Time Reversal Acoustics Research Articles

Selective source reduction to identify masked sources using time-reversed acoustics

This paper describes a time-reversed acoustics (TRA) method of spatially illuminating a source signal, which has been masked by another source signal. The selective source reduction (SSR-TRA) method employs a subtraction technique where one focus is selectively reduced to illuminate the masked focus. In this paper, numerical and experimental results are presented to demonstrate to what degree the SSR-TRA method is successful. The SSR-TRA method is demonstrated for two elastic wave pulses emitted simultaneously from two spatially separated sources of differing amplitudes. Results are presented from experiments conducted with two different solid samples: Aluminum and doped silica glass. Applying the SSR-TRA method, a stronger source, up to 13 times stronger than a weaker one, may be reduced to reveal information about the weaker source. Spatial and/or temporal characteristics of multiple close proximity sources can be resolved with the use of the SSR-TRA method. The results show that the SSR-TRA methods’ limitations are chiefly due to imperfect reconstruction of the source function in the time-reversed focal signal. [This work was supported by Institutional Support (LDRD) at Los Alamos National Laboratory.]

Pressure sensitivity kernels applied to time-reversal acoustics

Time-reversal has been found to be more robust to sound speed perturbations than a one-way transmission. Various explanations have been advanced to account for the lower sensitivity of time-reversal. In this contribution, the robustness of time-reversal is quantitatively examined using sensitivity kernels. A first-order Born approximation is used to obtain the pressure sensitivity of the received signal to small changes in medium sound speed. The pressure perturbation to the received signal caused by medium sound speed changes is expressed as a linear combination of single-frequency sensitivity kernels weighted by the transmit signal in the frequency domain. This formulation can be used to predict the response of a source transmission to sound speed perturbations. The stability of time-reversal is studied and compared to that of a one-way transmission using sensitivity kernels. In the absence of multipath, a reduction in pressure sensitivity using time-reversal is only obtained with multiple sources. This can be attributed both to the presence of independent paths and to cancellations that occur due to the overlap of sensitivity kernels for different source-receiver paths. [Work supported by ONR.]

Reference-Free NDT Technique for Debonding Detection in CFRP-Strengthened RC Structures

This study attempts to develop a real-time debonding monitoring system for carbon fiber-reinforced polymer (CFRP) strengthened structures by continuously inspecting the bonding condition between the CFRP layer and the host structure. The uniqueness of this study is in developing a new concept and theoretical framework of nondestructive testing (NDT) in which debonding is detected without relying on previously obtained baseline data. The proposed reference-free damage diagnosis is achieved based on the concept of time reversal acoustics (TRA). In TRA, an input signal at an excitation point can be reconstructed if the response signal measured at another point is reemitted to the original excitation point after being reversed in the time domain. Examining the deviation of the reconstructed signal from the known initial input signal allows instantaneous identification of damage without requiring a baseline signal representing the undamaged state for comparison. The concept of TRA has been extended to guided wave propagations within the CFRP-strengthened reinforced concrete (RC) beams to improve the detectability of local debonding. Monotonic and fatigue load tests of large-scale CFRP-strengthened RC beams are conducted to demonstrate the potential of the proposed reference-free debonding monitoring system.

Time reversal acoustic focusing with liquid-filled reverberator

The use of solid acoustic reverberators for temporal and spatial focusing of acoustic energy based on time reversal acoustic (TRA) principle was described in the literature. We experimentally and theoretically demonstrated that the liquid-filled TRA reverberators can also efficiently focus acoustic energy and compared the advantages of solid and water-filled reververators. A theoretical model was developed to explore the factors defining the efficiency of TRA focusing, providing the design constraint for the water-filled reververators. The model allows evaluation of the spatial and temporal characteristics of TRA focused acoustic field formed as a result of numerous reflections of ultrasound within the reverberator. The TRA focused field structure, its dependence on the shape and size of the resonator, ultrasound frequency, and attenuation were investigated for the standard and binary modes of TRA focusing. The experiments with custom built liquid-filled acoustic resonators confirmed the predictions of a developed theory, including sharpening of the focal area and increasing of the focal intensity in a binary mode of TRA focusing. The conducted research supports suggestion that such liquid-filled reverberators can be considered as a base for a miniature limited range ablation devices for biomedical applications.

Active acoustic time reversal for underwater acoustic barriers

This work addresses the possibility of using successive transmissions of time delayed channel probe pulses between two closely spaced acoustic sensor arrays for forming an acoustic barrier for target detection in shallow water. One array is a transmit-receive array (TRA) while the other is a receive only vertical line array (VLA). The two arrays are connected via cable or wireless. Time reversed replicas of the acoustic channel response to the probe signals are retransmitted into the ocean propagation plane to form focus peaks at each VLA element. It is shown both theoretically and with simulated data that an optimum disturbance detector can be built from the data received at the VLA. This detector becomes sub-optimal due to usual time reversal drawbacks such as ocean non-stationarity and spatial sampling limitations. Real data tests are foreseen to take place during summer 2007 to answer questions such as allowable ranges and frequencies of operation [Work supported by FCT (Portugal) under project POCI/MAR/59008/2004].

