Time Reversal Acoustics Research Articles

Time reversal acoustic focusing in a reverberant room: Elongating a focus, and the effect of head-scattering

Time reversal acoustics (TRA) has tremendous potential for focusing, or sound concentration, in reverberant architectural environments. While TRA provides an inherently simple means of achieving a high degree of spatiotemporal focusing, its appropriateness for listening applications in the built environment is challenged by the need to accommodate a person within a sufficiently large focal area. At its simplest, a reciprocal time reversal focusing system for audible range sound will produce a focus with a spatial spread corresponding to one-eighth of the wavelength, which is inconveniently small for speech frequencies. However, an elongated focus is introduced by deploying a directional impulse response, which can be created with a directional microphone, or synthesized by time-delay-mixing of multiple omnidirectional impulse responses. This paper reports on results of physical experiments conducted in a reverberant room, complemented by finite-different time-domain simulations in which the effect of introducing a head-sized scatterer was investigated.

A single-channel virtual receiving array using a time-reversal chaotic cavity.

Chaotic reverberation in a cavity, when coupled with time reversal acoustics, can be harnessed to build a perfect time-reversal mirror for transmitting and receiving highly focused sounds with a small number of transducers. In this article, a virtual receiving array, comprised of a single receiving transducer and a chaotic cavity, is developed based on time reversal processing of the reverberation inside the cavity. A prototype array, having 10 × 10 virtual receiving elements, is built and evaluated against a comparable physical array in terms of its localization and waveform reproduction capabilities. It turns out that the most crucial factor in the success of a virtual array is the ergodicity of its chaotic cavity, the exact mathematical expression for which is also derived. The virtual receiving array presented here may find some niche applications in reverberant environments, where a physical array turns out to be too costly or cumbersome to operate.

Векторная декомпозиция сейсмических данных методами волнового обращения во времени

The paper analyzes the stage of decomposition of the initial seismic data in the methods of wave reversal in time when constructing seismic attributes. Within the framework of the formal approach of mapping the data of one space into the data of a space of a higher dimension, a classification of existing approaches in seismic exploration is given. Identification of the decomposition stage in the seismic data processing workflow makes it possible to highlight the differences in existing approaches to building seismic attributes and predict the future direction of seismic data processing. The concept of vector decomposition, originally used in the RTH method, is introduced. The variety of depth seismic attributes obtained in the RTH method based on vector decomposition allows solving a wide range of problems in the exploration and development of hydrocarbon deposits at a new qualitative level. The RTH method includes, as a special case, the PSDM, AVO, AI methods and is an alternative to the MVA, FWI methods, as well as the method of a velocity model bilding based on fast beam migration algorithms. A close connection between the technique of wavefront time reversal in seismic exploration and analogous time reversal in optics and acoustics is noted. Examples of seismic data processing using vector decompositio to identify zones of natural fracturing in shale oil are given.

Exploring ways to maximize the amplitude of steady state broadband noise signals using time reversal in order to excite structures acoustically

Exciting structures with a broadband noise signal is a method used to assess a structure’s health and how it responds to the noise. Past work has shown that time reversal (TR) acoustics has provided a robust method to focus impulses at high amplitudes. Maximization of the highest possible amplitude while maintaining the desired spectral shape can be challenging. This study explores the benefits of using TR with equalization methods to a steady state broadband noise signal to acoustically excite structures. Tradeoffs exist between the possible amplitude of the focused noise and the duration of the noise and the amount of equalization employed. The spatial extent of the focusing will also be discussed. The use of TR to generate high amplitude broadband noise is compared to standard point application of noise equalization with the overall goal to use this high amplitude noise to excite structures under test.

Non-Destructive Evaluation of Preload Loss of Bolted Spherical Joints Based on Time Reversal Acoustics: A Numerical Simulation

Bolted spherical joints are widely used in space grid structures. However, the preload loss, which may arise from fabrication error, insufficient twist of the screw and some other unknown reasons can undermine the structural integrity and even lead to the collapse of the whole structure. Therefore, a method for evaluating the preload status of bolted spherical joints is vital to the structural health monitoring of the space grid structures. To this end, a non-destructive evaluation of the preload loss of bolted spherical joints based on time reversal acoustics (TRA) is presented. In this method, an excited signal generated by a lead zirconate titanate (PZT) patch at the bolted sphere is reconstructed after the signal recorded by another PZT patch at the bar is time-reversed and re-emitted. The deviation of the reconstructed signal from the initial signal allows identification of preload loss of the bolted spherical joint. This method is demonstrated to be effective by a finite element simulation. Furthermore, the effect of preload loss of the bolted spherical joint on its ultimate flexural capacity is studied, which highlights the necessity for the non-destructive evaluation.

