.]

Time‐frequency analysis of low diffraction beam pulses

Recent work in low diffraction beams has given our group some new sets of wave fields to investigate. Theoretical predictions matched experimental measurements closely, and several comparison techniques were included in the analysis. The field experiments have now been conducted at the NUWC Keyport facility and, most recently, at the University of Texas’ Applied Research Laboratory facility in Lake Travis. In cooperation with ARL, a series of low diffraction beam pulses was launched to a wide band width receiver 600 feet distant. Among the comparison methods used was a time‐frequency analysis of the received pulse. For each angular position the frequency arrival times via time‐frequency analysis are investigated. It is hoped this analysis will shed light on this very interesting phenomena.

Localized wave generation with a standard underwater array

Recent work at the NUWC Keyport has generated localized waves using Navy field equipment. The array, a rectangular arrangement of tonpiltz elements, was driven with precompensated signals to generate sound covering a decade of usable bandwidth. The elements were grouped by radius from the center and excited by signals designed specifically for each radius, for a total of 7 radii. Results of the angular scans show a narrowed beam pattern and lowered side lobes relative to design signals. Results of axial range scans show an increase in directivity of more than an order of magnitude over array specific tone bursts. Frequency analysis shows that the main beam is an octave wide while the surviving side lobes are narrow bandwidth.