Combination of a Time Reversal Process and a Consecutiv Outlier Analysis for Baseline-free Damage Diagnosis

A structural health monitoring system is developed for continuous online monitoring of delamination initiation and growth in composite structures. Structural health monitoring problems are often cast in the context of a statistical pattern recognition paradigm, in which a damage state of a structure is inferred by comparing test data with baseline data. However, subtle signal changes due to damage can often be masked by larger ambient variation of operational and environmental conditions of an in-service structure. Therefore, it is critical for the development of a robust monitoring system to minimize false-positive indications of damage caused by the undesired operational and environmental variation of the structure. The issue of minimizing damage misclassification has been addressed in this article by developing an instantaneous damage detection scheme that does not rely on past baseline data. The proposed instantaneous damage diagnosis is based on the concepts of time reversal acoustics and consecutive outlier analysis, and the proposed damage diagnosis has been tested for detecting delamination in composite plates.

Acoustic impact localization in plates: properties and stability to temperature variation

Localizing an impact generated by a simple finger knock on plate-shaped solid objects is made possible in an acoustic time reversal experiment. It is shown that the technique works with a single accelerometer. To better understand the phenomenon and to know exactly the nature of the created waves, a two-dimensional (2-D) elastic simulation is used, showing that in a very good approximation the A0 Lamb mode is the only propagating one. However, it is shown that, within one wavelength distance from the edges, evanescent waves must be taken into account. As a first consequence, the ability to distinguish two neighboring impacts improves when the plate thickness decreases and the frequency increases. As a second consequence, it is expected theoretically that temperature variations lead to a stretching or a contraction of acoustic signatures. The experimental demonstration used a heterodyne interferometer to measure the impulse responses created by a knock on a plate during the cooling. A simple algorithm is shown to perfectly compensate for temperature impacts, which demonstrates the feasibility of the technique for outdoor time reversal interactive experiments.

Flexible tools for time reversal acoustics focusing applications

Time reversal focusing of acoustic waves has numerous biomedical applications including medical imaging, HIFU treatment, and targeted drug delivery. These various applications pose different requirements. Imaging requires a small spatial focused spot, which can be scanned in 2‐D or 3‐D; therapy requires high deposited energy and controlled‐shape extended focus. Artann Laboratories is developing flexible tools including hardware, algorithms, and software with user‐friendly interface as a universal platform for wide‐range applications, based on the use of the time reversal focusing of acoustic waves. We have built a ten‐channel electronic system with sampling frequency up to 33.3 MHz. It enables simultaneous broadcasting of independent arbitrary signals and recording of the response by a scanning receiver/beacon. A set of functions were developed to enable the control of this setup within Matlab, which offers a high level computing language for numerical computation. With a developed system, a diverse variety of experiments and data analysis can be conveniently conducted within the same platform. Illustrative results on numerous biomedical applications of the time reversal focusing obtained within this framework will be presented.

Comparison of time reversal acoustic and prefiltering methods of focusing of tone burst signals

The concept of time reversal acoustics (TRA) provides an elegant possibility of both temporal and spatial concentration of acoustic energy in highly inhomogeneous media. TRA-based focusing is typically used for generation of short acoustic pulses, however, in some medical and industrial applications, longer pulses are required. TRA focusing of longer signals leads to an increase of side lobes in temporal and spatial domains. Another method for focusing, known as prefiltering, is based on measurements of the impulse response, which relates the signal at the TRA transmitter to that at the focusing point. After evaluating the impulse response, the excitation signal may be calculated to generate the desired waveform in the focus point. This method allows signal generation with any desired form including long tone-burst signals. Experiments on comparison TRA and prefiltering methods of ultrasound focusing were conducted in the frequency band of 200–1000 kHz. In the experiments, focused acoustic pulses with various forms and duration were generated: triangular, rectangular, and amplitude-modulated tone burst signals. The prefiltering modes provide better temporal compression of the focused signal, and the signal energy outside the main pulse in the prefiltering mode was shown to be much lower than that in standard TRA focusing.