Graduate programs in physical, engineering, and underwater acoustics at the Naval Postgraduate School

The Departments of Physics and of Electrical and Computer Engineering at the Naval Postgraduate School offer graduate programs in acoustics leading to MS and PhD degrees in applied physics and engineering acoustics. Engineering acoustics degrees can be completed in either traditional or distance learning modes. The departments also offer stand-alone academic certificate programs in fundamentals of engineering acoustics, underwater acoustics, and sonar system applications, with a set of three certificates leading to a MS degree in engineering acoustics. MS and PhD programs are interdisciplinary, with courses and laboratory work drawn principally from the fields of physics and electrical engineering. Subjects covered include waves and oscillations; fundamentals of physical and structural acoustics; the generation, propagation, and reception of sound in the ocean; civilian and military applications of sonar systems; and acoustic signal processing. Topics of recent theses and dissertations include development and field testing of novel sensors for atmospheric and ocean acoustics, modeling and measurements of ambient noise and sound propagation in the ocean, sound scattering in underwater waveguides, acoustic vector sensors and vector field properties, acoustic communications, noise interferometry, time reversal in acoustics, geo-acoustic inversion, acoustic remote sensing of the ocean, and acoustics of autonomous underwater and aerial vehicles.

Development of software for performing acoustic time reversal with multiple inputs and outputs

At Brigham Young University, acoustic time reversal experiments are conducted in solids and fluid media. The experimental setup involves synchronized generation and acquisition hardware. The synchronized hardware allows for multi-channel generation, and in some cases, multi-channel acquisition of time reversal focusing. A LabVIEW application has been compiled to increase ease of use and repeatability for the students conducting experiments. Forward and backward steps of time reversal are conducted through this simple user interface. The software is also able to control a 2D positioning system that allows for the measurement of the spatial extent of a time reversal focus. Modifications to traditional time reversal processing, such as inverse filtering and one-bit processing, may be easily implemented in the software. This presentation describes the hardware and software that help to facilitate time reversal research. Ideas from this presentation may help others develop time reversal data acquisition systems.

Time reversal acoustics for communications, focusing, and source localization

Time reversal is a signal processing method used either to communicate privately between two locations, focus high amplitude energy to a selected spatial location, or to locate and characterize an unknown source. It has been used in acoustics for various applications including secure underwater communications, biomedical applications (e.g., acoustic lithotripsy), and for nondestructive evaluation of structures and mechanical parts. Time reversal techniques require that a calibration signal (i.e., Green’s function, impulse response, transfer function, etc.) be obtained between the source and receiver location(s). Assuming that conditions do not change, the calibration signal(s) may be used to send wave energy to the target location to provide a coherent reconstruction of a source or constructive interference at that selected location. This presentation will discuss how time reversal techniques work, introduce various types of processing used, and review various applications of the technique. If that does not sound interesting enough, videos will be shown of LEGO minifigures being knocked over with focused vibrations and targeted candles will be blown out with focused loud sound.

Incomplete acoustic time reversal, frequency shifting, and matched field processing

In a reciprocal time-invariant medium, sound can travel in both directions along acoustic paths. This bi-directionality enables acoustic retrofocusing via time reversal (TR) and acoustic source localization via matched field processing (MFP). Given this common basis, the simplest frequency-domain formulations of acoustic TR and MFP are essentially identical; both involve a quadratic product of forward- and backward-propagating complex fields. For this presentation, all scenarios begin with the acoustic field from a remote source that propagates forward through a potentially-complicated acoustic environment to a transducer array where it is recorded. The difference between TR and MFP then arises because the second backward-propagating (time-reversed) acoustic field may be (i) a genuine field in the actual environment as in acoustic TR, or (ii) a predicted field developed from a model of the environment as in MFP. This presentation describes a third possibility, frequency-difference MFP or incomplete acoustic TR, that combines (i), (ii), and a multiplicative property of harmonic plane waves to produce frequency-downshifting. The resulting unconventional formulation involves a cubic product of complex fields and allows robust reduced-resolution source localization with sparse arrays in complicated uncertain environments. Example results from simulated, laboratory, and ocean propagation data are shown. [Sponsored by ONR, NAVSEA, NSF].

An Acoustic Time Reversal Technique to Locate a Partial Discharge Source: Two-Dimensional Numerical Validation

In this paper, the time reversal (TR) technique is extended to localize a partial discharge (PD) source in the acoustic regime. The proposed acoustic TR method consists of three steps, a) the acoustic waves emitted by the PD source are recorded by at least one sensor, b) the recorded signal(s) are reversed in time and back injected into the considered medium via numerical simulation, and c) an appropriate criterion is applied to identify the focal spot, which corresponds to the PD source location. We present a numerical validation of the proposed method in which the acoustic wave equations are solved numerically using the two-dimensional (2D) finite-difference time-domain (FDTD) method. The minimum entropy criterion is used to localize the PD source. The numerical results reveal that the proposed acoustic TR method is very efficient in localizing the PD source in the presence of the transformer windings.