Time-reversal acoustic focusing of waves produced by seismic vibrators

A land mine detection system has been developed based on excitation of seismic waves by four mechanical shakers and measurements of ground vibration by a 16-beam, scanning laser Doppler vibrometer (LDV). Time-reversal acoustic (TRA) focusing was applied to concentrate seismic waves using the above system. Linear swept frequency signals and orthogonal initial signals in the frequency band of 100–500 Hz were used in outdoor testing. The system impulse response from any shaker to the LDV output was computed by cross correlating the original and recorded signals. With orthogonal signals, the system impulse responses between multiple shakers and the LDV were measured simultaneously, thus reducing the overall measurement time. Each shaker then reradiates the time-reversed impulse responses focusing the seismic energy to a point on the ground. Experiments demonstrate that the TRA focusing provided a high concentration of elastic wave energy in the test area with typical focusing spots about 20 cm in diameter. [Work supported by U.S. Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate.]

Efficiency of time-reversed acoustics for nonlinear damage detection in solids

Time-reversed acoustics (TRA) has been developed in the last few years as a powerful tool for several applications, based on the theoretical properties emerging from the time reversal invariance of the wave equation. TRA is expected to be a good basis for the development of imaging techniques in the field of nondestructive evaluation. For this purpose, however, data processing is necessary to discriminate between images due to defects (in general nonlinear scatterers) and images due to linear inclusions, boundaries, etc. We propose here an approach based on the filtering of the time signals. The image of the scatterer is obtained through numerical simulations of the back propagation in a fictitious reference specimen. We validate the approach using the inversion of synthetic data. We also estimate the robustness of the procedure in the presence of constraints that can occur in any experimental procedure.

The focused acoustic fields time reversal experiment series

We review a progression of at-sea experiments that brought the concept of underwater, acoustic time-reversal (TR) focusing in the sea from an unverified hypothesis to reality. When the experiments began, it was completely unknown whether the time-varying nature of the oceanic environment would support sufficient temporal stability for the method to succeed, even at long (λ>3 m) acoustic wavelengths. A 10-year progression of experiments showed that not only was this possible, but that various applications could be implemented using higher frequency sound (up to 18 kHz), including phase coherent digital communications. Our most recent experiment, FAF-06, investigated multiuser applications for passive TR communications using small transmit and receive arrays. We demonstrated throughput of 10 kbps at several km range in a shallow-water propagation channel where the transfer function had an ISI span of several hundred symbols. [Work supported by NATO and ONR.]

A new nonlinear elastic time reversal acoustic method for the identification and localisation of stress corrosion cracking in welded plate-like structures – A simulation study

In order to reduce the costs related to corrosion damage in aircraft structures, it is vital to develop new robust, accurate and reliable damage detection methods. A possible answer to this problem is offered by newly developed nonlinear ultrasonic techniques, which monitors the nonlinear elastic wave propagation behaviour introduced by damage, to detect its presence and location. In this paper, a new nonlinear time reversal technique is presented for the detection and localization of a scattered zone (damage) in a multi-material medium. In particular, numerical findings on a friction stir-welded aluminium plate-like structure are reported. Damage was introduced in the heat affected zone and modelled using a multi-scale material constitutive model (Preisach–Mayergoyz space). Studies were conducted for two different transducer configurations. Particular attention was devoted to find the optimum time-reversed window to be re-emitted in the structures. The methodology was compared with traditional time-reversal acoustics (TRA), showing significant improvements. While the traditional TRA was not able to clearly localise the damage, the developed technique identified in a clear manner the faulted zone, showing its robustness to locate and characterize nonlinear sources, in presence of a multi-material medium.

Microscale multiple scattering of coherent surface acoustic wave packets probed with gigahertz time-reversal acoustics

The multiple scattering of coherent surface acoustic wave packets in a microstructure is studied using an ultrafast optical technique. By recording a set of acoustic transfer functions, we show that it is possible to implement time-reversal acoustics and refocus the wave packets up to the GHz range, two orders of magnitude higher than usual. Many applications in time-reversal acoustics are thus transposable to correspondingly smaller structures, opening the way to efficient nondestructive characterization and manipulation of multiple scattering on the microscale.

Correlation of random wavefields: An interdisciplinary review

This paper presents an interdisciplinary review of the correlation properties of random wavefields. We expose several important theoretical results of various fields, ranging from time reversal in acoustics to transport theory in condensed matter physics. Using numerical simulations, we introduce the correlation process in an intuitive manner. We establish a fruitful mapping between time reversal and correlation, which enables us to transpose many known results from acoustics to seismology. We show that the multiple-scattering formalism developed in condensed matter physics provides a rigorous basis to analyze the field correlations in disordered media. We discuss extensively the various factors controlling and affecting the retrieval of the Green’s function of a complex medium from the correlation of either noise or coda. Acoustic imaging of complex samples in the laboratory and seismic tomography of geologic structures give a glimpse of the promising wide range of applications of the correlation method.

Time-reversal acoustics in complex environments

Time-reversal mirrors (TRMs) refocus an incident acoustic field to the position of the original source regardless of the complexity of the propagation medium. TRMs have now been implemented in a variety of physical scenarios from megahertz ultrasonics with aperture sizes of the order of centimeters to ocean acoustics at hundreds to thousands of hertz with aperture sizes of the order of hundreds of meters. Common to this broad range of scales is a remarkable robustness — exemplified by observations at all scales — that the more complex the medium between the probe source and the TRM, the sharper the focus. The relationship between the medium complexity and the size of the focal spot is studied in this paper. This relationship is certainly the most exciting property of TRM compared to standard focusing devices. A TRM acts as an antenna that uses complex environments to appear wider than it is, resulting in a broadband pulse with a refocusing quality that does not depend on the TRM aperture. In this paper, we investigate both the role of the time-reversal window duration and the bandwidth of the time-reversed signals for various media (waveguide, closed cavity, random medium).

Wide-bandwidth time-reversal techniques for landmine detection

Detection of landmines using acoustic to seismic wave propagation has shown that accurate landmine localization is possible by sensing vibrations of a soil surface above the mine. Time-reversal acoustic (TRA) focusing allows concentration of elastic energy at a location in the soil that increases landmine vibration and improves signal/noise levels. Previous experiments with TRA landmine detection utilized narrow-band signals, but in the most recent experiments wide-bandwidth signals provide clear excitation of the mine resonant frequencies. A loudspeaker array broadcasts these signals and the resulting soil vibration is measured by a laser Doppler vibrometer. The experiment utilizes linear frequency sweeps and orthogonal signals with frequency content between 50 and 500 Hz. Using the orthogonal signals, the suggested TRA procedure can be implemented simultaneously with multiple transmitters to increase the scanning speed. The spatial distribution of energy on and off mine demonstrates that the TRA system concentrates elastic wave energy in the desired area with the focal spot dimension comparable to a wavelength. The spectral density of the TRA focused signal increases in the area near the resonant frequency of the landmine.

A novel application of time-reversed acoustics: Salt-dome flank imaging using walkaway VSP surveys

In this paper we present initial results of applying Time-Reversed Acoustics (TRA) technology to salt-dome flank, seismic imaging. We created a set of synthetic traces representing a multilevel, walkaway VSP for a model composed of a simplified Gulf of Mexico vertical-velocity gradient and an embedded salt dome. We first applied the concepts of TRA to the synthetic traces to create a set of redatummed traces without having to perform velocity analysis, moveout corrections, or complicated processing. Each redatummed trace approximates the output of a zero-offset, downhole source and receiver pair. To produce the final salt-dome flank image, we then applied conventional, poststack, depth migration to the zero-offset section. Our results show a very good image of the salt when compared to an image derived using data from a downhole, zero-offset source and receiver pairs. The simplicity of our TRA implementation provides a virtually automated method to estimate a zero-offset, seismic section as if it had been collected from the reference frame of the borehole containing the VSP survey.

In solid localization of finger impacts using acoustic time-reversal process

Time reversal in acoustics is a very efficient solution to focus sound back to its source in a wide range of materials including reverberating media. It expresses the following properties: A wave still has the memory of its source location. The concept presented in this letter first consists in detecting the acoustic waves in solid objects generated by a slight finger knock. In a second step, the information related to the source location is extracted from a simulated time reversal experiment in the computer. Then, an action (turn on the light or a compact disk player, for example) is associated with each location. Thus, the whole system transforms solid objects into interactive interfaces. Compared to the existing acoustic techniques, it presents the great advantage of being simple and easily applicable to inhomogeneous objects whatever their shapes. The number of possible touch locations at the surface of objects is shown to be directly related to the mean wavelength of the detected acoustic wave.

Advantages of time reversal acoustic focusing system in biomedical applications

The development and biomedical applications of time reversal acoustics (TRA) systems for focusing and manipulating ultrasound beams are reviewed. The TRA focusing system (TRA FS) is capable to deliver ultrasound energy to the chosen region in highly inhomogeneous medium (including soft tissues and bones) with focusing efficacy hardly achievable using conventional phased array transmitters. TRA FS is able to focus and stir ultrasound beams in a 3‐D volume using just a few piezoceramic transducers glued to the facets an aluminum block. Another advantage of TRA FS is its ability to produce pulses with arbitrary waveforms in a wide frequency band. A custom‐designed compact multichannel TRA system operating in a wide frequency range from 0.01 to 10 MHz has been developed. Measurements of TRA field structure were conducted in a large variety of inhomogeneous tissue phantoms and ex vivo bones and soft tissues. Principles of TRA focusing optimization based on acoustical properties of the resonator material, parameters of the sonicated medium, and the coupling of the TRA resonator with the medium were developed and applied in the tested TRA systems. [Work was supported by NIH.]