Structural health monitoring of liquid filled above ground storage tank floors: a time-reversed approach to acoustic emission source location

Acoustic Emission (AE) is a proven successful Non-Destructive Testing (NDT) method to assess the state of storage tank floors. Traditional AE source location in tanks floors is performed using only the wavefronts that have traveled directly from the source to the sensor (direct hit). The wavefronts captured after reflected from the tank walls are identified and discarded. This paper proposes a new AE source location algorithm in tanks that considers a combination of reflections and direct hits. The proposed algorithm is based on time-reversed acoustics and ray theory. The methodology uses the concept of time-reversed acoustics in which a wave detected at any location can be directed back to the source when re-created at the detection place. Therefore, the developed approach takes the time at which each wave arrives to the sensor and sends it back as if time had reversed. Ray theory is used in the methodology to account for the way in which the wavefront is reflected when encounters an obstacle such as the walls of the tank. Then, the point of intersection of all wavefronts is identified using an optimization algorithm. This point where all wavefronts intersect is considered the location of the source. The location algorithm considers the first path or direct hits from the source to the sensors combined with reflections obtained by wavefronts bouncing from the tank walls. The proposed location algorithm was validated using numerical data from 176ft diameter tank and experimentally using AE data from a tank 55ft diameter.

Time reversal focusing of high amplitude sound in a reverberation chamber: Optimizing the placement of transducers

Time reversal (TR) is a signal processing technique that may be used to intentionally generate high amplitude focusing of sound. The use of time reversal in room acoustics has been studied previously, but an analysis of the dependence of spatial positioning of the transducers in the room, and its effect on high amplitude focus signals has not previously been explored. Experiments are carried out in two reverberation chambers of differing size with eight compression drivers. Using these drivers, the impulse response is found and reversed in time. The spatial dependence of the transducers position is measured by calculating the time reversed impulse response at various transducer positions and then measuring the peak focal amplitude generated when the time reversed signal is broadcast to those positions. Multiple measurements suggest that the amplitude of the focus signal changes with both room size and proximity to a reflecting surface in the chambers.

Green's theorem in seismic imaging across the scales

Abstract. The earthquake seismology and seismic exploration communities have developed a variety of seismic imaging methods for passive- and active-source data. Despite the seemingly different approaches and underlying principles, many of those methods are based in some way or another on Green's theorem. The aim of this paper is to discuss a variety of imaging methods in a systematic way, using a specific form of Green's theorem (the homogeneous Green's function representation) as a common starting point. The imaging methods we cover are time-reversal acoustics, seismic interferometry, back propagation, source–receiver redatuming and imaging by double focusing. We review classical approaches and discuss recent developments that fully account for multiple scattering, using the Marchenko method. We briefly indicate new applications for monitoring and forecasting of responses to induced seismic sources, which are discussed in detail in a companion paper.

Nonlinear focusing of high amplitude sound using time-reversal

Time reversal (TR) is a signal processing technique that may be used to intentionally generate high amplitude focusing of sound. The use of time reversal in room acoustics has been studied by others, but the application to generating high amplitude focusing has not previously been explored. The purpose of this study is to generate high amplitude sound waves in order to mimic a virtual spherical source with enough intensity to observe nonlinear wave propagation. Experiments have been carried out in a reverb chamber with eight compression horn drivers. Using these drivers, the impulse response is calculated, reversed in time, and modified using the clipping technique. When these signals are broadcast from the sources, a focus is generated at the receiver location with peak levels reaching 198 dB (ref 20 μPa). As the waves superpose at the focus, the amplitudes observed do not scale linearly when the experiment is repeated at increasing amplification levels. The compression peaks are higher in amplitude than expected, and the rarefaction troughs are lower in amplitude (less negative) than expected, when compared to linear scaling. Additionally, the diverging waves from the focus resemble the propagation of a single spreading shockwave.

Time reversal acoustics applied to rooms of various reverberation times.

Time Reversal (TR) is a technique that may be used to focus an acoustic signal at a particular point in space. While many variables contribute to the quality of TR focusing of sound in a particular room, the most important have been shown to be the number of sound sources, signal bandwidth, and absorption properties of the medium as noted by Ribay, de Rosny, and Fink [J. Acoust. Soc. Am. 117(5), 2866-2872 (2005)]. However, the effect of room size on TR focusing has not been explored. Using the image source method algorithm proposed by Allen and Berkley [J. Acoust. Soc. Am. 65(4), 943-950 (1979)], TR focusing was simulated in a variety of rooms with different absorption and volume properties. Experiments are also conducted in a couple rooms to verify the simulations. The peak focal amplitude, the temporal focus quality, and the spatial focus clarity are defined and calculated for each simulation. The results are used to determine the effects of absorption and room volume on TR. Less absorption increases the amplitude of the focusing and spatial clarity while decreasing temporal quality. Dissimilarly, larger volumes decrease focal amplitude and spatial clarity while increasing temporal quality.

Super-Resolution in Near-Field Acoustic Time Reversal Using Reverberated Elastic Waves in Skull-Shaped Antenna

Super-Resolution in Near-Field Acoustic Time Reversal Using Reverberated Elastic Waves in Skull-Shaped Antenna

Localization of sources of noise in moving atmospheres using time reversal and geometric acoustics

Due to acoustic refraction and convection in moving, inhomogeneous atmospheres, the backazimuths of recorded signals can be significantly different from the source backazimuth, posing a significant difficulty in source localization. Although an inverse problem, localization of sources of infrasound in such atmospheres has mostly been made using the forward ray-theoretic model of sound propagation. This approach uses search grids for the unknown model parameters that are subsequently estimated by optimizing an objective function. This approach can give rise to a large computational time as well as to the issue of uniqueness, stability, and accuracy of the solutions. The current paper presents a technique which avoids these issues, and which combines time reversal and geometric acoustics for source localization in such atmospheres. This technique, which can also be useful for source localization of other forms of acoustic signals in refractive media, effectively transforms the inverse problem into a forward one by utilizing a flow reversal theorem in the ray tracing equations. Beside the atmospheric conditions, the only parameters needed in this technique are the trace velocities of the signals recorded by at least two arrays. Comparison of numerical simulations with observational data will be presented.

Using time reversal focusing for active control of magnetic resonance imaging noise

The noise produced by a Magnetic Resonance Imaging (MRI) scan can cause discomfort to patients. Active Noise Control (ANC) systems have been developed to help reduce the amount of noise a patient experiences. However, due to the small enclosure of the MRI machine, ANC equipment can be cumbersome to use, and is limited to non-ferrous material. Time Reversal Acoustics (TRA) is a technique that can focus sound to a selected position in space that may be far from actual sound sources. We are exploring using TRA to deliver an ANC signal by broadcasting “anti-noise” signals that are focused to the patient’s ears through TRA to cancel the MRI noise. This presentation will explore the applicability of using TRA to deliver different types of ANC signals to determine limitations in this process and show some proof of concept results.

Calculation of Volumetric Sound Field of Pulsed Air-Coupled Ultrasound Transducers Based on Single-Plane Measurements.

Quantitative and reproducible air-coupled ultrasound (ACU) testing requires characterization of the volumetric pressure fields radiated by ACU probes. In this paper, a closed-form reradiation method combining the Rayleigh-Sommerfeld integral and time-reversal acoustics is proposed, which allows calculation of both near- field and far-field based on a single-plane measurement. The method was validated for both 3-D (circular, square) and 2-D (rectangular) planar transducers in the 50-230 kHz range. The pressure fields were scanned with a calibrated microphone. The measurement window was at least four times the size of the transducer area and the grid step size was one third of the wavelength. Best results were observed by acquiring the measurement plane at near-field distance. The method accurately reproduces pulsed ultrasound waveforms and pressure distributions (RMSE <2.5% in far field and <5.5% in near field), even at the transducer radiation surface. The effects of speed of sound drifts during the scan in the pressure were negligible (RMSE <0.3%). The reradiation method clearly outperforms conventional baffled piston models. Possible applications are transducer manufacture control (imperfections at radiation surface) and calibration (on-axis pressure, side lobes, and beamwidth) together with generation of accurate source functions for quantitative nondestructive evaluation inverse problems.

Stress corrosion crack imaging in stainless steel using the time reversed elastic nonlinearity diagnostic and nonlinear resonant ultrasound spectroscopy

Spent nuclear fuel is often stored in stainless steel canisters in the United States. Stainless steel is susceptible to Stress Corrosion Cracking (SCC). This presentation will discuss progress on the use of the Time Reversed Elastic Nonlinearity Diagnostic (TREND) and Nonlinear Resonant Ultrasound Spectroscopy (NRUS) to determine whether SCC is present and attempt to quantify the depth of the cracking. NRUS is the measurement of the amplitude dependence of a sample’s resonance frequency, which occurs because of a softening of the elastic modulus in damaged media. NRUS provides a global indication of damage in a sample. TREND employs time reversal acoustics, which focuses wave energy at various points of interest to excite localized high amplitude. The amplitude dependence of this localized energy allows pointwise inspection of a sample. [This work was funded by the Nuclear Energy University Program of the U.S. Department of Energy through a subcontract from Los Alamos National Laboratory.]